One of the biggest existential questions that has puzzled humanity for as long as humans have been around is simply, “Where did all this come from?” After countless centuries of wondering and speculating, the 20th century brought with it our first scientific answers to that question. We learned that distant objects in the Universe are speeding away from one another: evidence that our Universe is expanding. We discovered that more distant galaxies appear younger, less massive, and with greater rates of star formation: evidence that our Universe evolves with time. And we discovered a near-uniform background of blackbody radiation: evidence of an early, hot, dense, radiation-dominated state. All of these puzzle pieces, when put together, indicate that our Universe originated from a hot Big Bang some 13.8 billion years ago.

But our Universe has a very curious property that not everyone appreciates. If you add up the mass and energy of all the particles contained within the visible Universe, you can ask the question, “How big would the event horizon of a black hole with this mass be?” And the answer, perhaps surprisingly, is very close to the actual size of the cosmic horizon for our observable Universe. Additionally, there’s another related idea, made famous by Stephen Hawking, that each time we create a black hole in our Universe, it could give rise to a “baby Universe” that’s only accessible to an observer that crosses inside that black hole’s event horizon. Could our Universe, then, actually have been spawned by a black hole that was created in some sort of grand “parent Universe,” and do we give birth to a new Universe each time a new black hole is created?

It’s a fascinating idea worth exploring. Here’s what the science, at present, has to say about it.

The defining feature of a black hole is the existence of an event horizon: a boundary that tells a very different story for an object outside of it versus one inside of it. Outside of a black hole’s event horizon, any object will experience its gravitational effects, as the space will be curved by the black hole’s presence, but it can still escape. If it moves fast enough or accelerates quickly enough in the proper direction, it won’t necessarily fall into the black hole, but it could break free of the black hole’s gravitational influence.

Once an object crosses over to the other side of the event horizon, however, it’s immediately doomed to be subsumed into the black hole’s central singularity. Because the fabric of spacetime is severely curved inside a black hole, an infalling object will reach the singularity within seconds of crossing the event horizon, growing the black hole’s mass in the process. To someone located outside the event horizon, the black hole appears to form, gain mass, and grow over time.

What does this have to do with our Universe, though? If you were to take all of the known, measurable forms of matter and radiation in the observable Universe, you’d have to add up all of the following:

• normal matter, made from protons, neutrons, and electrons,

• neutrinos, ghostly fundamental particles that rarely interact with normal matter,

• dark matter, which dominates the Universe’s mass but has so far eluded direct detection efforts,

• photons, or particles of light, which carry energy from every electromagnetic event throughout cosmic history,

• and gravitational waves, which are created every time a mass moves and accelerates through the curved fabric of spacetime.

At the farthest limits of what our instruments can possibly detect, we can see up to about 46 billion light-years away in all directions. If you add up all the energy from all of these forms throughout the entire observable Universe, you can arrive at an equivalent “mass” for the Universe using Einstein’s most famous relation: E = mc².

Then, if you like, you can ask a rather profound question: if the entire Universe were compressed into a single point, what would happen? The answer is the same as it would be if you compressed any large-enough collection of mass or energy into a single point: it would form a black hole. What’s remarkable about Einstein’s theory of gravity is that if this collection of mass-and/or-energy isn’t charged (electrically) and isn’t rotating or spinning (i.e., without angular momentum), the total amount of mass is the only factor that determines how large the black hole is: what astrophysicists call Schwarzschild radius.

Remarkably, the Schwarzschild radius of a black hole with the mass of all the matter in the observable Universe is almost exactly equal to the observed size of the visible Universe! That realization, on its own, seems like a remarkable coincidence, raising the question of whether our Universe might actually somehow be the interior of a black hole. But that’s only the beginning of the story; as we dive deeper, things get even more interesting.

In the mid-1960s, a discovery revolutionized our concept of the Universe: a uniform, omnidirectional bath of low-energy radiation appeared from all locations in the sky. This radiation had the same temperature in all directions, now determined to be 2.725 K, just a few degrees above absolute zero. The radiation had a practically perfect blackbody spectrum, as though it had a hot, thermal origin, and appeared identical to within 1-part-in-30,000 no matter where you looked in the sky.

This radiation — originally called the primeval fireball and now known as the cosmic microwave background — represented critical evidence that our Universe is expanding and cooling because it was hotter and denser in the past. The farther back we extrapolate, the smaller, more uniform, and more compact things were. Going all the way back, this picture of the hot Big Bang appears to approach a singularity, the same condition found at the central interiors of black holes: a location where densities, temperatures, and energies are so extreme that the laws of physics themselves break down.

Something remarkable happens when you look at the equations that govern a black hole as well. If you start just outside the event horizon and escape to an infinite distance away from the black hole, you’ll find that your distance (r) goes from R, the Schwarzschild radius, to infinity: ∞. On the other hand, if you start just inside the event horizon and track your distance from the black hole to the central singularity, you’ll find that same distance (r) instead goes from R, the Schwarzschild radius, to zero: 0.

Big deal, right?

No, it actually is a big deal, for the following reason: if you examine all the properties of space outside of a black hole’s event horizon, from R to ∞, and compare them to all the properties of space inside the black hole’s event horizon, from R to 0, they are identical at every single point. All you have to do is replace the distance, r, with its reciprocal, 1/r (or, more accurately, to replace all instances of r/R with R/r), and you’ll find that the black hole’s interior is mathematically identical to the black hole’s exterior.

It’s almost like taking a spherical orb that’s 100% reflective — a perfect mirror — and noticing that the entirety of the Universe that’s located outside of that sphere is now contained, albeit distorted, in the mirror image that’s reflected on the sphere’s surface.

As our understanding of the Universe has improved and been refined over the past few decades, two new discoveries have rocked the foundations of cosmology. The first was cosmic inflation: instead of arising from a singularity, it now appears that the Universe was set up by a rapid, relentless state of constant, exponential expansion that preceded the hot Big Bang. It’s as though there were some sort of field that provided an energy inherent to space itself, causing the Universe to inflate, and only when inflation ended did the hot Big Bang begin.

The second was dark energy: as the Universe expands and becomes less dense, distant galaxies start to recede from us at an accelerating rate. Once again — albeit, with a much smaller magnitude — the Universe behaves as though there’s some sort of energy inherent to space itself, refusing to dilute even as the expansion of space continues. People have speculated that there might be a connection for as long as inflation and dark energy have both been around.

The fact that there’s a fundamental difference between the expansion rate of the Universe that you infer depending on which of the two classes of methods you use to measure it only strengthens that conjecture. One potential explanation that stubbornly persists for reconciling this discrepancy is that there was a stronger form of dark energy early on: one that existed after the end of inflation but decayed away before the cosmic microwave background scattered off of the primeval plasma for the final time. Perhaps inflation and dark energy have more in common than we realize, and perhaps black holes will provide a critical insight into the nature of that link.

What might that connection be? Once again, black holes could be the answer. Black holes gain mass as material falls into them, and decay, losing mass, via Hawking radiation. As the size of the event horizon changes, is it possible that this changes the “energy” inherent to the fabric of space to an observer located inside the event horizon? Is it possible that what we perceive as cosmic inflation marks the creation of our Universe from an ultramassive black hole? Is it possible that dark energy is somehow connected to black holes as well?

And does this mean that, as astrophysical black holes have formed within our Universe, that each one gives rise to its own “baby Universe” somewhere inside of it? These speculations have been around for many decades, and although we lack a definitive or provable conclusion, there are certainly some mathematically compelling pieces of evidence that suggest a link. Nevertheless, many models and ideas abound, and this line of thought continues to be compelling to many who research black holes, thermodynamics and entropy, general relativity, as well as the beginning and end of the Universe.

Unfortunately, every physical model put forth — at least so far — has failed to make unique predictions that can do the following three things.

1. Reproduce all the successes, like the already-observed phenomena, that the inflationary hot Big Bang has already successfully accounted for.

2. Explain and/or account for observed phenomena that the prevailing theory cannot.

3. Make new predictions that differ from those predicted by the current leading model, that we can then go out and test.

Perhaps the most famous attempt at this is Roger Penrose’s Conformal Cyclic Cosmology (CCC), which does make a unique prediction that differs from the standard cosmological models: the existence of Hawking points, or circles of unusually low temperature variance in the cosmic microwave background. Unfortunately, these features do not robustly appear in the data, relegating the idea that our Universe was birthed from a black hole — and the idea that black holes give rise to baby Universes — back to one that’s purely speculative.

There’s a lot to like about the idea that there’s a connection between black holes and the birth of Universes, from both physical and mathematical points of view. It’s plausible that there’s a connection between the birth of our Universe and the creation of an extremely massive black hole from a Universe that existed before our own; it’s plausible that every black hole that’s been created in our Universe has given rise to a new Universe within it.

What’s missing, unfortunately, is the key step of a uniquely identifiable signature that could tell us whether this is the case or not. That’s one of the most difficult steps for any theoretical physicist: to determine the imprint of a new idea on our observable Universe, distinguishing that new idea from our old, prevailing ones. Until we successfully take that step, work will likely continue on these ideas, but they will only remain speculative hypotheses. We don’t know whether our Universe was birthed by the creation of a black hole, but at this point, it’s a tantalizing possibility that we would be foolish to rule out.

This story was originally published in January of 2021. It was updated in September of 2026.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Greetings readers,

The Universe is a lot like a detective story: We can only see a tiny fraction of the clues that piece together the entire cosmic story, but with a combination of theory, observation, instrumentation, clever techniques, simulation, and inference, we can come up with a successful approximation for how reality truly is. After all, we can’t directly observe the expanding Universe, the Big Bang, the fabric of spacetime, or dark matter and dark energy, but we’ve learned what they are and how they behave from the full suite of all that science has learned.

We can see out to 46 billion light-years away: the edge of the observable Universe, set by the finite speed of light, the time that’s elapsed since the Big Bang, and the expansion of the Universe. But if our Big Bang was just one “bubble universe” that arose within an inflating multiverse, what would we have seen, instead, if we had been born near the edge of that bubble, with that bubble’s edge contained within our observable Universe? Dark matter is our leading theory for what makes up most of the mass in the Universe, but a rival idea, MOND (MOdified Newtonian Dynamics), makes very different predictions on large cosmic scales. In an all-new video, I explore a new test that can tell them apart! Even though the earliest galaxies cannibalized by our Milky Way are long gone, some ancient relics — globular clusters — still persist as the evidence, allowing us to reconstruct the earliest merger in our galaxy’s history.

In addition, we all know that the sky appears blue and the oceans do as well: from the ground as well as from space. But the most common reason people are given which is that “one reflects the other,” has nothing to do with actual reality. And on a quantum level, the act of measuring reality really does alter the outcome of whatever experiment we dare to perform. It’s a bizarre and counterintuitive Universe out there, but if we pay close enough attention to what’s going on, we truly can make sense of it. May we always let truth, science, and reality be our guides!

All the best,
Ethan

ASK ETHAN

Ask Ethan: Can someone possibly see the edge of spacetime?

In all directions, there seems to be more Universe, with the only edge set by how far back in time we can see. But our Big Bang may not be all there is. It could represent one region where cosmic inflation ended, creating a bubble universe within a much larger sea of inflating space. While we see no evidence of such an edge, an observer born near a bubble wall might. Here’s what they would see.

READ MORE

THE OBSERVER EFFECT

The act of measuring reality alters what you’ll observe

Pass light through a double slit, and an interference pattern appears, revealing its wave-like nature. That pattern persists even when photons are sent through one at a time, but disappears if you measure which slit each photon passes through. The simple double-slit experiment reveals the strange wave-particle nature of reality, and how the act of observing can change the outcome.

READ MORE

Curiosity doesn’t stop with the universe. Explore the ideas behind Stephen Hawking’s A Brief History of Time, Carl Sagan’s Cosmos, and more with Shortform’s in-depth book guides.

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MILKY WAY ORIGINS

NASA’s Hubble opens the earliest window into the young Milky Way

Our modern Milky Way is a large spiral galaxy filled with evolved, metal-rich stars, primarily concentrated in the galactic plane. But throughout the galaxy, ancient globular clusters persist as relics of the early Universe. A new Hubble study links 12 of them to a progenitor galaxy the Milky Way devoured just 2 billion years after the Big Bang. It may be the earliest galactic relic ever uncovered.

READ MORE

WHY SO BLUE?

The sky is blue. The ocean is, too. But the reasons why are very different.

Most of us have heard the myth that the sky and oceans are blue because one reflects the other. But the real explanations are very different. The sky appears blue because atmospheric scattering sends blue light in more directions than red, while the oceans preferentially absorb other colors, leaving blue light to penetrate the deepest. Here’s the science behind why both appear blue.

READ MORE

PUT TO THE TEST

New video: Largest-scale cosmic test ever favors dark matter over MOND

The foundational principle of cosmology is the Copernican principle: the idea that we occupy no special place in the Universe, whether in space, time, or orientation. We can test this by searching for preferred directions or asymmetries across the cosmos, including in the Big Bang’s leftover light, the CMB. If light’s polarization is inherently rotated, even by a fraction of a degree, it could reveal a preferred direction in the Universe, pointing to exotic new physics or even a flaw in our cosmic picture.

READ MORE

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Anytime there’s a chance to subject our Universe to a new cosmic test, it’s imperative that we do it. First off, it gives the Universe a chance to surprise us: to show us something that we never could have learned unless we looked. Second off, it gives us a chance to test our underlying assumptions about the Universe: what it’s made of, what laws and rules it obeys, and how it behaves in never-before-probed ways. And third — and perhaps most importantly — if there are rival ideas for making sense of the Universe, a new test has the potential to answer the question definitively: with data from the cosmos itself.

With unprecedented data about both:

• the large-scale structure of the Universe, arising from galaxy surveys like SDSS and DESI,

• and the cosmic microwave background radiation, from Planck, the Atacama Cosmology Telescope, and the South Pole Telescope,

we can finally conduct a long-awaited cosmic test: we can measure the kinetic Sunyaev-Zel’dovich effect. These measurements have the potential to test the idea of dark matter against the idea of MOND (MOdified Newtonian Dynamics), testing gravity on the largest scales and at the smallest accelerations ever. In a brand new video, I summarize the results and put them in their proper cosmic context. Have a watch below, and read the transcript beneath it to make sure you catch every detail!

Curiosity doesn’t stop with the universe. Explore the ideas behind Stephen Hawking’s A Brief History of Time, Carl Sagan’s Cosmos, and more with Shortform’s in-depth book guides.

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Transcript

Is the Universe made up of an enormous amount of dark matter, and is that responsible for the gravitational effects that we see on galactic and cosmic scales that normal matter alone can’t explain?

Or is there no extra matter, and was Einstein wrong about how gravity works on the largest scales in the Universe?

The leading hypothesis is dark matter, but a rival one – MOdified Newtonian Dynamics, or MOND – paints a very different picture.

In a brand new study, the largest-ever direct test of dark matter vs. MOND has just been conducted, by looking at both the large-scale structure of the Universe and the temperature properties of the cosmic microwave background together, and inferring a very difficult-to-measure effect: the kinetic Sunyaev-Zel’dovich effect. These measurements enabled astronomers to determine, on average, how quickly two galaxies separated by a given distance accelerate towards one another: a measure of the acceleration due to gravity on the largest scales ever.

When the results are obtained, on enormous cosmic scales – between 100 million and up to 750 million light-years – they were able to show that the laws of gravity behaved as Einstein’s general relativity predicted, and not as the alternative, MOND, predicts on those same cosmic scales.

It’s a strong piece of evidence in our quest to uncover both how the Universe works and exactly what it’s made of, posing another colossal difficulty for alternatives to our standard picture of how reality truly is.

Scientific foundations

Our main goal, always, in physics and astrophysics, is to understand the Universe as it actually is. Two of the biggest questions we know how to ask are:

• what are the laws that govern reality,

• and what are the ingredients that make up our Universe?

Over recent decades and centuries, we’ve built up a consensus picture that represents humanity’s best approximation to reality: with general relativity and quantum field theory providing the laws, and the Standard Model plus dark matter and dark energy making up the ingredients.

However, it’s important to recognize that there are alternative formulations and viewpoints that challenge this consensus picture, and that ability, to challenge the consensus, is central to the advancement of science.

Testing those alternatives against our present consensus model, side-by-side with actual data, is one of the most powerful ways to reveal how the Universe is and isn’t.

How we gain knowledge

Anytime you make a new observation – anytime you view or analyze the Universe in a fundamentally different or novel way – it’s an opportunity to test our best answers to these questions, and to test alternatives against the consensus wherever their predictions differ.

If the new observations and the prevailing theory are compatible, that’s good: the theory is validated.

If the new observations and the prevailing theory are incompatible, that’s also good: they expose a flaw or incompleteness in our current understanding, and may pave the way for a new, superior picture of reality.

And if the new observations can tell two fundamental ideas about what the Universe is made of or what laws govern it apart, that’s the best possible outcome of all. It points the way towards a major success of one idea while simultaneously identifying a major flaw in the other.

This brings us to an exciting new piece of research that puts the question of “dark matter vs. MOND” to its largest-scale ever direct test. First, a little background.

Dark matter vs. MOND

According to the dark matter hypothesis, we’ve got the laws governing gravity correct with general relativity, but we need an extra ingredient to the Universe, some form of dark matter, to explain all that we see. This idea first came about all the way back in the 1930s when looking at the motions of individual galaxies within galaxy clusters, but it grew to prominence in the 1970s with observations revealing the too-rapid rotation at too great of a distance within spiral galaxies. It was those internal motions of spiral galaxies, when we could measure them well enough, that first brought the idea of dark matter to mainstream prominence.

Alternatively, according to the MOND hypothesis, there’s no need for a new ingredient, and instead we should just modify the law of gravity. This idea originated in the early 1980s by noting that individual galaxies would have their rotation curves match the data without a new ingredient if instead a new modification to gravity was introduced: one with a “fundamental acceleration” scale. At larger accelerations, you’d recover Newton’s 1/r² force law, but at small accelerations, you would transition to just 1/r, as that “fundamental acceleration scale” would lead to a change in behavior. That MOND scale only appears, however, at very small accelerations. The acceleration of gravity here on Earth is 9.8 m/s², but for comparison, the MOND scale is 1.2 Angstroms per second-squared (Å/s²).

Within the Solar System, there’s no way to test MOND, because all of our accelerations are far too large. Even Pluto’s acceleration around the Sun is many orders of magnitude too large; you’d need for the separation distances to be far greater than what we can actually observe.

Astrophysical tests

People have, in recent years, attempted to look at what we call “wide binary” systems of stars, because at very large separation distances, the accelerations actually drop to low enough values that you could potentially test for the effects of MOND. Indeed, several different groups have identified large numbers of systems and looked at their motions statistically, reaching wildly different and mutually incompatible conclusions in the process. That’s because it would take centuries-to-millennia of observations to actually determine the orbits of these systems individually, and confirm that they are indeed gravitationally bound, rather than just coincidentally close objects, while ruling out the presence of a higher-multiplicity (like a trinary or quaternary) system. Since we can’t do that, we have to make guesses as to which points we keep in and which ones we throw out. And – surprise, surprise – the conclusion you draw depends on which guesses and assumptions you make. So we can’t draw any surefire conclusions from that.

But if we could go to larger scales, we’d be able to test these two hypotheses against one another directly: where accelerations are small, at large cosmic distances, and where the two competing ideas make vastly different predictions for how quickly any two pairs of massive objects should be seen accelerating towards one another on average. There’s a chance to do this not just on solar system, stellar, or galactic scales, but on cosmic ones. The key is through an effect known as the kinetic Sunyaev-Zel’dovich effect, which has taken us decades to actually be able to measure.

Astrophysical effects

In our Universe, we have a background source of light that comes to us from all directions in space from our perspective: the cosmic microwave background. When it passes through empty space, the light we observe is identical to the light that was emitted, save for the redshifting effects of cosmic expansion. But if that light passes through a cloud of ionized matter – particularly if it’s an electron-rich cloud – it’s going to have two different ways of altering that light.

First, depending on the temperature of those electrons, the photons that pass through them will get boosted to higher energies. This creates what looks like a “cold spot” in the background radiation, because there’s a smaller amount of energy in the wavelengths where you’ve been seeing the rest of that background. But in reality, that radiation is hotter, because it’s been altered, and shifted up to hotter temperatures and higher energies. Because it’s dependent on electron temperature, this is called the thermal Sunyaev-Zel’dovich effect.

But remember, we were interested in the large-scale motions of objects, relative to one another, caused by their gravitational accelerations. A thermal effect doesn’t shed light on that. If the galaxies that have these ionized electrons in them are also in motion, however, that motion will also imprint itself onto the background light that passes through it, either boosting or de-boosting the energy of that light in the process. In other words, there isn’t just a thermal Sunyaev-Zel’dovich effect, but also a kinetic (or, sometimes, kinematic) Sunyaev-Zel’dovich effect as well.

The new cosmic test

The way the researchers approached this problem was to take regions of the sky that had both really excellent cosmic microwave background data, using the Atacama Cosmology Telescope and really excellent large-scale structure data, using spectroscopic galaxy maps from the Sloan Digital Sky Survey. Although isolated galaxies don’t generally affect the CMB, or cosmic microwave background, in meaningfully observable ways, galaxies within galaxy clusters – which often do have large quantities of ionized electrons inside – makes a great laboratory for finding galaxies that can have their Sunyaev-Zel’dovich effects measured.

So that’s the procedure:

• You take large numbers of galaxies where the kinetic Sunyaev-Zel’dovich effect can be measured in that same region.

• You measure their properties, spectroscopically, to determine their 3D positions and also their velocities relative to the Hubble flow.

• Then, you correlate all pairs of galaxies together, where you can estimate both the relative velocities between different pairs, which gives you a window into their mutual gravitational accelerations, and the separation distance between each pair.

• And then, you see what the relationship is between all three of those things at once. You want to see the relationship between:

1. the kinetic Sunyaev-Zel’dovich effect, which you measured from the cosmic microwave background. You want to see the relationship between that and

2. the pairwise velocity of each set of galaxies as a function of your chosen theory of gravity, relative to the MOND theory, or relative to Einstein’s general relativity with dark matter,

3. and then you want to see how that evolves, Sunyaev-Zel’dovich effect versus pairwise velocity, as a function of separation distance between those pairs of galaxies. You’ll divide large numbers of galaxies at all different distances from each other into “bins” for analysis.

On the scales that the research teams looked at, which range from 30 megaparsecs up to 230 megaparsecs – or about 100 million up to 750 million light-years – the accelerations due to gravity are extremely low, so this should be a really good test of that low-acceleration MOND regime. If the acceleration doesn’t follow a 1/r² force law, but instead follows a 1/r law, it would provide evidence for MOND and not only against dark matter, but against Einstein’s general relativity itself being valid on cosmological scales. However, if it does follow a 1/r² law, and not a 1/r law, it would directly show that MOND’s predictions are invalid on cosmic scales, and that Einstein’s general relativity and dark matter remain consistent with all that we see.

Results from the new study

What you see here is the main result from the paper: Gallardo et al., Physical Review Letters, 2026. It shows a series of binned data points of galaxies at a variety of separation distances from one another, with the measured effects of the pairwise kinetic Sunyaev-Zel’dovich effect shown on the y-axis. Note how small this effect is: they are looking at ten-to-a-hundred billionths of a Kelvin in temperature fluctuation differences here! What you’ll notice is that at very large separation distances, the effect is extremely small, but at small separation distances, the effect gets stronger.

The key question is: by how much?

According to MOND, the increase should be slow: corresponding to a 1/r force law.

According to Einstein with dark matter, the increase should be faster: corresponding to a 1/r² force law.

Visually, the data looks like it agrees with the consensus picture extremely well, and looks like it disagrees with the MOND predictions. In more detail, what they’re plotting – known as the pairwise velocity – scales as the integral of the correlation function multiplied by the force law, and that’s what is represented by the blue and yellow lines (with uncertainties), as shown for dark matter and MOND, respectively.

But how do things stack up when we perform a more robust statistical analysis?

As you can see, the standard picture, where we get a 1/r² force law at large distances, looks exactly right: completely consistent. However, the MOND picture, with a 1/r force law, doesn’t match the data, and is ruled out, already, at around the 99.9% confidence level.

In private communication with the lead author of the study, Patricio Gallardo, I asked him what would happen if they used newer, superior DESI data instead of older Sloan Digital Sky Survey data, and he pointed me to a paper from earlier this year, where they showed that DESI data detects the existence of the kinetic Sunyaev-Zel’dovich effect at a whopping 9-sigma statistical significance, compared to just 6-sigma for the Sloan Digital Sky Survey data that was used in the present paper. This means that a future analysis using the already existing DESI data could rule out MOND at the 99.999% confidence level: tantalizingly close to the 5-sigma “gold standard” in detection significance for astrophysics and cosmology.

The rest of the evidence

Certainly, people who are wedded to their support of MOND will disagree. They’ll point to the external field effect and the baryonic Tully-Fisher relation, saying that “all galaxies follow this, so some analogous effect should also exist on cosmic scales.” Maybe it should, but when they say that, they’re surreptitiously ignoring the ultra-diffuse galaxies we’ve discovered that don’t follow that relationship at all, proving that it’s not universal.

They’ll look at the large-scale structure of the Universe or the fluctuations in the cosmic microwave background’s temperature, and say, “well you need some other modification to make MOND work on large cosmic scales anyway,” and then they’ll put forth modifications that – surprise surprise – behave identically to how dark matter behaves under Einstein’s general relativity.

Or they’ll look at strong independent evidence for dark matter, such as from colliding galaxy clusters, and resort to the ancient argument that “these collision speeds are too fast for general relativity and a dark matter-rich cosmology to explain,” while completely ignoring the revised collision speeds that we’ve been able to infer since we first spotted colliding galaxy cluster MACS J0018.5, which has the collision occurring along the line-of-sight, enabling us to measure cluster collision velocities and the emergence of shocks directly. This system showed that the actual collision speeds between clusters was much lower than previously inferred, because shocks depend on the circumcluster medium in a way that was not understood or appreciated until we made those line-of-sight observations in 2024. They reduce the estimated speeds of cluster collisions, including from the Bullet Cluster, to speeds that fall exactly in line with the predictions of a standard dark matter cosmology, while still allowing for the emergence of the shocks that we’ve seen.

Concluding thoughts

It’s always fashionable to challenge the scientific consensus, and to favor ideas that would lead to a scientific revolution. But to get there, you have to have data on your side: data that refutes the consensus position and that supports your alternative. The way you do good science is you extract predictions from a theory that lead to observables, and then you use those observables to test the validity of your theory. When your theory passes the test, it remains in the realm of validity; when your theory fails the test, you need to either revise or discard it.

What you mustn’t do – and this is a tantalizing temptation to have to resist – is move the goalposts, or as we say in cosmology, to “invoke the tooth fairy a second time,” to save a fundamentally broken idea. The new test is so powerful because it’s so simple and straightforward: in such a clean cosmic laboratory. It shows how gravity works at low accelerations on large cosmic scales: where the density in any location departs only slightly (by much less than 100%) from the average cosmic density. It shows that gravity still obeys a 1/r² force law, not the alternative 1/r force law that’s at the core of MOdified Newtonian Dynamics. And it shows that we can use easily accessible data, from the CMB and large-scale structure, to conduct this test, implying that as better data comes in, from ACT, from the South Pole Telescope, and from the Simons Observatory on the CMB side, and from SPHEREx, Euclid, Nancy Roman Telescope, and Vera Rubin Telescope on the large-scale structure side, we’ll be able to conduct an even more stringent version of this test. It’s a wonderful illustration of how science works and progresses, if only we can bring ourselves to accept the conclusions that the data is unambiguously telling us.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who’s been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

with Ethan Siegel • August 22, 2026

Greetings readers,

Our cosmic picture of the Universe is one of the most important and profound developments of the last 100 years. A full century ago, we had only recently discovered other galaxies beyond the Milky Way. General relativity was barely a decade old, and quantum theory was still in its infancy: when we still didn’t understand the concept of a wavefunction. Today, we have the Standard Model, dark matter, dark energy, and the inflationary hot Big Bang model of cosmology, but we continue to push the frontiers forward, revealing details near and far about our Universe in profound ways.

A half century ago, we performed one of the most direct tests of relativity ever: by flying two clocks around the world in opposite directions. We recorded that they not only measured time differently from a clock on the ground, but from each other as well, depending on whether they flew with or against Earth’s rotation. The first solar storm we ever recorded occurred only in 1859 with the spectacular Carrington event. But through the forensic study of ancient trees, we’ve discovered larger solar storms that occurred 1000+ years even before that. We’ve found many “multi-lobed” objects in the asteroid and Kuiper belts, including our first three-lobed object, teaching us how our primordial Solar System has evolved over the past 4.6 billion years.

Meanwhile, on cosmic scales, there has been some evidence (albeit dubious) that the Universe may not be the same in all directions. If that were true, what would it mean for the expanding Universe, and could it change our ultimate fate from a “heat death” to something different? And finally, there’s a new anomaly that’s been appearing in the polarization data from the oldest light in the Universe: the CMB. That anomaly is known as cosmic birefringence, and it shows that the polarization of that ancient light is rotating by only a fraction of a degree, but that could be enough to point the way to new physics. With future observatories, like the Simons Observatory and BICEP3, coming online soon, we’ll be able to test the Universe as never before. Let’s keep going; the greatest cosmic truths about reality await!

All the best,
Ethan

TIME TRAVELERS

Flying clocks in opposite directions around the world proved Einstein right

According to Einstein’s relativity, motion and gravity both affect how quickly time passes. Move relative to another observer and return, and you’ll have aged less; experience different gravitational conditions, and your clock will tick differently. By flying atomic clocks both with and against Earth’s rotation, then returning them to the same starting point, scientists put Einstein to a remarkable test. Here’s what they learned.

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WRITTEN IN THE RINGS

Earth’s oldest trees reveal history’s strongest solar storms

One of the greatest threats to humanity’s electronic infrastructure is a powerful solar storm, which could trigger a multi-trillion-dollar disaster. Today, we remain largely unprotected against an event like the great Carrington storm of 1859. But in 774–775, an even more powerful cosmic event struck Earth. After a decade-long investigation, we’ve learned the Sun may be even more violent, and more threatening, than we ever imagined.

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ASK ETHAN

Ask Ethan: Does uneven cosmic expansion change our fate?

If you can measure how fast the Universe is expanding today and how that rate has changed over time, you can learn what the Universe is made of and determine its ultimate fate. Our standard cosmological model is dominated by dark energy, pointing toward a likely heat death. But recent measurements suggest the Universe may not be expanding equally in all directions. Could that change our cosmic fate, and if so, how?

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SPACE SNOWBALLS

“Dirty snowballs” have joined together since the planets formed

Although our Solar System is rich with objects today, we see only the survivors from 4.6 billion years of evolution. Small bodies like asteroids and Kuiper belt objects offer us the best window into its past, preserving primordial material from the era of planet formation. We’ve discovered many conjoined objects, including snowman-like contact binaries. Now, in 2026, we’ve found our first contact trinary: the asteroid Nysa. Here’s what it reveals about where we came from.

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A COSMIC TWIST

Cosmic birefringence: the signal that could upend cosmology

The foundational principle of cosmology is the Copernican principle: the idea that we occupy no special place in the Universe, whether in space, time, or orientation. We can test this by searching for preferred directions or asymmetries across the cosmos, including in the Big Bang’s leftover light, the CMB. If light’s polarization is inherently rotated, even by a fraction of a degree, it could reveal a preferred direction in the Universe, pointing to exotic new physics or even a flaw in our cosmic picture.

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Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

One of the most enduring principles in the entire Universe is the cosmological principle: an outgrowth of the Copernican revolution. Copernicus, nearly 500 years ago now, hypothesized that the Earth occupied no special place in the Universe when he put forth the heliocentric picture, instead recognizing that Earth might be just an ordinary planet like all the others: revolving around the Sun. We’ve since expanded our view of the Universe substantially, and with it, our view of the cosmological principle. In every way fathomable, there’s nothing special or preferred about our location in space and time — in the here and now — relative to the vast, expanding Universe that we ourselves are a part of.

But any number of possible signals, coming from the Universe itself, could call that very principle into question. Such signals could include:

• a bulk “flow” to the Universe, where everything appears to move uniformly in the same direction,

• an overall directional field to the Universe, such as an electric field, that causes a uniform acceleration of certain particles,

• a preferred direction to the sky, where the Universe is anisotropic (or not the same in all directions),

• an overall rotation to the Universe, or a preference for galaxies rotating either clockwise or counterclockwise from our perspective, rather than random orientations,

• or an overall net polarization to the cosmic background of light remaining from the Big Bang: the cosmic microwave background (CMB) radiation.

While extremely large studies show excellent agreement with the longstanding assumptions of isotropy and homogeneity for the Universe, recent studies of the CMB — first from WMAP, then from Planck, and now from ACT as well — indicate a small but positive, real, and nonzero effect for cosmic birefringence of that light. Behind the Hubble tension, it’s probably the second most exciting tension we’re seeing in observational cosmology today, and the three leading explanations all give us something to look forward to with new, superior observations on their way.

One of the most foundational aspects of our Universe is the principle of relativity itself: that uniform motion itself is indistinguishable from being at rest. Dating all the way back to the time of Galileo, it’s the oldest physical principle that’s still thought to be 100% correct. Einstein built upon this to conclude that the speed of light in a vacuum was indeed invariant for all observers, and that was the key insight that led to special relativity. All observers, everywhere, no matter where or when they were or how they were moving, measured the same speed for all forms of light. That principle was later extended by Einstein to include acceleration and gravitation as well, leading to general relativity.

However, since light itself is an electromagnetic wave in nature — with oscillating, in-phase, mutually perpendicular electric and magnetic fields — different observers in relative motion to one another will observe the same light wave to have different wavelengths: the light will be redshifted or blueshifted depending on the relative motion of the observer to the direction of propagation of the light wave itself. The light’s speed is invariant (the speed of light in a vacuum), but the observed wavelength changes.

This is thought to be absolutely true in a vacuum. However, it isn’t always true when light passes through a material, as materials can slow down light in a wavelength-dependent fashion. This, quite famously, is why white light gets broken up into its individual wavelengths when you pass it through a dispersive prism.

For some materials, refraction goes a step further: to what we initially called “double refraction,” and now know as birefringence. The first birefringent material was identified way back in 1669: a calcite crystal, which literally splits the light that passes through it into multiple paths. The reason some materials are birefringent are because they’re both:

• anisotropic, which means that the effects of the material on light propagating through it are dependent on the direction of propagation through the material, and also

• polarization-dependent, where light that’s polarized parallel or perpendicular through the material behaves differently, and moves at different speeds and/or refracts at different angles.

We have excellent observational constraints against the vacuum of space being birefringent. The light we see of different wavelengths traveling from even tens of billions of light-years away exhibits no birefringent properties, even if the original light itself is heavily polarized. That rules out a birefringent vacuum, and rules out birefringence for a variety of light emitted from distant galaxies: optical, X-ray, ultraviolet, infrared, and radio light.

But if the leftover light from the Big Bang, the CMB, were found to exhibit birefringence, that would truly have profound implications.

You see, the light that we observe from the Universe’s earliest stages really is polarized, and is polarized in two fundamentally different ways.

• There are E-mode polarizations, where the polarization pattern can occur either along the line-of-sight or perpendicular to it, in a fashion that either points radially outward (like stars shooting out from a firework) or radially inward (like spokes from a bicycle tire connecting to the axel).

• And there are B-mode polarizations, where the polarization pattern occurs in a fashion that curls around the line-of-sight direction: either clockwise or counterclockwise, like a planet orbiting a star.

The radiation in the Universe, according to standard theory, should be born with both E-mode and B-mode polarizations (both seeded by cosmic inflation), where the E-modes arise from density imperfections and the B-modes arise from gravitational wave imperfections. According to our standard cosmological model — the inflationary Big Bang with dark matter and dark energy, also known as ΛCDM — these types of polarizations should arise and imprint themselves in the CMB’s light, but shouldn’t lead to an observation of cosmic birefringence in the CMB’s light on its own.

However, it’s important to check the CMB’s light for those effects, because there are three main explanations for cosmic birefringence if we do, indeed, observe it.

1. The standard model of cosmology is wrong, and something that we presently think is fundamental about the Universe is wrong or violated. This could include a violation of Lorentz invariance (basically violating the principle of relativity), or the introduction of a parity-violating field: basically some type of new physics that means all directions don’t equally obey the same rules, violating our concept of isotropy.

2. There’s some new type of exotic particle that couples to the photon, such as variants of the axion, a theorized dark matter candidate, that’s inducing this cosmic birefringence as the light travels through the space where those particles are present.

3. Or that there’s some sort of systematic effect that’s causing these observations, rather than a true cosmic signal. Such effects could include polarized dust within our own galaxy (a galactic foreground effect) or an imperfect calibration of the instruments used to measure the CMB and its polarization. This is the trickiest one, and must be robustly ruled out before we draw any wild conclusions.

The way you look for cosmic birefringence is to measure the polarization of the light from the CMB, and to see if it exhibits a non-zero rotation angle, indicating that the CMB light’s polarization is actually being affected by something — one of these three effects, most likely — happening during its journey from 46 billion light-years away until it reaches our instruments.

There are, to date, three main CMB experiments that have measured the polarization of the CMB’s light exquisitely.

• First was WMAP: NASA’s successor to COBE and the first observatory, more than 20 years ago now, to measure the CMB’s polarization. It placed strong constraints on the effects of cosmic birefringence, to the less-than-1° level in terms of rotation, but preferred a slightly positive, non-zero value over the no-birefringence option of 0°.

• Next was Planck in the 2010s: a more sensitive ESA space mission that was the successor to WMAP. Planck had two polarization-sensitive instruments: HFI and LFI. The strongest signal for birefringence came from HFI, but HFI is also most easily polluted by dust, so many were skeptical. Overall, Planck favored a value for β, the polarization rotation angle, of around 0.3°. However, because of the large errors and uncertainties, the statistical significance was very weak, not even rising to the 2σ (95% confidence level) threshold.

• Planck ended in 2018, and subsequent reanalyses of Planck and WMAP data combined — along with a new model for galactic foregrounds — led to similar values for the rotation angle β: between 0.2° and 0.3°, but with a more suggestive significance of around 3σ.

• However, with the latest data release from the Atacama Cosmology Telescope (ACT) collaboration here in the 2020s, a new set of analyses has arrived. From ACT alone, the significance rules out zero rotation angle (β = 0°) at 2.9σ, and by jointly combining WMAP, Planck, and ACT data together, a brand new paper accepted in Physical Review D in August of 2026 claims to see β = 0.277° ± 0.057°, which is non-zero at 4.8σ significance.

The latest data from ACT, despite not having all-sky coverage, is superior to anything Planck or WMAP has done in terms of smaller angular size and superior polarization measurements, which is primarily responsible for the recent spike in interest in cosmic birefringence.

One of the most interesting things you can do, as cosmologist Johannes Eskilt showed recently, is to try to correlate E-modes and B-modes in the CMB together. You can do this by comparing what are known as “EE” modes (where only the electric-type polarization patterns are considered) with “EB” modes (which links together E-modes and B-modes), and which should be zero unless there’s either some type of real cosmic birefringence or an instrumental alignment/calibration error.

If you only had one data set, you would worry very strongly about whether your instrument was aligned and calibrated properly, as everything would hinge on you having an exquisite understanding of that instrument and all of its potential systematics. However, with two independent instruments — such as ACT and Planck — you can correlate the “EE” modes with the “EB” modes in three different ways:

• of ACT “EE” modes with ACT “EB” modes,

• of ACT/Planck “EE” modes with Planck/ACT “EB” modes (mixing the two),

• and of Planck “EE” modes with Planck “EB” modes.

If you get consistent, non-zero EB modes in all three cases, then it’s unlikely that the alignment and calibration was the sole culprit. The independent confirmation of the signal, including of the same magnitude and with the same effect, serves as a sanity check that this may indeed be something real.

The reason for this is that there are two effects that can “appear” to cause the rotation of the polarization angle:

• α, which represents the effect of instrument calibration and the polarized foreground emissions (e.g., of the galaxy),

• and β, which is the actual primordial cosmic birefringence angle.

The actual signal that we measure is the combination of “α + β,” and so we have to make sure that α isn’t responsible for the entirety of what we’re inferring the rotational angle of polarized light is. The fact that we see a strong, positive signal for all three cases strongly suggests that either at least some of the effect is due to a real, non-zero (i.e., cosmic birefringence), or that some combination of miscalibration and galactic foregrounds are conspiring in both data sets, consistently, to mimic a birefringence signal.

Of course, the ACT data is better than the Planck data for these purposes: it’s more granular and it extends to smaller scales, or higher multipoles in the data. (Planck has more complete sky coverage, however, making it superior for all-sky studies.) The joint analysis is able to rule out the possibility that dust is causing the non-zero EB signal, and furthermore shows that if you want to recover a value where there’s no cosmic birefringence at all (β = 0), then both the dust modeling and the instrumental calibration must both be failing together across multiple data sets.

The latest ACT data is particularly exciting because of how revolutionary it would truly be if cosmic birefringence were real. In astrophysics as well as particle physics the “gold standard” for announcing a new discovery is the vaunted 5σ threshold for statistical significance. With ACT, WMAP, Planck LFI and Planck HFI data combined, we’re anywhere between 4.6σ and 4.8σ sigma in terms of significance, at maximum, depending on how you’re treating the data. That’s tantalizingly close to achieving the necessary threshold for announcing an actual discovery of cosmic birefringence.

But before you get too excited about that particular line of thought, it’s worth remembering the adage that “extraordinary claims require extraordinary evidence.” We have fooled ourselves many times in the past with claims that have achieved the necessary statistical significance to announce a discovery, but were instead plagued by a systematic error that went unidentified. The fiasco of faster-than-light neutrinos was due to a hardware error. The DAMA/Libra collaboration’s alleged detection of dark matter was most likely the result of poor conduct among the experimenters, and was ruled out by a reproduction test. And violations of lepton universality, which nearly reached the 5σ threshold at the Large Hadron Collider, were instead indeed identified to be a calibration error: one that was corrected.

In all cases, once the experiment or observation was correctly calibrated, the effect disappeared. It will take strong, robust, independent confirmation of the presence of cosmic birefringence before the community is ready to accept something so exotic.

Thankfully, that’s exactly what a brand new CMB experiment is going to do, with the new measurements from BICEP3 and then, likely definitively, from the Simons Observatory. These independent measurements, from two independent facilities, can help us gain control over the instrumental systematics that could be responsible for our current best measurements indicating that β, the polarization rotation angle of the CMB light, is non-zero. They can teach us whether we’ve calibrated our instruments correctly, and whether we truly understand the foreground dust emissions as well as we claim to.

If it turns out that this polarization rotation term is indeed non-zero and frequency-independent for the CMB, then that would hint at an axion-like field that couples to the photon, which could truly lead to a revolution in cosmology: either by shedding light on the nature of dark matter or by showing us that the Universe does exhibit a fundamental parity (mirror-symmetry) violation. In this Universe, it’s our observed reality — to the limits of our measurement capabilities — that’s the ultimate arbiter for how physical reality actually is. The suggestive evidence pointing toward cosmic birefringence, although it isn’t overwhelming just yet, could be the crack in our current cosmic picture that takes us to the next level in our understanding of the Universe.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

with Ethan Siegel • August 15, 2026

Greetings readers,

There’s a lot that can feel heavy here on planet Earth, but from a physics point of view, the reason for that is always the same: gravity. High up above our atmosphere in outer space, thousands upon thousands of satellites orbit our world, kept in stable orbits by Earth’s gravitational acceleration. But even though Earth’s gravity still persists, each satellite, and anyone on board them, experiences only weightlessness. There’s a profound reason for that! Back in the 1600s, when Newton formulated our first successful theory of gravity, he assumed that gravitation was an instantaneous force. But that’s not the case at all, as Einstein theoretically showed back in the 20th century and as gravitational wave observations verified in a profound 2017 event! And while we still don’t know whether gravity is quantum or classical at a fundamental level, we do have a hint that it might be quantum: the gravitational Aharonov-Bohm effect, demonstrated and measured back in 2022.

Meanwhile, the Daniel K. Inouye Solar Telescope (DKIST) just released not only the most incredible images of the Sun’s surface ever acquired, but they also took time-lapse images that have now been stitched together into videos, enabling us to visualize an ocean of plasma waves “breaking” on the Sun for the first time ever. And while the ESO is speeding ahead to build the ELT, the Extremely Large Telescope, which will shatter the current record for world’s largest optical telescope, there are two others that the world is trying to bring to fruition: the GMT, or Giant Magellan Telescope, and the TMT, or Thirty Meter Telescope, the last of which will be the only one in the Northern Hemisphere. Humanity, and the human enterprise of science, very much needs all three, but whether we’ll get what we need remains to be seen!

Finally, we’ve got an all-new episode of the Starts With A Bang podcast for your enjoyment, featuring Dr. Carlos Vargas about space telescopes in the ultraviolet, and the all-new (and soon-to-launch) Aspera mission. Remember: We didn’t learn all that we did about the Universe just by sitting around and waiting for the knowledge to come to us. We got curious, and we went out and investigated it for ourselves. That curiosity still calls out to us, compelling us to turn our gaze skywards and to ponder and investigate all that we find out there. It’s your Universe, too, so let’s keep on exploring it together!

All the best,
Ethan

ASK ETHAN

Ask Ethan: Why are astronauts weightless while still in Earth’s gravity?

Here on Earth, gravity accelerates us at a near-constant 1g, or 9.8 m/s². But in space, astronauts are completely weightless, whether in low-Earth orbit or traveling to the Moon. Yet, Earth's gravitational pull still acts on them, even hundreds of thousands of kilometers away. If gravity hasn't disappeared, why do astronauts experience weightlessness?

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THE GRAVITATIONAL DELAY

Gravity can’t be an instantaneous force

When Newton first formulated gravity, he imagined masses attracting one another instantaneously across any distance. But no signal, not even gravity, can travel faster than the speed of light: the cosmic speed limit for all particles and forces. Einstein restored the cosmic order, and recent observations have confirmed that gravity really does travel at the speed of light. Here’s how we know.

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SOLAR SWELLS

We caught an ocean of plasma waves “breaking” on the Sun

The Daniel K. Inouye Solar Telescope (DKIST) is our most powerful tool for viewing the Sun's surface, revealing it at unprecedented resolution. After capturing individual convective cells and the connections between sunspots, magnetic fields, and surface structures, new images reveal something never seen before: the Kelvin-Helmholtz instability. Familiar from ocean waves, rolling clouds, and gas giant atmospheres, this phenomenon has now been confirmed on the Sun, too.

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THE PODCAST

Starts With A Bang podcast #132 – Space in the ultraviolet

Here on Earth, we can see the Universe because our atmosphere is transparent to visible light. Most other wavelengths, including gamma-rays, X-rays, ultraviolet, infrared, and microwaves, require space-based observatories. Yet we’ve had precious few ultraviolet observatories, including Hubble and the now-defunct GALEX. With the ASPERA mission launching soon, principal investigator Carlos Vargas joins us to explore what this new window on the Universe could reveal.

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TELESCOPE TRIFECTA

Why the world needs all three 30-meter class telescopes

Today’s ground-based optical telescopes reach up to 10.4 meters across, but going to the next level requires bigger, sharper, and more sensitive facilities. Three next-generation, 30-meter-class telescopes, the GMT, TMT, and E-ELT, hope to begin soon observing the Universe. But astronomy’s future isn’t about which one will be best. All three are needed, for very different reasons. Here’s what each will bring to the world.

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A QUANTUM CLUE

The strongest experimental hint that gravity is quantum in nature

Three of nature's fundamental forces, the electromagnetic, strong, and weak nuclear forces, are known to be quantum in nature. Gravity, however, has only been described by Einstein's general relativity: a classical, continuous theory. But by demonstrating the Aharonov-Bohm effect for gravity, previously seen only with electromagnetism, we may finally have our first clue to gravity's quantum nature.

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Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

If you were to break down the matter in our Universe to its smallest and most fundamental subatomic constituents, you’d find that everything was made up of individual quanta, each of which possesses both wave and particle properties simultaneously. If you pass one of these quantum particles through a double-slit and don’t observe which slit it passes through, the quantum will behave as a wave, interfering with itself on its journey and leaving us with only a probabilistic set of outcomes to describe its ultimate trajectory. Only by observing it can we determine precisely where it is at any moment in time.

This bizarre, indeterminate behavior has been thoroughly observed, studied, and characterized for three of our fundamental forces: the electromagnetic force and the strong and weak nuclear forces. However, it’s never been tested for gravitation, which is the one remaining force that only has a classical description in the form of Einstein’s general relativity. Although many clever experiments have attempted to reveal whether a quantum description of gravity is required to account for the behavior of these fundamental particles, none has ever been performed decisively.

A long-studied quantum phenomenon, the Aharonov-Bohm effect, was thought to apply only to electromagnetism. However, a 2022 experiment demonstrated that it occurs for gravity as well: the strongest hint toward a Universe where gravity is inherently quantum in nature. This greatly underappreciated result, could serve as our greatest clue toward the ultimate nature of the weakest force in all the Universe.

The quantum question at the core of reality

In the world of quantum physics, few experiments are more demonstrative of the bizarre nature of reality than the double-slit experiment. Originally performed with photons more than 200 years ago, shining light through two thin, closely-spaced slits resulted not in two illuminated “lines” on the screen behind the slits, like you’d expect if light were just a beam, but rather in an interference pattern. The light that went through each of the two slits must be, somehow, interacting and interfering before that light reaches the screen, creating a pattern that displays light’s inherent wave-like behavior.

Later on, this same interference pattern was shown to be generated not merely by light alone, but

• by groups of electrons,

• by the quanta identified as photons,

• for single photons, even as you passed them through the slits one at a time,

• and for single electrons and even composite particles (like atomic nuclei), again even as you passed them through the slits one at a time.

As long as you don’t measure which slit the quantum particles go through as they’re going through it, the wave-like behavior is easily observable. It is evidence of the counterintuitive, but very real, quantum mechanical nature of the system: Somehow, an individual quantum is capable of going through “two slits at once” in a sense, where it must interfere with itself.

And yet, if you do measure which slit these quanta pass through as you perform the double-slit experiment, you see no interference pattern at all. Instead, you just get two lines of “clumps” on the far side of the screen, which correspond to the set of quanta that went through slit #1 and slit #2, respectively.

This is an extraordinarily weird result that gets at the heart of what makes quantum physics so unusual, and yet, so powerful. You cannot simply ascribe definite quantities like a “position” and a “momentum” to each particle, as you would in a classical, pre-quantum treatment of those quantities. Instead, you have to treat position and momentum as quantum mechanical operators: mathematical functions that “operate” (or act) on not a particle-like entity, but rather on a quantum wavefunction.

When you “operate” on a wavefunction, you get a probabilistic set of outcomes for what is possible to observe. When you actually make the key observation — i.e., when you cause the quantum you’re “observing” to interact with another quantum whose effects you then detect — you can recover only a single value, corresponding to your measurement of the system.

Let’s assume that you perform this experiment with electrons — particles with a fundamental, negative electric charge — and that you send them through these slits one at a time. If you measure which slit the electron goes through, it’s easy to describe the electric field generated by the electron as it goes through that slit: because the electron behaves as a particle, and we know how electric fields behave when generated by particles.

What happens, then, if you don’t make that critical measurement? It turns out that even if the electron goes through both slits at once — one way of interpreting how the electron’s wavefunction interferes with itself to produce the interference pattern that results — you can still describe the electric field that even a single electron, with an ill-determined position, generates. The reason you can do this is because it isn’t just the individual particles or waves that are quantum in nature, but the physical fields that permeate all of space are quantum in nature as well: they obey the rules of quantum field theory.

For the electromagnetic interaction, as well as the strong and weak nuclear interactions, we’ve verified and validated the predictions of quantum field theory many times over. The agreement between theoretical predictions and the results of experiments, measurements, and observations is spectacular, agreeing in many cases to better than 1-part-in-a-billion precision. For those three forces, the Universe really is quantum in nature.

However, if you ask a question like, “what happens to the gravitational field of an electron as it goes through a double-slit,” you’re bound to be disappointed. Theoretically, without a working quantum theory of gravity, we cannot make a robust prediction of any type; we can only make the general prediction that it ought to be different from the deterministic, point-like effect predicted by Einstein’s and Newton’s theories. Experimentally, detecting such an effect goes far beyond our current capabilities. At present, we do not know whether gravity is an inherently quantum force or not, as no experiment or observation has been able to make such a critical measurement.

The Aharonov-Bohm effect

There are so many subtle quantum effects that not only pop out of our equations, but also have been physically verified, that it’s sometimes difficult to keep track of them all. For example, in the classical Universe, if you have a charged particle in motion, it can be affected by both the presence of electric fields and magnetic fields.

• The electric field will accelerate the charged particle along the direction of the field, in direct proportion to the strength of the field and proportional to the charge of the particle, causing it to either speed up or slow down in the process.

• The magnetic field accelerates the charged particle perpendicular to both the magnetic field and the direction of motion of the particle, causing it to bend, but not to increase or decrease its speed.

If your electric and magnetic fields are both zero, your electron won’t accelerate; it will just continue along in constant motion, exactly as you’d expect from Newton’s first law about an object in motion remaining in constant motion.

However, in the quantum Universe, there’s another effect that comes into play that can change the behavior of your quantum particle, even when the electric and magnetic fields are both zero: the Aharonov-Bohm effect. The key to understanding it is to learn the relationship between electric and magnetic fields and a more abstract concept: electric and magnetic potential.

Electric potential is more commonly known as “voltage” in our everyday experience. Changes in voltage, from one region to another, are what creates electric fields and compels electric currents to flow. You can get the electric field from the electric potential simply by taking the gradient, which details how the field changes, directionally, throughout space.

Magnetic potential is a little more complicated because it doesn’t have a common analog like voltage, and also because the magnetic field itself doesn’t come about from a simple gradient, but rather from a mathematical operation known as the curl of the magnetic vector potential. It’s enough, at this level, to know that electric and magnetic potentials exist, and that different kinds of “rates of change” (or derivatives, in mathematical terms) allow you to derive the electric and magnetic fields from them.

Now, here’s where it gets interesting: You can have interesting, non-zero electric and/or magnetic potentials in a region of space even where the electric and magnetic fields are both zero. For a long time, physicists wondered whether the potential was actually a physical thing, since it appears to only be the fields, not the potentials, that affect the motions of particles in a measurable way.

This turns out to be true in classical physics, but not 100% of the time in quantum physics. In particular, the potential of a field couples to the phase of a charged particle’s wavefunction, and if you measure the phase of that charged particle — which you typically do with interference experiments — you’ll find that it does depend on the electromagnetic potential, not just on the electric and magnetic fields.

The way we typically measure the Aharonov-Bohm effect is to set up a cylindrical region of space that contains a substantial but highly confined magnetic field: something that’s easy to create with a long coil of wire, like a solenoid. You then set up a charged particle in motion around that magnetic field, but carefully, so that the particle itself only passes around, and doesn’t pass through, the region containing the field.

The wavefunction will still experience a phase shift that can be — and has been — observed experimentally. This is true even though the electric and magnetic fields are negligible outside the confined region containing the field, and the probability of finding the particle within the field-containing region is also negligible. It might seem like yesterday’s news. After all, the original work by Aharonov and Bohm dates back to 1959, with an earlier paper by Ehrenberg and Siday predicting the same effect back in 1949.

However, the same effect that’s been observed for the magnetic potential should be observable for any force that arises as a consequence of a potential. This includes not only the electric force and the other known quantum (strong and weak nuclear) forces, but also the gravitational force. If a clever enough setup could be devised, it should be possible to search for evidence of a gravitational Aharonov-Bohm effect as well.

What about gravity?

When you want to experiment with the gravitational force, the biggest problem is always that gravitational effects are so maddeningly small. Although people have been designing experiments for many decades with a view toward detecting this effect, an enormous breakthrough arrived in 2012. A team of researchers led by Michael Hohensee came up with the idea for an experiment that could feasibly be performed with current technology.

The idea was that you can create ultra-cold atoms and control their motion by pulsing a laser beam, including into a region where the gravitational potential — but not the field — is different from other locations. Even in regions where the gravitational force is zero, which can be arranged by a careful setup, the non-zero potential could still have an effect. If you can then split a single atom into two matter waves, move them into areas with different potentials, and then bring them back together, you could observe an interference pattern, measuring their phase and, therefore, quantifying the gravitational Aharonov-Bohm effect.

If the effect turns out to be non-zero, it must be a purely quantum phenomenon in nature. Only, for the first time, it’s an effect that would be entirely dependent on the gravitational force, rather than any other interaction.

A decade later, a team led by Chris Overstreet succeeded in experimentally performing the experiment and measuring the gravitational Aharanov-Bohm effect. As published in the January 13, 2022 issue of Science, the team took multiple ultra-cold rubidium atoms, put them into quantum superpositions with one another, and compelled them to trace two different paths inside a vertical vacuum chamber. Because there was a heavy mass at the top of the chamber — but one that was axially symmetric and completely outside of the chamber itself — it only changed the gravitational potential of the atoms, with the atoms that reached a higher trajectory experiencing a greater change in potential.

Then, the atoms are brought back together, and from the interference pattern that is produced, a phase shift emerges. The amount of the phase shift that’s measured should correspond to:

• how separated the two atoms are from one another,

• how close they each come to the top of the chamber,

• and whether the external mass which alters the gravitational potential is present or not.

By performing this experiment over and over with a variety of such conditions, Overstreet’s team was able, for the first time, to measure the phase shifts of these atoms and compare them with the theoretical predictions for the gravitational Aharonov-Bohm effect. Lo and behold, not only has it been detected, but the match is dead on.

With that in mind, we come to the big question: Does the detection of this quantum mechanical phase shift, owing to the gravitational potential and not to either the gravitational field or any of the known quantum forces, demonstrate the inherently quantum nature of gravity?

Unfortunately, it does not prove that assertion; it only provides a hint. We have created a phase shift, shown how the shift accumulates owing to gravitational potential and not the gravitational field, and measured it to be in agreement with theoretical predictions using atom interferometry. This establishes the same thing for gravitation that was previously established for electromagnetism: a demonstration that it isn’t simply the gravitational force or field that’s real, but that the gravitational potential itself has real, physical effects on the quantum mechanical properties of a system.

This is a remarkable achievement. But the analysis could be applied to any force or field that’s derivable from a potential: both a quantum force/field as well as a classical force/field. (Only the particle in question, in this case, the Rubidium atom, is necessarily quantum in nature.) It’s a tremendous triumph for quantum mechanics under the influence of gravity, but it isn’t quite enough to demonstrate the quantum nature of gravity itself. Perhaps someday we’ll get there. In the meantime, the quest for a deeper understanding of gravitation itself continues.

This article was first published in January of 2022. It was updated in August of 2026.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

with Ethan Siegel • August 8, 2026

Greetings readers,

Of all the worlds we’ve ever discovered, including the hundreds we know of in our Solar System and the many thousands of exoplanets around other stars and in interstellar space, Earth remains the only one that we know harbors any forms of life at all. Despite the immense vastness of space — an observable Universe that extends for 46 billion light-years in all directions and that may even be infinite if you include the unobservable part — there’s no evidence that anyone is coming from anywhere else to save us from ourselves. Our planet, a fragile dot in the vast cosmic expanse, is the only place we can survive and thrive in all the known Universe.

That saddles us with a tremendous set of responsibilities. It is up to us to steward our planet and to keep it habitable for future generations of humans, plants, animals, and more. We can collectively choose to build a path to a new golden age of science right here in my home country of the USA by returning to the three keys — one each for Bohr, Pasteur, and Edison — that brought us to prominence in the first place. By studying our planet, we can learn that gravity, despite being “universal” in nature, actually varies by up to 0.7%, from the location of greatest to smallest gravitational acceleration, across our planet’s surface. And by looking all across our Solar System, we can discover how rare not only Earth-like atmospheres are, but planetary atmospheres in general.

We’ll have an all-new slate of stories for you next week, along with a brand-new podcast to share. It’s about an exciting new space mission that seeks to measure our galaxy and Universe in a wholly underappreciated wavelength of light: the ultraviolet. Remember to remain curious about the Universe, and if you get a chance on the night of August 12th/morning of August 13th, don’t forget to look up to catch the Perseid meteor shower at its peak!

All the best,
Ethan

SCIENTIFIC PROGRESS

3 forgotten keys to a real scientific golden age for the USA

Vannevar Bush’s “Science: The Endless Frontier” inspired America’s federally funded scientific dominance from the end of WWII to the present. Michael Kratsios, the current head of the Office of Science and Technology Policy (OSTP), wrote “A New Golden Age,” a document similar in style to Bush’s but vastly different in substance. And yet neither got it fully right. The key gap was filled by a lesser-known thinker, and his “quadrants” complete the story.

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ASK ETHAN

Can “following the science” save the Earth?

Planet Earth, as large and vast as it is, is also finite and fragile. With the power of scientific discovery at our disposal, we can fight a great many maladies, but our very nature can be our worst enemy as unchecked competition and overconsumption threaten our ability to survive, long-term, on Earth. Can following the science be the answer?

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THE GREAT BEYOND

Is the Universe truly infinite in size?

The limits of our observable Universe are set by three things. The expansion rate throughout history, the time elapsed since the Big Bang, and the constant speed of light take us back 13.8 billion years in time and 46.1 billion light-years in distance. But beyond that limit, there ought to be more Universe, and no one knows how far it goes. Is the full Universe finite or infinite? Many have their suspicions.

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GRAVITATIONAL PULL

Where is gravity the strongest and weakest on Earth?

Our Earth has spent over 4 billion years in hydrostatic equilibrium, pulled into a rounded shape by its own gravity, yet it still isn't a perfect sphere. Thanks to the crust, the mantle, and our planet's rotation, gravity varies significantly across the surface. Here's where it's strongest and weakest, and why.

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RANKING THE SOLAR SYSTEM

Which 10 atmospheres are the Solar System’s thickest?

The thickest atmospheres in the Solar System, unsurprisingly, belong to the four gas giants, followed by Venus, Titan, and Earth. Mars comes in eighth, with less than 1% of Earth’s surface pressure. Beyond that, the pressures drop spectacularly. Learn about all ten, plus some “honorable mentions” that are the most surprising of all.

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Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Although Jupiter and Saturn rank #1 and #2 in terms of having the thickest atmospheres of any planetary bodies in the Solar System, the pattern of “mass determining atmospheric thickness” doesn’t continue to hold as we move to lower and lower masses. By the time we get down toward the end of the top 10, the worlds with the thickest atmospheres in our Solar System can get downright surprising.

Our Solar System has giants, rocky planets, moons, asteroids, and more.

By size, it’s clear that the gas giant worlds vastly outstrip any of the terrestrial planets, and this is true for mass as well. Although Saturn is nearly the same physical size as Jupiter, it contains just 21% of the planetary mass in the Solar System, compared to Jupiter’s whopping 71%. Mass is the largest factor in determining whether a planet has an atmosphere or not, but other factors such as temperature, history, and composition can play major roles as well.

But few of these worlds keep and maintain an atmosphere.

Although Earth and Venus are the two largest rocky objects in the Solar System, Mars, Mercury, as well as over 100 of the largest moons, asteroids, and Kuiper belt objects have all achieved hydrostatic equilibrium. Ganymede and Titan are larger than Mercury, but Callisto, at 99% of Mercury’s size, has just one-third of Mercury’s mass. Although it may come as a surprise, the largest bodies on this list don’t necessarily have the thickest atmospheres, as Mercury and the Moon have very thin atmospheres compared to Triton and Pluto, despite their larger sizes.

Which ones, then, have the thickest atmospheres?

Of the eight planets in our Solar System, the four gas giant worlds are the least dense, with less than half the density of the least dense rocky planet (Mars), and with Saturn being even less dense than water. While the four gas giants have the thickest atmospheres, there are many moons and other worlds with thicker atmospheres than either the planet Mercury or the Moon possesses.

1-4.) Jupiter, Saturn, Uranus, and Neptune: with thick, volatile envelopes.

These four images show the four gas giants as imaged by JWST, with the sizes of the images rescaled to reflect their relative actual size to one another, rather than the observed sizes perceived by JWST’s cameras. Jupiter has the thickest atmosphere, followed by Saturn’s, with slightly larger-and-less-massive Uranus thought to possess a thicker atmosphere than Neptune does.

Mass enables thick hydrogen-helium atmospheres, with denser Neptune’s atmosphere thinner than Uranus’s.

Although it will take a dedicated orbiter mission (and perhaps also a probe) to Uranus and Neptune to know for certain, models indicate that Neptune, although smaller than Uranus, is denser, and has a hydrogen-and-helium atmosphere that’s thinner than that of Uranus, implying that Uranus has the third-most substantial atmosphere in the Solar System, about 3000 miles (5000 km) thick, behind Jupiter and Saturn, and that Neptune’s, at about two-thirds the thickness of Uranus’s, is fourth overall.

5.) Venus: with 92 times Earth’s barometric pressure.

The WISPR data from the Parker Solar Probe, in monochrome, clearly matches the surface features seen by the significantly longer-than-optical wavelengths of the orbiter Magellan, shown in assigned color. The view of Venus from the Parker Solar Probe represents the first time the clouds of Venus have ever been seen parting, revealing the surface below. Venus’s atmosphere is the thickest of all worlds in the Solar System, by far, after the four gas giants.

Volcanically created, the pressure equals being 3000 feet (900 meters) underwater on Earth.

Venus has about 4 times as much nitrogen in its atmosphere as Earth does in ours, but its much thicker atmosphere is dominated by carbon dioxide: thought to be driven by extensive volcanic activity on Venus. Only the gas giant worlds possess a thicker atmosphere than Venus does, which has approximately 92-93 times the pressure at its surface than Earth experiences at its surface.

6.) Titan: with 1.5 times Earth’s atmospheric pressure.

This ultraviolet image of the limb of Titan was acquired by the Cassini spacecraft in 2005, which discovered a large number of layers to the tholin-rich haze atop the nitrogen and methane-rich atmosphere. Among the rocky worlds in our Solar System, only Venus’s atmosphere is thicker.

Despite Titan’s low mass, its thick nitrogen-methane atmosphere enables liquid methane on its surface.

At an estimated mass of 9 quintillion kg, Titan’s atmosphere is nearly double the mass of Earth’s and, despite Titan’s smaller size and lower gravity, creates about 1.5 times the pressure at Earth’s surface. Even at Titan’s cold temperatures, this allows methane to exist in the liquid phase on its surface, with a thick atmosphere of nitrogen, methane, and sunlight-induced hazes atop its surface.

7.) Earth: whose nitrogen-oxygen atmosphere enables surface liquid water at Earth-like temperatures.

As light strikes the Earth from space, it must pass through the various layers of the atmosphere, where it’s distorted by turbulence, clouds, and molecules, and can be absorbed and/or reflected as well. The atmosphere distorts incoming light, which is a huge obstacle for ground-based astronomers. Our thin-but-substantial atmosphere is very rare in the Solar System, with only Titan having comparable pressures to Earth, and with only Earth possessing the right pressure-temperature combination for liquid water on its surface.

8.) Mars: with a current atmosphere just 0.7% as thick as Earth’s.

Mars, like most rocky worlds in the Solar System, is full of craters and has an impact-rich history. Despite how easily visible the Martian atmosphere is from orbit, it’s very thin, just 1/140th the thickness of Earth’s atmosphere and less than 1/200th of the mass of Earth’s atmosphere.

With pressures below water’s triple point, the liquid phase is a past relic of Mars.

Oxbow bends only occur in the final stages of a slowly flowing river’s life, and this one is found on Mars. While many of Mars’s channel-like features originate from a glacial past, there is ample evidence of a history of liquid water on the surface, such as this dried-up riverbed: Nanedi Vallis. However, today’s atmosphere on Mars is too thin, with the pressure below the triple-point of water, to enable water to be in the liquid phase on the Martian surface.

9.) Triton: whose nitrogen-dominated, methane-rich atmosphere is 0.0014-0.005% of Earth’s.

Triton’s south polar terrain, as photographed by the Voyager 2 spacecraft and mapped to a spheroid of the appropriate shape and size. About 50 dark plumes mark what are thought to be cryovolcanoes, with those dark trails colloquially called ‘black smokers.’ Triton is a captured Kuiper belt object, having most certainly cleared out almost all of Neptune’s original moons. Of all moons in the Solar System, Triton has the 2nd thickest atmosphere.

Thin layers of frozen (CO₂) volatiles sublimate into gas during Tritonian summer, driving Triton’s weather.

This portion of JWST’s NIRCam image of Neptune focuses on its giant moon, Triton. Appearing blue in this assigned color image, ice-rich Triton brightly reflects the near-infrared light emitted by the Sun to the tune of ~70%. By comparison, Neptune only reflects a tiny fraction of the incident sunlight in the infrared, as methane, one of Neptune’s primary components, is a spectacularly successful infrared absorber. The eight-spike pattern unique to JWST is clearly visible around Triton, but Triton’s atmosphere was best revealed by Voyager 2 measurements and during occultation events.

10.) Pluto: whose nitrogen-dominated atmosphere has 0.00014-0.001% Earth’s pressure.

As New Horizons flew by Pluto, it acquired views of this distant world from several different angles. Here, 15 minutes after closest approach, New Horizons looked back and captured this image, revealing layers of atmosphere and planetary hazes, along with shadows cast along the planetary surface by towering ice mountains reaching up to 11,000 feet (3500 meters) high. Pluto’s atmosphere is about two-thirds as thick as Triton’s atmosphere, owing to its lower overall mass and greater distance from the Sun.

Conditions mirror Triton’s, but Pluto is smaller and colder.

Just 15 minutes after passing by Pluto on July 14, 2015, the New Horizons spacecraft snapped this image looking back at the faint crescent of Pluto illuminated by the Sun. The icy features, including multiple layers of atmospheric hazes, are breathtaking, and provide us with the best-ever measurements of Pluto’s atmospheric properties. As Pluto rotates on its axis and ventures closer-and-farther from the Sun, certain volatiles can vaporize and condense, leading to various forms of precipitation, such as nitrogen and methane snow. It is yet possible that Eris, as well as other Kuiper belt worlds whose atmospheres remain unmeasured, will have substantial atmospheres as well.

Io’s sulfur and salt-rich atmosphere (0.5-40 nanobars) is next,

These images of Jupiter’s innermost major moon, Io, showcase a volcanic eruption (center), coming from the volcano Prometheus. When Galileo was in Jupiter’s shadow (upper left), it showcased Io’s sodium cloud, while sulfur is the primary component of Io’s atmosphere. Overall, Io has the 3rd thickest atmosphere of any moon in the Solar System, behind only Titan and Triton.

followed by Ganymede, Callisto, and Europa,

Although Earth contains the most liquid water on its surface of any of the 8 planets in our Solar System, the most water in any form is found on Jupiter’s moon Ganymede. Next in order is Saturn’s Titan, Jupiter’s Callisto, and Jupiter’s Europa. When ultraviolet sunlight splits apart water molecules on Ganymede, Callisto, and Europa, the heavy oxygen persists, forming the 12th-through-14th thickest atmospheres in the Solar System.

whose oxygen atmospheres arise from split-apart water molecules.

NASA’s Galileo spacecraft was an orbiter mission around Jupiter in the 1980s and 1990s, and took these four images of Jupiter’s four largest moons: Io, Europa, Ganymede, and Callisto. Io is the closest to Jupiter and has the thickest atmosphere: #3 among moons in the Solar System and #11 among all Solar System worlds, followed by Ganymede and Callisto, and then Europa, as sunlight splits water molecules apart on those latter three worlds, where hydrogen escapes but oxygen remains behind in a thin atmosphere.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

with Ethan Siegel • August 1, 2026

Greeting readers,

In almost every phenomenon within our physical Universe, if you look hard enough, there are secrets of nature just waiting to be unlocked. From the quantum phenomena at play within every atom to the vast expanse of the cosmos and everywhere in between, the Universe provides us with no shortage of natural laboratories where our curiosities are free to run wild. As long as we’re willing to interrogate the Universe with a scientific spirit of open-mindedness, and to let whatever it tells us observationally, experimentally, and through our measurements be the guiding light to our conclusions, we’re bound to not only learn new lessons about nature itself, but to open up new frontiers as far as the next generations of inquiry are concerned.

For Ask Ethan, I received a wonderful question about what the first forms of life — perhaps arising just 1 or 2 billion years after the Big Bang, rather than the 13.8 billion that have elapsed from our perspective — would see in their night skies. The answers go deeper than you might realize! Down on subatomic scales, the Universe became filled with neutral atoms and transparent to radiation from the Big Bang after only 380,000 years had elapsed. If not for a very rare and special quantum transition, it would have taken hundreds or even thousands of times as long! I went through the last 200 years of astronomy’s history and picked out the best 20 discoveries, one from each decade, that helped bring our Universe into focus. I then went back even before that to highlight who I think is one of the most underappreciated heroes of science: Johannes Kepler. Will you agree that what he did was truly profound?

Finally, a contentious issue: the U.S. Government just released a brand-new report titled Science: A New Golden Age, and while the reaction among scientists was largely to sound the alarm, I wanted to see what was in there for myself. Go ahead and read my summary — or the full 100-page report, if you like — and see what plans are in store for the future of U.S. science. There’s plenty to explore, discover, and plan for. And remember: It’s your Universe too. If you believe that basic research and frontier knowledge are worth fighting for, your voice can make a difference just as much as anyone’s can. I’ll keep doing my best to bring it all to you either way!

All the best,
Ethan

ASK ETHAN

Ask Ethan: What would “first life” see in their night sky?

In our night sky, we can see the Moon, the planets, thousands of stars, the Milky Way, and even a few nebulae and distant galaxies. With telescopes, we can peer to the edge of the observable Universe. But if life had arisen much earlier in cosmic history, when the Universe was younger, hotter, denser, and more uniform, the night sky would have looked vastly different. What would observers have seen 1 or 2 billion years after the Big Bang, and could life have even existed back then?

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QUANTUM LOOPHOLE

The rare quantum reason that neutral atoms are possible

After the hot Big Bang, the Universe was hot, dense, and ionized: a primeval plasma. It took hundreds of thousands of years to cool enough for neutral atoms to form. But every newly formed atom emitted a high-energy ultraviolet photon capable of exciting and reionizing another atom. Only a rare, mostly forbidden process called the two-photon transition allowed neutral atoms to survive at all. Without it, our Universe would look nothing like it does today.

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BEHIND THE HEADLINES

An honest look inside the “Science: A New Golden Age” report

In July 2026, White House Office of Science and Technology Policy Director Michael Kratsios released a report to the President titled “Science: A New Golden Age.” Spanning more than 100 pages, it lays out an unprecedented vision for the future of U.S. science, one that many scientists are sounding the alarm about. But what does the report actually say? Are scientists overreacting to its language and priorities, or are their concerns well-founded?

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THE KEPLER WAY

Johannes Kepler, not Newton or Einstein, is science’s best role model

History is filled with scientists who had revolutionary ideas, found evidence to support them, and sparked scientific revolutions. Far rarer is someone who discovers the evidence doesn't quite fit, abandons a cherished idea, and embraces a better one instead. That's what set Johannes Kepler apart from nearly every other great scientist, and why he remains one of history's greatest scientific role models.

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COSMIC MILESTONES

The 20 biggest cosmic discoveries of the last 20 decades

Two hundred years ago, we didn't know the distances to the stars, what they were made of, whether other galaxies existed, or how the Universe began or would end. Step by step, each discovery laid the foundation for the next, revealing how our Universe came to be. Today, in 2026, we've learned more than ever before, yet the latest observations have uncovered mysteries that even our best data can't resolve. Here's the cosmic journey that brought us here.

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Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

It’s hard to fathom just how far we’ve come in our understanding of the Universe over the past 200 years. Back in the early 1800s, we didn’t even know how far away the stars were, what they were made of, or how they worked. We didn’t know what the Universe was, how big it was, or even what the laws were that governed it. We were up to seven planets in the Solar System, with Uranus being discovered in 1781, and had uncovered several asteroids: objects in between the orbits of Mars and Jupiter. We didn’t know the nature of the Milky Way, or whether there was anything at all beyond it. And we didn’t know if the Universe was infinite and eternal, or finite across either space or time.

All of those things have changed and more. Today, we not only know how stars and galaxies work, but can retrace our cosmic history back billions of years: to the Big Bang and even the inflationary period that came before it. We know not only of the matter that we ourselves are made out of, but also radiation, neutrinos, plasma, dark matter, and even dark energy. We see the Universe not only in visible light, but in wavelengths beyond what our eyes can see, as well as in particles and gravitational waves, too. Although these scientific advances have occurred through slow and careful research, there are giant leaps we’ve taken as well.

From 200 years ago until today, here are 20 great achievements and discoveries from the past 20 decades that have truly opened up the Universe to humanity.

1830s: Parallax is discovered

If the Earth truly orbited around the Sun, then it should be possible to see the closest stars shift their apparent positions relative to the more distant background stars as the Earth changes its relative position by up to 300 million km over a calendar year. After centuries of not being able to see a parallax at all, Friedrich Bessel finally does it: for the star 61 Cygni in 1838. We later learn that Thomas Henderson observed Alpha Centauri in 1832-1833 and saw a parallax, but was too afraid to publish his findings until after Bessel did so, with Henderson only publishing his results in 1839. The Earth truly does orbit around the Sun.

1840s: Neptune is discovered

With a seventh planet to test Kepler’s laws against, astronomers noticed a major anomaly: it appears to violate Kepler’s 2nd law. Instead of making an ellipse that “sweeps out equal areas in equal times,” it first moves too fast, then goes at the right speed, and then moves too slowly. Several theorists explore the idea that a more distant, eighth planet is gravitationally tugging on Uranus, but Urbain Le Verrier gets it right and sends his predictions to the Berlin observatory, where Johann Galle and Heinrich d’Arrest discover Neptune the same night the letter arrives: September 23, 1846. It is the first-ever discovery of matter through its gravitational effects alone: “dark matter.”

1850s: the Carrington Event occurs

Arguably the most important event in the history of solar astronomy and heliophysics, astronomer Richard Carrington was tracking a large, irregular sunspot when it suddenly made a “white light flare,” with unprecedented brightness, that lasted about 5 minutes. 17 hours later, the largest geomagnetic storm ever recorded on Earth occurred, with aurorae coming down to the equator, and early electrified systems (like telegraphs) activated and caught fire: the first electrical grid disruption. Solar astronomy has advanced tremendously since then, but a Carrington-like event today would be a multi-trillion dollar disaster. We have, frighteningly, put no substantive countermeasures in place at all.

1860s: spectroscopy enables the discovery of helium

In the early 1860s, astronomer William Huggins applied the technique of spectroscopy to starlight for the first time: splitting light into its component wavelengths. Just a few years later, in 1868, astronomers Pierre Janssen and Norman Lockyer detected an unknown spectral line in the Sun’s atmosphere: the first detection of helium, discovered in space before it was isolated here on Earth. (The first time this would occur for any element.) Today, spectroscopy is used to identify all sorts of atomic, ionic, and molecular fingerprints, but the 1860s marked the first time it was ever used for cosmic purposes, and with profound results.

1870s: stellar classification is pioneered

Although today we understand the complex relationship between a star’s color and temperature, as well as the relationship between spectral lines, composition, and ionization, it was a long, slow process that took us several decades before we settled on our modern scheme of it. But stellar classification began in the 1870s with the work of Angelo Secchi. Secchi created the foundational four classes:

• I: of white and blue stars with thick hydrogen lines,

• II: of yellow stars with prominent metal lines,

• III and IV: orange, red, and carbon-rich stars,

dividing stars into classes based on measurable properties like color and spectra line prominence. Astronomer Hermann Vogel would shortly thereafter unify two of Secchi’s classes and subdivided categories to track fine differences in lines, also in the 1870s: the birth of stellar classification.

1880s: nebular and galactic astrophotography begins

Although photography had been around for decades, and the first photograph of stars was taken way back in the 1850s, it was the combination of telescopes and cameras that first began revealing structures in deep-sky objects too faint for the human eye to detect. Representing a huge improvement in the amount of detail that we were aware of in the Orion Nebula and, most spectacularly, the Andromeda Nebula (now the Andromeda Galaxy, including the discovery of its spiral arms), it set the stage for all of modern long-exposure astronomy, from photographic plates to today’s modern digital images.

1890s: the discovery of the Lorentz transformations

Arguably the most important step on the road to relativity, the Lorentz transformations brought the earlier concept of the invariance of the speed of light (from Maxwell’s equations), the null results of the Michelson-Morley experiment, and the concept of length contraction (from FitzGerald) together, predicting time dilation for the first time and correctly detailing how velocities sum together when close to the speed of light: in defiance of Newton’s laws. This would pave the way for Einstein’s development of first special relativity and then general relativity, which would lead to our modern picture of reality in the 20th century.

1900s: high-energy astronomy is born

In 1900, in a laboratory experiment involving radium, physicist Paul Villard discovered gamma-rays: the highest-energy form of radiation of all, exceeding X-ray energy. Max Planck, that same year, proposed a solution to the puzzle known as the ultraviolet catastrophe: that energy is quantized, even energy in the form of light. Einstein’s photoelectric effect (in 1905) explained how we can quantify and measure it, and numerous theoretical predictions set expectations for radiation of all wavelengths to exist in space. The next decade, balloon-borne astronomy would begin measuring cosmic particles, and the rest of the 20th century would see astronomy develop in every known wavelength of light.

1910s: a new theory of gravity is confirmed

In 1905, Einstein put forth special relativity into the world, and then after a decade of further work, general relativity: a new theory of gravity. Although there were indirect tests of relativity, like the orbit of Mercury, it would fall to a novel prediction — of starlight seeming to bend due to the Sun’s gravity during a total solar eclipse — to test the new theory directly. On May 29, 1919, expeditions across multiple continents, led by Arthur Eddington, conducted exactly that critical test. The results proved Einstein right and Newton wrong, and ushered in the era of general relativity. Over a century later, it’s still the finest description of gravity we’ve ever developed.

1920s: spiral nebulae are proved to be galaxies outside of the Milky Way

Back in 1920, a great debate occurred, where the subject was the nature of spiral nebulae. Were they protostars, or other objects within our own Milky Way? Or were they galaxies unto themselves: “island universes” that resided outside of our own galaxy? The debate was only settled three years later: in 1923, when Edwin Hubble observed what were determined to be individual stars in Andromeda. That allowed us to calculate a distance, and determine how far away the object was: far beyond the distances of Milky Way objects. This would lead to many other such distance measurements, and eventually, to the discovery of the expanding Universe. Cosmology would never be the same.

1930s: the first hint of dark matter

All sorts of fascinating developments happened in the 1930s. The expanding Universe became well-accepted. Supernovae and supernova remnants were identified, and the name was coined. The original idea of the Big Bang, or “primeval atom” as it was called, was developed. But perhaps most importantly was the first hint of dark matter: discovered by observing the relative speeds of individual galaxies within a single galaxy cluster. The mismatch between:

• the mass inferred from starlight, or stellar mass, and

• the mass inferred from the motions of the galaxies, or gravitational mass,

was severe, and represented the first evidence for dark matter. Although this wouldn’t be taken seriously until the work of Vera Rubin and the measurement of rotation curves of individual galaxies nearly 40 years later, the cosmological importance of Fritz Zwicky’s work can’t be overstated.

1940s: the Big Bang theory is developed

Although the idea of extrapolating the expanding Universe backward had been around for a long time, it was in the 1940s that George Gamow and his collaborators, Ralph Alpher and Robert Herman, began working out the consequences of a hot, dense, rapidly expanding and much more uniform initial state. This led to a series of cornerstone predictions:

• a cosmic web of structure that forms hierarchically and galaxies that are more primitive in the past: with smaller-scale structures forming first and larger-scale ones maturing later on,

• a leftover bath of radiation — a primeval fireball — that redshifts and cools to create a background of low-energy radiation today,

• an early period of nuclear fusion, where elements heavier than hydrogen are formed,

• and even a relic population of neutrinos and antineutrinos, forming a bath that’s even colder than the leftover background of radiation.

By today, all of these predictions have been confirmed, tracing their roots back to Gamow and his colleagues: the first to explore these lines of thought.

1950s: we make sense of the nuclear physics within stars

The Big Bang wasn’t the only game in town, however, as far as explaining how the elements came to be. Elements are also produced via nuclear fusion in stars, and for a long time, we didn’t know how. In the 1950s, a team of scientists figured it out. On the astronomical side, Martha and Geoffrey Burbidge team with Fred Hoyle on the theoretical physics side, and Willie Fowler on the nuclear experiment side to develop a theory for how heavy elements are built up in stars from hydrogen alone. The discovery of the triple-alpha process and the predicted Hoyle state of excited carbon validated the whole idea, and now we know, except for the lightest elements, stellar nucleosynthesis is what’s responsible for the majority of heavy atoms in the Universe.

1960s: the “smoking gun” for the Big Bang, the CMB, is found

In the mid-1960s, a new telescope, designed for radar, came online: the Holmdel Horn Antenna. While calibrating it, its operators — Arno Penzias and Bob Wilson — discovered an annoying hum, a source of noise, that appeared in all directions in the sky. They tried everything: recalibrating it, turning it off-and-on, pointing it at the ground (the hum went away), even cleaning it and removing roosting birds from it. Finally, they got a visit from a reviewer who pointed them to a paper by Bob Dicke’s group at Princeton: a paper that allowed them to identify this “hum” with the predicted background radiation from the Big Bang. The cosmic microwave background, or CMB, had just been discovered, and the Big Bang has been with us ever since.

1970s: the birth of inflation

Throughout the 1970s, many important developments occurred. Dark matter rose to prominence with the observations of rotating galaxies, the cosmic web began to be mapped out, the first imperfections were discovered in the CMB, and spaceborne telescopes began to be planned: accessing wavelengths that cannot be seen from the ground. But by considering puzzles about the Universe — things that we have observed but can’t explain or predict — like why it’s the same temperature everywhere, why the expansion rate and energy density balance so perfectly, and why there are no leftover high-energy relics, the idea of cosmic inflation was born. Although Alan Guth is normally credited as the founder of the idea (in 1979), the idea was considered by many others at around the same time: Robert Brout, Alexei Starobinskii, and Rocky Kolb among them. Today, it has superseded the Big Bang as our ultimate cosmic origin story, but the idea goes all the way back to the late 1970s.

1980s: the birth of neutrino astronomy

Back in the 1980s, we built enormous tanks of liquid surrounded by photomultiplier tubes: not to detect neutrinos, but rather to search for decaying protons. Of course, the proton didn’t decay, so all we were seeing was the “background” of solar neutrinos. Then, an unprecedented event occurred: a supernova went off in the Large Magellanic Cloud, a satellite galaxy just 165,000 light-years away. Dozens of neutrinos arrived in the detector, and did so a full 4 hours before the first light from the supernova arrived. Just like that, it was no longer solely light that we were detecting, but particles as well. At present, we’ve extended our reach to many millions of light-years away, and neutrino astronomy is a bona fide science all unto itself.

1990s: the accelerating Universe and dark energy

The Universe is expanding, sure. But would it expand forever, or would it stop expanding and someday recollapse? By measuring standardized objects at a variety of distances, we could track the evolution of our cosmic expansion, and arrive at the answer. By using Type Ia supernova data, two independent teams first arrived at the answer in the late 1990s. Surprisingly, they found that not only would the Universe keep expanding, but the expansion itself was accelerating: demanding a new form of energy that dominates the cosmic energy budget, known as dark energy. Although we’ve pinned down dark energy’s properties very well here in the early 21st century, it was its 1998 discovery that marks the greatest cosmic advance of the 1990s.

2000s: detailed measurements of the CMB reveal our cosmic inventory

What is the Universe made out of? To answer this, we can look at the imperfections — just microkelvin-level variations in a ~3 K background — in the cosmic microwave background, and how those imperfections correlate on a variety of cosmic scales. Although other lines of evidence have since joined them, and although ACT, SPT, and Planck have all superseded WMAP’s initial precision from their first results in 2003, it was these imperfections in the CMB that first gave us our cosmic picture of a Universe that’s:

• expanding at around 70 km/s/Mpc,

• about 13.8 billion years in age,

• made up of about 70% dark energy,

• around 25% dark matter,

• and only about 5% normal matter,

with a tiny addition of photons and neutrinos as well. Although many puzzles have arisen in the time since, that overall concordance picture still holds true even today.

2010s: the birth of gravitational wave astronomy

Sure, many other important things happened this decade, too, like the uncovering of the Hubble tension, a boom in exoplanet discoveries, our first up-close views of Pluto, and the first image of a black hole’s event horizon. But the biggest revolution was in the detection of merging black holes through a whole new method: gravitational wave astronomy. The first gravitational waves were seen when the twin Advanced LIGO detectors first turned on in 2015, and were later announced in 2016. Now, 10 years later, we’re up to 390 confirmed detections, with enough evidence to even determine where these black holes come from. Instead of light alone, we now have three methods — light, particles, and gravitational waves — to view the Universe in.

2020s: the discovery of “Little Red Dot” objects

In 2021, we launched the JWST: the most powerful space telescope of all-time. It’s shown us many fascinating aspects of the Universe, but one big surprise, still not fully explained, are the Little Red Dot galaxies that it sees. Are they galaxy-quasar hybrids? Are these coming from the aftermath of enormous, supermassive stars? They seem to be indicating that black holes came before the galaxies we find them in; could that really be true? And they may be explained by enhanced supermassive black hole activity; is that the true resolution?

With just one discovery highlighted from each of the past 20 decades, we can see how spectacularly our understanding of the Universe has advanced. With each new discovery, there’s so much more that we then know how to ask. As long as we keep investing in the endeavor of asking the Universe questions about itself, we’re bound to keep discovering new facts about our shared reality. Although we can’t predict what we’ll find in the coming decades, we know we must keep looking. In the end, the act of discovery is the greatest part of conducting science of all.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

with Ethan Siegel • July 25, 2026

Greetings readers,

For me, one of the most exciting moments in science is when theoretical predictions and direct evidence — observational or experimental — finally come together to deliver a verdict on a speculative but profound idea. One longstanding prediction is that sugars, an essential ingredient and near-universal nutrient for all forms of life on Earth, should be formed in space. Another is that massive pairs of stars, typically born in binary systems, should go supernova one after another, leaving remnants that should be linked within the same complex across interstellar space. Excitingly, both predictions were recently validated with new, state-of-the-art observations. Erthyrulose, a four-carbon sugar, was just discovered close to the galactic center, and two supernova explosions were uncovered in the opposite direction: pointing away from the Milky Way’s core. It’s a big win for both ideas, as the observations validated the proposed ideas.

Other times, we only have some of the pieces in place for what we know how to predict, and there’s still a lot of guesswork involved in the ultimate resolution. From a terrestrial perspective, one such question is how spectacular this year’s upcoming Perseids are going to be. While they’re putting on a lousy show right now, by the time the peak arrives in the pre-dawn hours of August 13, the meteor shower should shape up to be the best of the year: a sight that nearly everyone on Earth should be able to enjoy. From a cosmic perspective, the positive, non-zero value to the energy of empty space itself raises the dreadful possibility that our Universe could quantum tunnel into a more stable state: triggering the catastrophic scenario of vacuum decay. I was recently asked if we should stop worrying about that scenario, given how large the Universe is and how unlikely we are to be affected by it anytime soon. And yet, because “worrying about it” leads to the possibility that we could actually prevent or delay it, I think worrying is immensely justified. Will you agree?

And finally, despite all of the advances we’ve made here in 2026, there are still some incredibly pervasive myths that lead many of us, despite our best intentions, to abandon science just when we need it most. There is a path back, however, and I hope the resources I’m providing can help you guide any close friends or family members back to evidence-based reality. As always, it’s my absolute pleasure to be sharing these stories with you week-after-week, with plenty more to come!

All the best,
Ethan

SWEET BEGINNINGS

Sugar in space: another step toward life’s origins

Three types of molecules form the foundation of life as we know it: sugars, fats, and amino acids. While amino acids and nucleobases are common in meteorites and can be readily synthesized in prebiotic experiments, sugars have been far more elusive, making this latest discovery especially exciting. Because sugars are such an efficient energy source for even primitive life, they may have played a pivotal role in life's emergence on Earth and perhaps elsewhere. Here's what we've learned.

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SKY SHOW

2026’s Perseids, lousy this July, will peak at the perfect time

Each year, Earth passes through the same debris streams from the same comets and asteroids, giving rise to our recurring meteor showers. Some years favor the Quadrantids or Leonids, while the Geminids and Perseids are usually the most prolific. In 2026, however, one shower stands above the rest. Here’s why.

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ASK ETHAN

Ask Ethan: Should we stop worrying about vacuum decay?

Although dark energy appears consistent with a cosmological constant, its positive, non-zero value may not be the Universe’s true ground state, but only a false minimum. If that’s the case, it could someday transition to a lower-energy state, triggering the ultimate cosmic catastrophe: vacuum decay. Such a bubble of destruction would race outward at the speed of light, giving us no warning. Is there any reason to worry, or any possible way out?

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MYTH BUSTER

4 myths cause us to abandon science. Here’s the path back.

There are many legitimate reasons to mistrust institutions, especially when corruption and bad behavior are easy to spot. But in recent years, public trust in science has eroded even around findings that are robust and undisputed. Here are four of the most common myths that “merchants of doubt” use to sow distrust, and how to see through them.

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EXPLOSIVE REUNION

A cosmic first: separated sibling stars reunited by supernovae

Stars are rarely born in isolation, but in large groups, with many ending up in binary or other multi-star systems. The highest-mass stars are especially likely to have similarly massive companions, and both are destined to die as supernovae. For the first time, astronomers have traced a new supernova remnant back to an older one in the same stellar family: the first-ever “supernova reunion” of long-separated sibling stars. Here’s what we know.

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Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

Out there in the Universe, a similar cosmic story plays out time and time again: the stellar life cycles. A massive, cold cloud of gaseous material, under the power of its own self-gravitation, begins to contract. As clumps within that gas get denser, the cloud fragments, with each fragment collapsing on its way to become a protostar. The most massive and dense clumps, initially, attract surrounding matter the fastest, growing into the most massive of these new stars, where around 50% of all such stars wind up developing in multi-star (binary, trinary, or higher multiplicity) systems.

When a star forms with more than about 8-10 solar masses, initially, it’s destined to die in a core-collapse supernova explosion: one of the most energetic cataclysms that occurs within our Universe. And yet, despite the fact that:

• binary systems are common,

• stars of similar masses are often found together (especially at high masses),

• and that the highest-mass stars are the shortest-lived ones before dying,

we’ve never witnessed or traced back a “sibling supernova” event: where two core-collapse supernovae occurred from the same original system. In a remarkable new study led by Stanford scientist Miltiadis Michailidis, two different supernovae have just been linked — through their gamma-rays — across the same great molecular cloud complex. We may have just witnessed the first pair of sibling stars, separated when the first went supernova, reunited when the second went supernova just a little bit later. Here’s the story.

What you see, above, is an actual image of many protostellar cores forming within the same obscured star-forming region. This is one of the highest-resolution images of protostars still being formed within a dusty, obscured region of a star-forming nebula: only revealed because of the incredible power of the ALMA observatory. Later on in life, we can see the newborn star clusters that emerge, as well as the singlet, binary, trinary, and higher multiplicity systems that eventually emerge from those dense stellar nurseries.

Of the stars that form, it’s the highest-mass ones that burn through their fuel the fastest, and that die in a core-collapse supernova after the smallest amount of time elapses. For a high-mass binary that forms, that means the more massive star dies first, with its supernova blast wave expected to “kick” its companion across interstellar space. However, if the less massive star is close to the more massive one in mass and lifetime, only a short amount of time might elapse before it goes supernova, too. If the remnant from the first supernova hasn’t faded away, these two supernova remnants — from stars that were initially siblings — could overlap, leading to a magnificent reunion.

The way to look for such an event isn’t visually, with the type of light that our eyes can see, but rather at the highest energies of all: through gamma-rays. The Fermi Gamma-Ray Space Telescope, launched by NASA in 2008, is our greatest, most sensitive, highest-resolution gamma-ray telescope to ever open its eyes on the Universe, giving us an unprecedented view of our galaxy (and beyond) at the highest energies of all. Pulsars, black holes, and supernova remnants stand out at these energies, with one specific region pointed almost directly away from the galactic center containing several notable gamma-ray sources within it:

• IC 443, a bright supernova remnant also known as the Jellyfish Nebula,

• the Crab nebula, a pulsar inside of a supernova remnant, and

• Geminga, one of the brightest and closest pulsars of all.

If you take a close look at IC 443 and the region around it in a variety of wavelengths, however, you’ll find that it isn’t just an isolated supernova remnant out there in deep space, but rather is interacting with its environment. There’s a nearby star-forming region filled with ionized hydrogen, Sharpless 249, that the supernova remnant interacts with, as well as another nearby supernova shell: G189.6+3.3.

With not just Fermi data, but also including data from Planck and MWISP in the microwave/radio, from the Digitized Sky Survey in the optical, from WISE in the infrared, from Swift in the ultraviolet, and from eROSITA in the X-ray, we can see the vast scale at which IC 443 interacts with its surroundings. The greater the number of different data sets we fold in, the greater the amount of detail emerges not only within this region of space, but the greater the details we can reconstruct about the interactions between different sources of energy and different regions within this volume of our Milky Way.

Because there are multiple supernova remnants within this one region of space — remnants that may be separated by many light-years in space but only by a few tens of thousands of years in terms of time-of-detonation — there’s a remarkable scenario that’s worth considering. Could it be possible that:

• there was, initially, a binary system of high-mass stars,

• where the first, more massive star exploded,

• sending the companion journeying rapidly away from it through space,

• where, tens of thousands of years later, the second star exploded,

• and finally, the remnants expanded and interacted,

reuniting these companions in the aftermath of their demise? A multiwavelength view of this region is certainly suggestive of precisely that scenario.

It isn’t generally appreciated why this is the case, or why gamma-rays are such an important puzzle piece in putting this picture together. Even with the incredible energies that go into a core-collapse supernova, including nuclear processes like electron capture, neutrino production, and a high-energy blast wave, none of those events produce radiation at the highest energies: gamma-rays. Instead, there are only two mechanisms that can get you there:

• high-energy proton collisions with interstellar gas,

• or high-energy electrons that collide with low energy photons and boost them to gamma-ray energies.

In addition, gamma-rays come in both low-energy and high-energy varieties, and those different energies encode different pieces of information. Low-energy gamma-rays are thought to emerge primarily from one bright gamma-ray beacon in the sky: IC 443, in this case. This is because, at those low energies, the entire region looks blended together, with IC 443 being the location where the emission signals appear strongest.

However, at higher gamma-ray energies, you can see separate, distinct structures: structures that become even more apparent when you subtract the gamma-ray light from IC443 out.

Over on the left of the image above, you can see the low-energy gamma-ray signature, while the right shows the high-energy gamma-ray signature. Where the bright pink glow appears around the bubble (IC 443), that shows the cumulative gamma-ray emission, including IC 443. However, when the pink glow disappears, that showcases the signal from IC 443 subtracted out: leading to a wealth of structure that’s apparent, particularly at the highest gamma-ray energies. It isn’t just one instrument that sees this, either, as both NASA’s Fermi and ESA’s eROSITA detect these important signals.

When they take a closer look, they see that IC 443 isn’t the only structure that contributes to the signal. Instead — and very importantly — they indicate the presence of a second, independent supernova remnant: G189.6+3.3, a source that, up until recently, was hidden in the shadow of the much brighter remnant in its vicinity: IC 443. eROSITA detects a shell from this supernova remnant, while the shape traced out by the high-energy gamma-ray emissions indicates an origin from within the remnant. In particular, a very striking feature that emerges is a bright, ionized wave at the edge of the bubble: consistent with a shock front. As much as there can be one, this is what we can consider “smoking gun” evidence for a second, recent supernova.

The gamma-ray emission that you see above — prominently in purple — clusters at the northern boundary of the second supernova remnant: exactly where it overlaps with the dense cloud of interstellar material. There’s a filament that emerges, and that filament is also apparent in ultraviolet light and in visible light, particularly in its hydrogen-alpha emissions. The fact that this filamentary feature appears in a slew of energetic forms of light: in gamma-rays and X-rays, in the ultraviolet, and from the de-excitation of excited atoms in visible light, all points in one direction: this is a radiative shock.

The ejecta from the second supernova departs from the core-collapse event extremely rapidly, with the fastest-moving ejecta leading the way and colliding with material in the interstellar medium first. When it collides, it slows down: producing gamma-ray particles (because they’re fast-moving initially) at the start, but then ceasing to produce gamma-ray particles after the slowdown. So if we’re seeing gamma-rays coming from this region after all, that teaches us something: there must be a re-acceleration of material — perhaps from previously accelerated protons — that’s producing these bright signature gamma-rays.

This was validated in the new paper by Michailidis et al., which explored a crash cloud model for this scenario. All throughout the remnant, you have cosmic ray electrons that collide with low energy photons — from the CMB, from infrared (thermal) radiation, and from starlight directly — which can boost photons up to high, gamma-ray energies. This produces small numbers of gamma-rays at low energies, but as you look to greater and greater energies, the gamma-ray flux increases. This leads to a relatively even flux of gamma-rays spatially, but one that’s energy-dependent: with few low-energy gamma-rays but more high-energy gamma-rays.

However, at the northern boundary, where you have that filamentary feature resulting from a shock wave colliding with a population of dense molecular gas, the interaction of cosmic ray protons with that gas produces gamma-rays: but a big excess of gamma-rays at low and medium energies over the other (inverse Compton scattering) process. The clouds and the filament provide abundant targets for proton-proton interactions, which leads to an increased overall flux of gamma-rays. That explains why, when we look in gamma-ray energies, we see a much greater overall count of gamma-ray photons where the filament and dense molecular cloud overlap.

One worry you might have is this: how confident are we, even if we see evidence for two supernova remnants, that these remnants actually overlap in three-dimensional space, rather than just coexist along the same line-of-sight? After all, the pulsar Geminga, close by in the sky, is only around 800 light-years away, whereas IC 443 and the other features within that gas cloud are more like 5000-6000 light-years distant. The way we can be confident of their nearby nature is that even if the two remnants aren’t interacting with and crashing into one another, the new supernova remnant is definitely interacting with the same molecular cloud that IC 433 interacts with, and therefore, they’re at comparable distances.

You might also worry that, just possibly, these are two unrelated supernova remnants, rather than being remnants whose progenitor stars have a common, shared history with one another. This is not something we can answer definitively, unfortunately, but what we can do (and what Michailidis and his team did in the paper) is to run simulations that allow us to estimate the probabilities that this event arose from one of two different scenarios.

1. Can two unrelated supernovae occur, leading to two remnants that appear this close to one another in the sky: whether co-located in 3D space or just along the same line of sight?

2. Can a binary pair of massive stars form of sufficient mass that one of them goes supernova, kicks the other one, and then, just tens of thousands of years later, the second one goes supernova as well?

As it turns out, both scenarios are plausible, but with vastly different probabilities and likelihoods. The first scenario, in all of its permutations, has a likelihood between 0.1% up to 1.2%: not negligible, but not particularly likely. The second scenario, on the other hand, is extremely well-motivated on physical grounds, and could in particular result from a star of between 30 and 40 solar masses exploding, creating the newfound (and older) remnant, and then, between 20,000 and 100,000 years later, a star that was initially about 5 solar masses lighter (between 25 and 35 solar masses) would go supernova, creating IC 443. That second scenario, for the two supernova remnants to have been born in a binary, has a probability of approximately 99%.

What isn’t supported by the data — and this is interesting to consider — is a scenario where the older remnant exploded first, and as the second star hurtles through space and then explodes, that the second explosion’s shockwave catches up with the first. However, that would lead to a different morphology, or shape for the remnants, than what we observe. The bi-directional shape of the ejecta instead indicates that the shockwave from IC 443 has not yet caught up with the earlier supernova: it doesn’t show up in gamma-rays, X-rays, or the ultraviolet. There’s a questionable (albeit, probably spurious) signature that could point to those interactions, but it has yet to be confirmed.

On the other hand, two related supernova remnants, that overlap and that originate from the same set of initially bound stars, have never been found before. This system, therefore, represents the first candidate scenario for stars that were:

• born together,

• were separated by the death of the first one,

• then the second one died,

• and now, tens of thousands of years later, are reunited by the ejecta of their remnants.

We have never observed two supernova remnants that originated from the progenitors within a binary system before. As our views of the Universe improve — in resolution, in sensitivity, in observing time, and in wavelength coverage — we can be sure that more and more rare events will appear. In time, we may either confirm or refute this picture, but as long as we’re willing to look, we’ll continue to come closer and closer to piecing together how the Universe actually behaves: when it comes to common, rare, and even unique events.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

with Ethan Siegel • July 18, 2026

Greetings readers,

They often say that not only is the Universe stranger than we’ve ever imagined, but stranger than we can imagine. The stories I’ve got for you this week definitely illustrate that, but might also inspire us to push the power of our imaginations past their usual limits, and to dream up scenarios and solutions to puzzles whose full extent isn’t yet known.

When it comes to the most energetic cataclysms in the Universe, core-collapse supernovae are hard to beat: dying in brilliant flashes that can shine brighter than 10 billion suns put together. But when you add up all the energy from all the forms of light that a supernova emits, as well as the kinetic energy of the blast wave and the explosion itself, it only accounts for 1% of that supernova’s total energy. The rest is carried away by the ever-elusive neutrino, making 99% of a supernova’s energy almost completely invisible. We fully expected that massive stars born in binary systems would eventually lead to pairs of black holes that inspiraled and merged, but gravitational wave data has shown us a surprise: beyond about 40-45 solar masses, just about all of the black holes we’ve seen merging didn’t come from that scenario at all. Instead, they’re “second generation” (or later) black hole mergers, indicating that even among black holes, the cosmos is hierarchical.

We also have a new episode of the podcast live for you. It’s on the baryon life cycle and features the newly minted Dr. Ava Polzin. The physical Universe, as much as we find utility in describing it mathematically, isn’t made of pure mathematics alone. Instead, to take our understanding from the realm of pure math into the world of physical relevance, we need a little bit more. Although many of us use AI, we can be pretty certain, from what he said, wrote, and thought, that Einstein, were he to come back today, would turn away from AI. Instead, he would choose to struggle himself, as the joy and reward he sought was to grow his own mind. And finally, have you ever wondered if a human colony on Mars is even possible, or whether the red planet’s low gravity would be a long-term dealbreaker? Come investigate that topic with me on this week’s Ask Ethan. There’s a grand Universe out there to explore, so let’s keep going! What we uncover next just might pleasantly surprise us all!

All the best,
Ethan

AN INVISIBLE EXPLOSION

99% of a core-collapse supernova’s energy is invisible

Core-collapse supernovae are among the most energetic explosions in the Universe. Although they can shine as brightly as ten billion Suns, that dazzling light represents only about 1% of the explosion's total energy. The other 99% escapes almost invisibly as neutrinos. Why do these elusive particles carry away nearly all the energy, and how can the remaining 1% produce such an extraordinary cosmic spectacle? The answer lies deep within nuclear physics.

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BORN AGAIN BLACKHOLES

Big evidence for “second generation” black hole mergers

Stars are the best-studied objects in astronomy, from how they form to how they ultimately die. The most massive stars often leave behind black holes, many of which are born in the same stellar clusters. When two black holes merge, they produce ripples in spacetime known as gravitational waves. With nearly 400 such events now detected, astronomers are beginning to uncover unexpected patterns, including the first compelling evidence that some black holes are themselves the products of earlier mergers.

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ASK ETHAN

Ask Ethan: Does Mars’s gravity prohibit human colonization?

Ever since Yuri Gagarin became the first human in space, the dream of settling other worlds has felt within reach. But even the most promising destinations, such as the Moon and Mars, differ from Earth in one critical way: gravity. After studying hundreds of astronauts who have spent extended time in low gravity, we’re beginning to understand its profound effects on the human body. The question is no longer whether we can reach Mars, but whether humans can truly live there.

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COSMIC EQUATIONS

The physical Universe isn’t made of pure mathematics alone

We've arrived at many profound ideas through mathematics alone, and some have proven remarkably well suited to describing reality. But mathematics by itself isn't reality; every idea must ultimately be tested against what we can observe and measure. Mathematical play is invaluable, but if you want to understand the Universe, you have to confront the Universe itself.

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THE PODCAST

Starts With A Bang podcast #131 – The baryon life cycle

Shortly after the Big Bang, the Universe contained only hydrogen and helium: the simplest elements, incapable of forming the complex chemistry needed for life. Everything else came later, forged inside stars, scattered by stellar explosions, and recycled into new generations of stars and planets. We're only now beginning to piece together that remarkable story in detail. In this episode, Ava Polzin explores how the Universe evolved from its simplest beginnings to the rich cosmic landscape we see today.

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MIND OVER MACHINE

The big reason Einstein would never have used AI

Many claim that AI, especially LLMl chatbots, will soon provide “intelligence as a service,” replacing the need for human experts. But whether AI is truly intelligent or not, outsourcing your critical thinking comes at a cost: The skills you don’t practice begin to fade. In fields like physics and astrophysics, where progress depends on experiments, observations, and new data, AI has clear limits. That’s where human curiosity and reasoning still matter most.

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Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

For today’s students, the temptation to cheat is at an all-time high, with AI tools making it easier than ever. In one recent class at Brown University, an economics class of 86 students, students scored higher on the (take-home) midterm exam than ever before: with an average score of 96 percent. Only 5 students out of 86 scored below 90 percent on the midterm: in a class where midterm scores had never exceeded 80 percent before. Strongly suspecting cheating, the professor announced that the final would be in-person instead.

18 students dropped the class. 9 students didn’t show up for the final. And of the 59 who took the final, the average score was 48.6 percent, with only one student achieving a score of 90 percent or more. The professor, Roberto Serrano, put it bluntly:

“We cannot afford to have a society in which a significant fraction of our best young minds think that cheating is OK. That leads to a declining society, to a failed society … We cannot choose to become idiots.”

Despite all that AI can actually do, the most dangerous thing it can do is erode our skills: a real danger if we outsource our actual learning, our critical thinking, and the time we spend struggling with puzzles and problems to a tool that purports to do it for us. This is not a new problem, but merely one that has worsened in the era of Large Language Models (LLMs). In fact, Einstein warned about this problem himself, leading us to be confident that if he had been around for the modern era, Einstein wouldn’t have used AI at all. Here’s how we know.

On the one hand, there’s the myth of Einstein that most of us grow up hearing. According to the myth:

• Einstein was an outsider: interested in physics, but flunked and left his academic studies.

• Einstein was working at a patent office, rather than engaging in formal training in physics.

• Einstein was a lone genius, working on his own, when he came up with relativity, mass-energy equivalence, Brownian motion, and the photoelectric effect.

• And because of his unique and singular brilliance, he was able to single-handedly revolutionize physics, first with his “miracle year” in 1905, and then, 10 years later, with his greatest achievement of all: the publication of general relativity, overthrowing more than two centuries of Newtonian gravity.

It’s important to recognize that very little of this is actually true. Einstein was no outsider, but rather completed his physics degree at one of the top Universities in Europe: what is today ETH Zürich. He finished his undergraduate studies in 1900, and then — contrary to the myth you may have heard — he remained there, continuing his studies at the same place.

“But didn’t he work at the patent office?”

Yes, but that was because he didn’t earn the equivalent of either a Teaching Assistantship (TA) or Research Assistantship (RA) at his University.

Instead, it was Einstein’s friend and classmate, Marcel Grossmann, who reached out to his father to try to get Einstein a job. Grossmann’s father had connections that could open up a position for a graduate student to work at the Swiss Patent Office: the same way any student would get work/study or a part-time job to help support them throughout the 20th century. Since Grossmann himself already had an assistantship, he used his connections to get his friend a job, enabling Einstein to concurrently continue his studies.

Einstein also was far from a lone genius in any regard. He wasn’t working alone at any point, but rather founded a group in 1902 known as the Olympia Academy: a study and discussion group about physics, philosophy, literature, and mathematics. Along with Conrad Habicht and Maurice Solivine (and sometimes also joined by Grossmann and others), the group studied:

• their own works,

• books by scientists Karl Pearson, Ernst Mach, and Henri Poincare,

• and philosophy and literature works by John Stuart Mill, Baruch Spinoza, and Miguel de Cervantes.

In other words, it is true that Einstein was working at the patent office in the years between completing his undergraduate degree (in 1900) and his miracle year (in 1905), but he was also engaged in formal graduate studies and augmented that with group study with others, in addition to his independent work.

It’s also a myth that Einstein flunked his studies, although there is a grain of truth there. Einstein’s most famous teacher at the Eidgenössische Polytechnikum (today’s ETH Zürich), the great mathematician Hermann Minkowski, had Einstein as a student during his undergraduate years. Minkowski remembered Einstein as:

• “always skipping lectures,”

• “being a real lazybones… who never bothered about mathematics at all,”

• and as having a “mathematical education [that] was not very solid.”

However, that is not atypical of a large fraction, and perhaps even a majority, of people who go on to become excellent scientists as adults. Very few teenagers (and Einstein was 17 when he began his undergraduate studies) have the discipline, study habits, and foundation to immediately be successful in a formal academic setting.

In other words, Minkowski’s evaluation of Einstein was based on the only thing that a college professor can justifiably evaluate a student on: performance. But performance is not the same thing as potential. Minkowski couldn’t see the large, sustained effort that Einstein would put into his education into the future, into improving his mathematical foundation, or into learning how to think long and deeply about problems. He couldn’t see the skills that Einstein would develop, or foresee what would sufficiently motivate Einstein to make that big effort that would lead to his revolutionary discoveries.

Einstein would later talk about the value of the time he spent getting a formal education, and discussing just what it was that made the investment of his time and efforts so valuable. In contrast to Thomas Edison, who derided a college education as useless, Einstein wrote the following in 1921, as recorded by Philipp Frank in his biography of Einstein:

“It is not so very important for a person to learn facts. For that he does not really need a college. He can learn them from books. The value of an education in a liberal arts college is not the learning of many facts, but the training of the mind to think something that cannot be learned from textbooks.”

This powerful statement, on its own, is sufficient to illustrate that Einstein would have chosen not to use AI/LLMs, even if they were available to him, to do anything other than look up facts (which he would then, of course, have to verify weren’t AI hallucinations). He absolutely would not have outsourced his critical thinking skills — or any endeavors that helped develop them — to a tool that would have taken those valuable efforts away from him. After all, thinking and struggling with the material, and mutual sharing in the expertise of a group of colleagues, is how Einstein grew his mind and attained his great achievements in the first place.

Despite the fact that Einstein indeed did recognize the unparalleled value of a formal education, that’s something that normally gets forgotten by history. When we quote Einstein, his most famous quote tends to get summarized as, “imagination is more important than knowledge,” but taking it at face value — suggesting that imagination is what’s important and that knowledge is unimportant — completely misunderstands the full meaning and context of what Einstein was saying. That quote arose in the context of a 1929 interview with George Sylvester Viereck, where Einstein was discussing the validation of general relativity by the 1919 eclipse expedition.

Einstein: “I believe in intuitions and inspirations. I sometimes feel that I am right. I do not know that I am. When two expeditions of scientists, financed by the Royal Academy, went forth to test my theory of relativity, I was convinced that their conclusions would tally with my hypothesis. I was not surprised when the eclipse of May 29, 1919, confirmed my intuitions. I would have been surprised if I had been wrong.”

Viereck: “Then you trust more to your imagination than to your knowledge?”

Einstein: “I am enough of the artist to draw freely upon my imagination. Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world.”

In context, it is simply Einstein expressing his conviction that his ideas were correct before they were validated: no different than any imaginative thinker believing that their novel ideas can take us beyond the current frontiers of our existing understanding. This is a requirement for anyone seeking to advance what we know, but it is not sufficient on its own.

Without the knowledge to underpin the foundation for his ideas — which, in turn, grew out of his imagination — and without the requisite knowledge and skills to formally and fully develop those ideas, Einstein’s novel predictions never could have come to be. The expectation that he would be correct is not the revolutionary part of Einstein’s quote about imagination and knowledge, but rather that his deep theoretical understanding meant that any other outcome would have greatly surprised him.

And yet, because he was a physicist at heart, and physics itself is a science that’s rooted in experiment and observation, he remained open to being surprised in exactly that regard.

This is something that no AI or LLM can replace. No matter how confidently an LLM tells you what the outcome of an analysis or study is going to be, there is absolutely no way to ever replace what can be learned by conducting the study, particularly by collecting the relevant data and letting a fully correct and rigorous analysis be what determines the conclusion. When LLMs tell you what the outcome will be for an experiment or set of observations that has not yet been conducted, you must absolutely not believe it. It is, at best, an educated guess that has dressed itself up as a confident conclusion: an impostor posing as something it can never be a substitute for.

This isn’t something physicists and astronomers tell themselves to continue to justify their existence in the era of widespread AI and LLMs, but rather a conclusion reached by a consortium of world-leading computer science and AI researchers. As the leader of the Stanford Trustworthy AI Research (STAIR) lab, Dr. Sanmi Koyejo addressed the American Astronomical Society at their 248th meeting in June of 2026, and presented an important distinction: doing well on a test — or in the case of LLMs, what we call “performing well on a benchmark” — is not the same as doing good science, or as doing science at all.

In order for AI to work in any sense at all, that’s precisely what it needs: benchmarks, and in particular, benchmarks for validation. Is there some kind of test or check you can perform for your data? If so, then you might be able to leverage AI in some useful way, as then your AI can run quickly and often on your data, even as you acquire it. However, these benchmarks assume three things.

1. Cheap verification: that checking whether your benchmark is met (or a threshold is crossed) is fast and nearly free.

2. Short feedback loops: that you learn whether “you were right” or “you were not right” immediately.

3. And that the questions are fixed: the set of questions and the benchmark tests are given up front, and there are no revisions (or moving targets) to those benchmarks.

The big limitation of AI for science is this: none of these assumptions are true when it comes to physics and astronomy.

In real science, things are a lot messier. There are often mislabeled items and mismeasured points: for example, when two galaxies at two different distances overlap along the same line-of-sight. There are often several plausible correct answers, not one unique solution, such as when fitting the photometric redshift of a distant object. Sometimes, there are structural ways that a benchmark breaks: when a model is marked wrong (because it crosses or doesn’t cross some threshold) despite being correct.

As Koyejo elaborated, a benchmark for AI is like a measuring instrument, but with noise. Some percentage of the signal that it picks up will be usable signal, while the rest will be noise and artifacts. But as you go to stronger AI models and more frontier models, the instrument that is AI actually loses resolution, and is more likely to provide noise (and not usable signal) exactly when you need to only extract the signal the most.

To put it bluntly, of the three assumptions you need for AI benchmarks to be useful, a physical science like physics or astronomy actually breaks all three of them. When you need more (or better) data — as you often do in science — having extra computing power does not help. This leads to an extremely grave danger: that AI models will reach agreement, and converge on what they can conclude based on all current inputs, without actually approaching the truth of factual reality. AI consensus can measure agreement, but cannot tell you anything about correctness.

AI models cannot evaluate; you need an expert capable of critical thinking to do that. AI models cannot identify unique events that you didn’t pre-specify that it should look for; it will overlook them. AI models cannot engage in critical thinking the way an expert-level human can; they can only reach agreement by comparing to what they were trained on. They severely underperform, compared to humans, at reviewing and evaluating the work of others, and moreover can be gamed: an AI’s evaluation score can be increased significantly simply by rewriting the veneer (surface) of a submission, without changing the science underneath it. In Koyejo’s exact words:

“When the thing being measured and the thing doing the measuring share the same blind spots, the errors do not cancel. They reinforce.”

To be sure, humans are fallible as well. We have gaps in our knowledge. We have blind spots. We have biases. But we have tools and abilities that AI does not. We have critical thinking skills. We can think beyond our training data. We have a culture of blind analysis, of rigor, and of reproducibility. And, perhaps most importantly, we have a collaborative community that works together to strengthen each other, and to strengthen the field as a whole. Einstein relied on it, benefitted from it, and contributed to growing it for the next generation.

Every time you eschew critical thinking for yourself and outsource it to AI, you fundamentally cheat yourself out of any intellectual challenge, and hence, any opportunity to actually grow your mind. People like Einstein actively sought such challenges out, and that enabled him to flourish, transforming him from the below-average student that Minkowski knew to one of the greatest geniuses in all of recorded history. Einstein would have rejected the use of AI, and you should too. As Nicholas Carr summed it up, “Armed with generative AI, a B student can produce A work while turning into a C student.” Once the class or the evaluation is over, your mind will be the only thing of value you’re left with. Don’t cheat yourself out of the opportunity to grow the most capable one you can.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

with Ethan Siegel • July 11, 2026

Greetings readers,

Now that we’ve entered the summer months, there are not only plenty of new science developments to bring to your attention, but also Big Think’s brand new quarterly print issue: The Opt-Out Issue, available to all Big Think members. Become one today if you aren’t already! As part of the online version of the issue, I’ve written a piece on whether the U.S. will opt out of the field it helped invent: gravitational wave research, with a story titled “America pioneered gravitational wave research. Now it may leave it behind.” It’s not the only thing that people, institutions, and nations are opting out of, and there’s still hope to maintain the course we initially set and continue advancing it. Let’s advocate for all the right things!

Beyond that, we’ve taken a look at some incredible science stories this week. The beginning of the Universe, before the Big Bang and before cosmic inflation, is something we can only speculate about today; the information that would tell us what actually occurred has already been erased from what we can ever observe. A study came out just a couple of weeks ago claiming that the Universe isn’t the same in all directions: an anisotropic Universe. Unfortunately, that study was fatally flawed, and I’ve got the lowdown on just how they messed up and what the data actually indicates. The smallest of the three big proposed next-generation flagship observatories, the Giant Magellan Telescope (GMT), will turn out to be the best of the three on many fronts, and now you can see why for yourself. The U.S. may have broken records with this year’s July 4th (and 5th) fireworks shows, but we still only pale in comparison to the power generated by our planet’s aurorae.

And finally, if you’ve ever wondered just how we know what the size of the Universe was at any moment in cosmic history, it turns out you’re not alone; that’s the subject of this week’s Ask Ethan! We’ve got a new podcast coming to you next week, and a whole lot more spectacular science on the way as well. Make sure you don’t miss any of it, and I’ll make sure to deliver as much as I can each week as we continue on our cosmic voyage together!

All the best,
Ethan

SMALL BUT MIGHTY

Why the “smallest” giant telescope may be the world’s best

The current generation of the world's largest optical telescopes spans 6.5 to 11 meters in diameter, but the next era belongs to a new class of 25- to 39-meter observatories. While the 39-meter European Extremely Large Telescope is the largest, the 25-meter Giant Magellan Telescope may ultimately outperform them all. Thanks to its clever optical design, fewer reflections, and more efficient instruments, it could become the most powerful telescope of its generation. Here's why.

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THE MISSING BEGINNING

Forget seeing the Universe’s beginning. It’s already been erased.

In the original Big Bang model, the expanding Universe could be traced back to a single point: a singularity marking the birth of space and time. But that picture has since been replaced by cosmic inflation, which preceded the hot Big Bang and left observable imprints on our Universe. Unfortunately, inflation also erased nearly all traces of what came before it, leaving only the final tiny fraction of a second within our reach and our ultimate cosmic origins frustratingly out of view.

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ASK ETHAN

Ask Ethan: What sets the size of the observable Universe?

Here in our modern cosmos, we often say the Universe is 13.8 billion years old and that the observable Universe extends 46.1 billion light-years in every direction. While the full Universe may be vastly larger, or even infinite, the size of the observable Universe is remarkably well known, not just today but throughout cosmic history. What determines its size, and how do we know it so precisely? The answer is surprisingly straightforward, even in a Universe governed by Einstein's general relativity.

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FALSE ALARM

Famous study in error: the Universe isn’t anisotropic

Although the Universe is clumped into galaxies, galaxy clusters, filaments, and cosmic voids, it should appear uniform on the largest scales. While past hints of large-scale anisotropies have surfaced, they’ve consistently disappeared as better data became available. Now, using data from DESI, two researchers claim to have found evidence that the Universe isn’t the same in every direction. But a closer look reveals their conclusion doesn’t hold up.

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LIGHT SHOW

What has more power: Earth’s aurorae or fireworks shows?

While fireworks are often thought of as Earth’s greatest visual spectacle, nature offers an even more dazzling show: the aurora. When charged particles from the Sun are funneled by Earth’s magnetic field into the atmosphere near the poles, they create the Aurora Borealis and Aurora Australis, ranging from faint glows to spectacular outbursts. Even as fireworks displays have shattered records in the 2020s, one question remains: Which produces more power, fireworks or the northern lights?

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OPTING OUT

America pioneered gravitational wave research. Now it may leave it behind.

The United States has led gravitational wave research for decades, with LIGO making the first-ever detection in 2015. But that leadership is now at risk. Proposed federal budget cuts threaten LIGO’s future, while the next-generation Cosmic Explorer has received no funding at all. The U.S. doesn’t have to surrender its leadership in this groundbreaking field, but the decisions made now will shape the future of gravitational wave astronomy for decades to come.

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Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

This article is part of Big Think’s newest special issue, The Opt-Out Nation.

Over the past 75 years, an entirely new type of astronomy has emerged, one where it isn’t the light from distant objects that reveals their presence and properties, but ripples generated in the fabric of space itself: gravitational waves.

In many ways, the United States has been at the forefront of gravitational wave research since the very beginning. But facing proposed budget cuts, the nation’s role as a leader in this area of research is all but certain.

A new way to see the universe

First predicted as a consequence of Einstein’s general relativity, gravitational waves were initially thought to be a mere mathematical artifact of the theory. But in 1957, Richard Feynman theoretically demonstrated that they must carry energy whenever they are generated. Soon after, Joseph Weber designed and built the first primitive gravitational wave detectors. In 1974, Russell Hulse and Joseph Taylor detected the orbital decay of a binary pulsar — a decay that must be due to the emission of gravitational waves — leading to a Nobel Prize.

Theorists and experimentalists then worked together to design and build the Laser Interferometer Gravitational-Wave Observatory (LIGO), which detected gravitational waves from the merger of two black holes for the first time in 2015. One Nobel Prize, several upgrades, and hundreds of discoveries later, the twin LIGO facilities in the U.S. — one each in Hanford, Washington, and Livingston, Louisiana — remain the most sensitive gravitational wave detectors in the world.

Then, in 2023, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration revealed the world’s first positive detection of gravitational waves using a completely different method: through the exquisite timing of scores of millisecond pulsars over more than a decade of continuous monitoring. Along with other pulsar timing arrays around the world, including in Europe and China, humanity has now seen evidence of a completely different type of gravitational wave: the collective “hum” from pairs of co-orbiting supermassive black holes.

These developments heralded the dawn of gravitational wave astronomy, which is now a global affair. The ground-based Virgo and KAGRA detectors, in Italy and Japan, respectively, are now online to supplement LIGO, and the European Pulsar Timing Array (EPTA) complements NANOGrav. China’s FAST telescope — the largest radio telescope in the world — has allowed the nation to achieve pulsar timing accuracies from just three years of observations that rival what NANOGrav needed 15 years to accomplish. And the European Space Agency (ESA) plans to build and launch the Laser Interferometer Space Antenna (LISA), the first space-based gravitational wave observatory. That spacecraft will be capable of detecting massive objects that get swallowed by supermassive black holes for the first time.

The future on hold

These achievements could push us to invest in expanding the frontiers of this burgeoning field. We could be preparing for the next generation of observatories: developing a larger, more sensitive version known as LIGO II, building the next-generation Very Large Array (ngVLA) to improve our pulsar timing capabilities, and rebuilding a more advanced version of the Arecibo Observatory to exquisitely measure individual pulsars. Expanding our searches for gravitational waves from space with LISA is also in order, especially given the enormous success of the LISA Pathfinder mission, which demonstrated the capabilities of this type of technology.

This path for the future of American science, laid out in detail by the National Academies in their Astro2020 decadal study, was widely supported by NASA, the National Science Foundation (NSF), the Association of Universities for Research in Astronomy (AURA), and many other leading scientific organizations.

But then, the 2026 presidential budget request arrived, proposing massive funding cuts to LIGO and the closure of one of the two LIGO facilities, despite the fact that signals in both detectors are required to trigger an event alert. Fortunately, Congress rejected the closure and cuts earlier this year, allotting LIGO “not less than $49 million for the year,” a slight increase in the funding it had been receiving, and far above the $29 million budget the Trump administration had sought.

Supporting existing and new facilities is essential to the progression of science, said Corrina Jaramillo Feldman, speaking on behalf of the National Radio Astronomy Observatory (NRAO):

“The inability to use existing facilities, fund or create new ones, would mean that astronomy in general would not be able to move forward. New discoveries, or the growth of existing knowledge, simply wouldn’t be possible. The desire to build the next generation of astronomical instruments is fundamental to continued growth of astronomical knowledge. Whatever it takes to build them, they will require a significant amount of investment, and it has to come from somewhere. The careful process of design and planning that major facilities currently undergo can ensure that the tools made are the right ones to move science to this next level.”

LIGO’s cuts were rejected, but the international astronomical community has taken note of the turbulence, and many countries, collaborations, and organizations across the world are now preparing to continue gravitational wave astronomy without the U.S. Leaders at Laser Interferometer Space Antenna (LISA) — initially a NASA project that was deselected and then resurrected by our partners at ESA — have deemed the U.S. an unreliable partner. As a result, ESA scientists have expressed the desire to contract out NASA’s promised contributions to the space-based mission to other European countries. Although replacing NASA’s high-tech systems with similar systems developed by European companies would result in a two-year delay to LISA’s launch timeline, pushing it to 2037, the greater risk is that the mission continues to rely on NASA’s promises, only to see those promises broken when the critical time comes.

Events from 2025 and early 2026 — including the government shutdowns, DOGE eliminating science positions, the Office of Management and Budget (OMB) withholding approved funding, and the president’s catastrophic budget proposal — have eroded trust in NASA’s reliability as a funding partner. Given that funding for U.S.-based science projects and contributions to international ones is subject to the annual political whims of the president and Congress, there is a perception that promises by the U.S. government are far from guaranteed.

“We can hope for a system that has more stability to allow these major projects to move forward,” Feldman said. “Because of their size, and the amount of time that goes into research, design, planning, and construction, stability is required to move them forward.”

The cost of giving up leadership

It took 20 years of theoretical and experimental development before the prototype for LIGO was built. It was then another full decade before the Advanced LIGO Project — the twin detectors at Hanford and Livingston — were ready to come online. Just days after they began taking data, their first bona fide signal arrived: a gravitational wave from two black holes, of 36 and 29 solar masses, that merged into a 62-solar-mass black hole.

If you’re quick with addition, you might have noted that 36 plus 29 equals 65, not 62. Those three “extra” solar masses’ worth of mass were converted into energy (via Einstein’s E = mc²) in the form of gravitational waves. From over a billion light-years away, a tiny bit of that energy was deposited into each of the two LIGO detectors.

The cumulative work of thousands of people over multiple decades led to that vaunted detection, and in the years since, we have confirmed nearly 400 additional detections. Over 100 candidate detections are awaiting final analysis and publication. The detectors have been upgraded several times, pushing past prior limits on everything from the quality of the vacuum inside the laser arms to the quantum noise of the apparatus itself.

Yet, while the community plans for a larger and more sensitive successor detector (Cosmic Explorer) and the first international LIGO detector (LIGO India) prepares to come online, gravitational wave scientists are scrambling to avoid the closure of their existing flagship observatories. As early-career scientists lose opportunities and senior leaders are pressured into early retirement, the loss to the gravitational wave community has rippled throughout the country that was, until recently, the unquestioned global leader in astronomy.

Another gravitational wave scientist, speaking on condition of anonymity, shared the following thoughts:

“We’re on the brink of countless exciting revelations about the Universe facilitated by gravitational wave observations; it would be shameful to give up on this remarkable new mode of discovery. Large-scale scientific projects, including gravitational wave experiments, are one of the few remaining examples we have in the U.S. of broad cooperation towards a goal that serves humanity rather than capital. Big scientific endeavors, and the breakthroughs they facilitate, provide an opportunity for Americans to rally around something inherently collaborative rather than divisive.”

Finally, the science of pulsar timing has now advanced to the stage where — with enough high-accuracy pulsars from all directions within the Milky Way — we can use the variations in those timings to detect the background of low-frequency gravitational waves rippling throughout the Universe. Created by pairs of ultramassive black holes in orbit around one another at the centers of stupendously massive galaxies, these signals were first teased out of the data by the U.S.-based NANOGrav collaboration in 2023. In short order, other pulsar timing arrays published similar detections, with China swiftly surpassing NANOGrav’s best data, despite NANOGrav’s 15-year baseline of observations compared to three years for China.

The reason is simple: China has the Five-hundred-meter Aperture Spherical radio Telescope (FAST), which came online in 2016, surpassing Arecibo as the world’s largest. The larger your telescope is, the more accurately you can measure the timing of any individual pulsar, and for years, NANOGrav benefited from the quality of Arecibo. Its collapse in 2020 was a huge loss for the pulsar timing community, as it significantly reduced the capacity of NANOGrav to accurately make those quality single-dish measurements.

The other great tool used to measure pulsar timing — and the great strength of the European Pulsar Timing Array (EPTA) — is a coordinated array of radio telescopes. While EPTA has ALMA, the cutting-edge array located in South America, NANOGrav has only smaller single-dish radio telescopes and the Very Large Array in New Mexico. That facility is half a century old.

As NANOGrav, the first to the finish line of detecting the cumulative “hum” of all the ultramassive black hole pairs in the Universe, falls farther and farther behind the other pulsar timing collaborations in the world, prospects for a rebuilt Arecibo or a next-generation successor to the Very Large Array — the ngVLA — feel dimmer than ever. The impact on early-career scientists is perhaps the most profound, as the anonymous gravitational wave scientist pointed out:

“The never-ending cycle of budget fights, hirings and firings, and changing priorities in American science are rightly making young scientists question their career choices. If the U.S. began both investing seriously in science and protecting the funding that is allocated, the teams that drive major breakthroughs and next-generation technologies would be able to work at full efficiency and attract talent that might otherwise be driven away by the now-infamous instability of not-for-profit scientific work.”

In April, the Trump administration released its 2027 budget request to Congress. It again calls for reducing LIGO funding to $29 million, in addition to slashing NASA science and NSF science by approximately 50%. Scientists are gearing up to fight funding battles all over again, and every year for the foreseeable future.

The U.S. doesn’t have to opt out of its leadership role in gravitational wave research. Congress could again reject the administration’s proposed cuts, instead directing the nation onto a long-term path of well-funded scientific research. Although we’ll need to reckon with the whims of whatever political winds are blowing in the years to come, America’s role as a scientific leader demands that we remain pointed toward discoveries that help us understand the greatest cosmic mysteries of all.

Click here to check out The Opt-Out Nation in its entirety.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

with Ethan Siegel • July 4, 2026

Greetings readers,

Here in the United States today, we’re celebrating the country’s 250th anniversary in true American fashion: with loud bangs, glorious light shows, deliciously grilled foods, and frozen treats to help us beat the summer heat. But that’s nothing compared to the fireworks that we experienced long ago: when an asteroid struck our planet and drove our predecessors, the dinosaurs, to extinction. The dinosaurs, however, didn’t have telescopes, satellites, or the ability to predict what would be potentially hazardous far into the future. If we didn’t have those abilities either, and a similar event were headed our way, how much lead time would we have between when we could first detect that hazard and when the actual catastrophic impact occurred?

On the darker side of things, scientists just announced the discovery of DF9: the fourth allegedly dark matter-free galaxy of all. With four examples, you might think that we’ve now demonstrated the existence of galaxies that truly lack dark matter, but many are far from convinced. There’s a simple way out: Perhaps we’ve just mismeasured the distance to these few objects? You might think it’s easy to calculate the number of stars in the Universe: Just count the stars in the Milky Way and multiply by the number of galaxies we have across the cosmos. But if you do that, you’ll greatly overestimate the total number of stars. Here’s how to get it right, instead.

There’s a uniquely strong signature of the element silicon coming from a relatively nearby galaxy, NGC 1277, which hasn’t formed stars in over 10 billion years. Could that silicon be a compelling hint that will someday lead us to an understanding of the first stars? And finally, an ultra-massive galaxy cluster was found earlier in cosmic history than ever before, and it’s even been caught gravitationally lensing a set of even more distant objects. Here’s what that implies for dark matter. As always, it's a pleasure to share another week of awe, wonder, and joy of learning about the Universe with you, and we’ll be back with even more next week!

All the best,
Ethan

DARK MATTER DETECTIVE

Is it finally time to take dark matter-free galaxies seriously?

Less than a decade ago, astronomers identified the first seemingly dark matter-free galaxy: the ultra-diffuse dwarf galaxy NGC 1052-DF2. The discovery sparked enormous controversy, as different distance measurements led to dramatically different estimates of its dark matter content. Today, four galaxies appear to lack dark matter, but three of them lie on the outskirts of the same galaxy group. Are these truly dark matter-free galaxies, or have we been getting their distances wrong all along?

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COSMIC TIME CAPSULE

Could nearby relic galaxy NGC 1277 lead to the first stars?

Located in the Perseus Cluster, NGC 1277 is the nearest known example of a massive, “red-and-dead” galaxy, containing only ancient stars and no gas for new star formation. Astronomers typically search for the Universe’s first stars by looking to its most distant reaches, hoping to catch them during their brief lifetimes. But what if the evidence of those first stars has survived much closer to home? NGC 1277, just 240 million light-years away, may hold the answer.

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ASK ETHAN

Ask Ethan: Could we spot an extinction-level asteroid?

We know a great deal about the asteroid impact that triggered Earth's fifth great mass extinction: a massive asteroid struck the planet, carved out the Chicxulub crater, and wiped out roughly three-quarters of all species. But with only the naked eye, there would have been little warning before it arrived. If a similar object were headed for Earth today, how much advance notice would we get, and when would we know it was on a collision course? The answer reveals just how important planetary defense has become.

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COUNTING STARS

How to not overestimate the number of stars in the Universe

If you look across the observable Universe, stretching 46.1 billion light-years in every direction, you’ll find trillions of galaxies. Since the Milky Way contains a few hundred billion stars, it might seem reasonable to estimate the total number of stars by simply multiplying the two. But neither our galaxy nor our Sun is typical. Here’s a better estimate of how many stars fill the Universe, and how different they are from the ones we know best.

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A WARPED VIEW

The most distant lensing cluster reveals a dark matter surprise

In theory, the first mature galaxy clusters shouldn’t appear until about 3 billion years after the Big Bang. While Hubble previously imaged XLSSC 122, the most massive early cluster yet discovered, JWST reveals it in stunning new detail, including blue arcs created by strong gravitational lensing: the most distant lensing cluster ever found. Those observations reveal something unexpected: its dark matter is far more concentrated than in nearby galaxy clusters. Here’s what that surprising discovery could mean.

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Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

All across the Universe, the one force that matters the most on cosmic scales is gravitation. From a nearly perfectly uniform past, initially tiny imperfections — just a fraction-of-a-percent denser than average — eventually grew into the massive cosmic structures we see today: star clusters, galaxies, groups of galaxies, and even stupendously large galaxy clusters. While one such early cluster, XLSSC 122, had been previously imaged and identified with the Hubble Space Telescope, the extraordinary vision of the James Webb Space Telescope (JWST) revealed so much more, including the unexpected.

Sure, it’s an incredibly massive cluster: weighing in at about 300 trillion solar masses, or hundreds of times the mass of the Milky Way galaxy. In addition, it’s very far away: possessing a redshift of z = 1.98, which corresponds to a time just 3.3 billion years after the Big Bang, or 10.5 billion years ago. This big, organized, evolved object was already mature, and was mature at a time when most other clusters were still actively forming.

But with JWST’s power, scientists were also able to identify streaking blue arcs near the cluster’s center: evidence of strong gravitational lensing. This makes it one of the farthest gravitational lenses ever discovered (along with this distant lensing galaxy), and the most distant galaxy cluster seen to exhibit strong lensing. That encoded a plethora of valuable information about the cluster’s mass distribution, with severe implications for dark matter. Here’s what astronomers found inside.

The cosmic story for how a cluster like this comes to be always has the same standard components to it, even if the details such as timescales and masses vary. That story is known as hierarchical structure formation, which involves the following steps.

• First, the Universe gets seeded with a spectrum of initial fluctuations on all scales, where the initial overdensities typically provide the seeds that will, at late times, grow into massive structures.

• After neutral atoms form, those seeds begin growing, drawing matter in from their surroundings, leading first on very small scales (tens-to-thousands of light-years) to the formation of the earliest stars and star clusters, or even to early black holes.

• Next, as more time passes, these early, massive structures grow via accretion (where surrounding matter falls into them) and mergers (where various clumps gravitationally join together), leading to large galaxies and swarms of small galaxies: the first galactic groups.

• Then, at even later times, seemingly independent galaxies and galactic groups get drawn together by gravity, forming the earliest galaxy protoclusters, with some of the most precocious examples spotted less than a billion years after the Big Bang.

• And finally, much later on, typically no earlier than 3 billion years after the Big Bang, the earliest mature galaxy clusters emerge, with hundreds of trillions of solar masses and many large, massive galaxies inside of them.

The JWST has been revolutionary in revealing not only unprecedented examples of most of these stages, but in showcasing details about these objects at these stages that no other observatory can presently reveal.

In particular, the time period from about 3 billion years after the Big Bang until about 5 billion years after the Big Bang represents a very special epoch in cosmic history: a time period known as cosmic noon. If you were to track the Universe’s star-formation history over time, you’d find that:

• initially, the rate was zero,

• then it gradually and steadily rose (cosmic morning),

• until it reached a peak (cosmic noon),

• and then has steadily declined (cosmic evening/sunset) ever since.

Today’s star-formation rate is estimated to only be about 3% of what it was at its peak at cosmic noon, indicating that the vast majority of stars that will ever form have already formed, and did so long ago on top of that.

However, by looking back at the distant objects we see at those earlier times, we can indeed learn how our Universe grew up, and in turn, how it led to the modern cosmos we’re familiar with today. And in the case of XLSSC 122, it’s a remarkable system for a number of reasons. Perhaps most importantly, for its time in cosmic history, 10.5 billion years ago (or 3.3 billion years after the Big Bang), it’s the most massive galaxy cluster known so far. Although plenty of more massive galaxy clusters appear later on, this one is special because of how massive, centrally concentrated, and “grown up” it looks right at the time that the first “grown up” galaxy clusters ought to be appearing.

There’s a second reason that large, massive collections of matter are so useful for studying our cosmic past: with so much material collected in such a small volume of space, these masses can lead to gravitational lensing of the objects located behind them, but along the same line-of-sight from our perspective. Due to the nature of spacetime, and in agreement with the predictions of Einstein’s general relativity, the presence of a large-enough, compact-enough mass in space acts like a lens in two important ways.

• Through the overall effect of having that mass located in space, the background objects whose light passes near or through that massive region of space will appear distorted: with their shapes stretched along circles surrounding that mass. This is the effect of weak gravitational lensing.

• And when individual background objects happen to line up with those regions where the magnification and distortion effects are maximal, those background objects can be severely stretched, distorted, and magnified, often into arcs, rings, and multiple images. This is the effect of strong gravitational lensing.

While weak gravitational lensing is a vital tool for mapping out the mass distribution of galaxy clusters, including for XLSSC 122, it’s the strong lensing features that can probe the mass distribution most precisely at the cluster’s center: where the mass densities are highest.

Here in the late-time Universe, one of the things we’ve discovered about galaxy clusters, as you can see above, is that there isn’t just the masses of the individual galaxies — stars, gas, dust, plasmas, black holes, and enormous dark matter halos — that contributes, but a broad, smooth distribution of mass that dominates the entire cluster. That mass cannot be normal matter, but rather is dark matter, and it outweighs all of the normal matter combined by about a 5-to-1 ratio.

Initially, it was gravitational lensing, and mostly weak gravitational lensing at that, which allowed us to map out the masses of galaxy clusters in the nearby Universe. This method is very powerful because of how often it can be leveraged for galaxy clusters. However, it’s limited in its power. While it’s generally quite sensitive to the cluster’s overall mass, it’s less able to probe cluster substructure or the overall mass density (and variations in that density) toward the cluster’s center.

Another method that’s grown more powerful and ubiquitous in recent years is to use stars that have been torn out of their parent galaxies to trace the dark matter distribution in the intracluster medium: the space within galaxy clusters that lies between the individual galaxies. This intracluster light can augment lensing studies and provide key information about the dark matter structure and distribution inside of these massive galaxy clusters.

What we generally find, at least here in the late-time Universe, is a bit of a puzzle. While simulations — relying on a single-species model of dark matter that’s both cold and collisionless, but that does have mass and interacts gravitationally — generically predict that dark matter should be:

• of low density on the outskirts,

• should increase as you move toward the center,

• should “turn over” to increase more slowly within a certain distance,

• and then should lead to a cuspy-profile, peaked in dark matter density near the center itself,

observations don’t quite support that picture. The first three steps appear to align with observations fairly well, but then, in the innermost regions, it’s as though the density doesn’t rise anymore and there is no cusp.

Instead, the density toward the central regions appears to remain constant or, in some cases, to even decrease: as though something has caused the expected dark matter to be removed from those regions. Many theoretical ideas have been proposed to explain this phenomenon, including self-interacting dark matter, two (or more) species of dark matter particles instead of just one, or the very clever idea of dynamical dark matter heating (originally applied to individual galaxies): where gravitational interactions between normal matter and dark matter cause the dark matter to gain kinetic energy and be removed from the cluster’s center.

However, by being able to leverage strong gravitational lensing signals from a young, early, but massive-and-evolved galaxy cluster, we now have a new laboratory to observationally probe. The older Hubble image of XLSSC 122 was unable to reveal any signatures of strong lensing, but with JWST’s high-quality, high-resolution, and longer wavelength imaging, those characteristic arcs prominently appeared, in blue, around not only the galaxy cluster itself, but directly around the brightest galaxy found at the center of the cluster. From those arcs, the scientists were then able to:

• construct a rudimentary mass model for the cluster’s central region,

• finding an extreme central concentration of dark matter,

• and then using that mass model to predict the location, based on geometry, of any fainter radial arcs that might be present.

Because it’s just a geometry-based calculation, it’s as though astronomers now had an invisible “arrow” that screamed out, “look here” to them. When they did, they indeed found the radial arc very close to that bright galaxy’s center, and that arc’s visual properties then empowered the astronomers to construct an even more precise mass map of the cluster’s central region.

The multiwavelength data from JWST helped identify the individual arcing features from background galaxies, and also allowed astronomers to quantify the intracluster starlight that traces the dark matter near the central regions, with both sources of data combining to help construct the most accurate mass map — including dark matter — of this cluster ever. They furthermore were able to show that this isn’t just an isolated galaxy cluster, but rather a galaxy cluster that’s in the process of actively experiencing a merger between two or more components: a conclusion bolstered by multiwavelength (X-ray, microwave, and radio) data.

What’s most interesting about this cluster is that the dark matter is unusually heavily concentrated toward the center: higher than modern day, late-time clusters, higher than other galaxy clusters caught in the act of merging, and higher even than simulations predict. The central density of mass is very high, and this is based off of not only a variety of lines of evidence that support such a conclusion, but are bolstered by the exquisite strong lensing data, available only with the advanced imaging capabilities of JWST and its NIRCam instrument.

This is the Universe as seen at its most extreme in an impressive number of ways.

• It represents the earliest, most distant galaxy cluster ever observed to be behaving as a strong gravitational lens, and is a candidate for the most distant object, period, that acts as a strong lens.

• It’s also the most massive galaxy cluster for its age in the Universe ever spotted, with hundreds of trillions of solar masses (estimated: 300 trillion) at a time when the Universe was under a quarter of its current age, just 3.3 billion years old.

• It has a more severe concentration of dark matter at its center than any other massive galaxy cluster ever found. While that concentration is expected to evolve as the Universe evolves (note the difference between the solid lines and the dotted lines on the graph at the above-right), the actual, observed concentration is really at the upper limits, even exceeding them in places, of what simulations predict.

• And finally, it represents the earliest example of intracluster light ever detected within a galaxy cluster: an incredibly sensitive tracer of the global dark matter distribution inside of galaxy clusters.

When you fold in the weak lensing data, it allows for an accurate, inside-to-outside mass map of the extended cluster complex, demonstrating that it is, in fact, merging, with the constituent galaxies still in the process of coming together.

According to the lead author of the newest study, Kyle Finner, who presented this work at the American Astronomical Society’s 248th meeting in June of 2026 in Pasadena, CA, this really is the start a revolution in astronomy: one that’s just beginning to come to fruition with the new imaging of this cluster by JWST.

“Before JWST, we couldn’t do this level of science in the early, distant Universe. Strong lensing is a way to measure the dark matter without actually seeing the dark matter… it gives us a sensitive probe of our cosmological models. Weak gravitational lensing can constrain mass much further out, so you can get a better picture of the surrounding cluster area. [And then,] in this cluster, the intracluster light essentially traces the dark matter. That light tells us that the cluster is in a merging state. It’s still early in the JWST era, and if we can start to get data on tens or hundreds of these types of objects at this stage in the universe, then we can really start putting our cosmological models to the test.”

This is the most important part: JWST and Hubble, over all the time they’ve been operational, have surveyed less than 1% of the total sky combined. As we not only peer into the Universe deeply, but with wide-field and multiwavelength instruments, we’re bound to uncover a whole lot more examples of what are presently rare objects like XLSSC 122.

If just a few dozen, or even a few hundred, clusters like this are identified — massive galaxy clusters at early times in cosmic history — we’ll be well-equipped to not only learn how the Universe grows up in many regards, but to learn about its implications for dark matter, structure formation, and galaxy cluster collisions. When you find one extraordinary example when you only look at a tiny fraction of the sky, you can count on other examples being out there. If clusters like this one turn out to be common, we just may be on the verge of discovering a whole host of profound information about what composes our Universe at a fundamental level.

Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.

with Ethan Siegel • June 27, 2026

Greetings readers,

It’s hard to believe how time flies. In the time between you getting this newsletter and the time I send the next one, the current year will already be halfway over. The inexorable march of time isn’t necessarily something to be sad about, however. With each passing moment, the hard work of thousands upon thousands of scientists continues to pay off, leading to new discoveries, novel realizations, and a deeper understanding of our mutually shared reality.

Here in the Milky Way, for example, we’ve recently learned that one of the largest, most prominent and famed globular clusters in our skies, Terzan 5, might be an imposter after all. The third object of interstellar origin to ever be recorded passing through our Solar System, comet 3I/ATLAS, was not only confirmed to be older than our Solar System itself, but might even be more than twice as old as the Earth, Sun, and everything else we’re familiar with. Even beyond the Milky Way, there are larger structures that we’re a part of (like the Local Group), but many of the superstructures we’re often touted as belonging to, from the local sheet to the local volume to the Laniakea supercluster and beyond, might need a rethink in light of all we now know about our place in the Universe.

Even the beginning of our Universe might need a rethink, as a series of lines of evidence might point towards a rather stringy predecessor state: Could string theory provide the ultimate answer? And finally, Einstein’s most famous equation, E = mc², isn’t even the full story; it’s only half of the equation. When we include the other half, it’s remarkable just how far we can go! As always, it’s an absolute pleasure to have you along on the ride as we journey through the Universe together. I’m not sure what the next thing we’ll uncover will be or where it will lead us, but, like you, I can’t wait to find out!

All the best,
Ethan

BEFORE THE BANG

One stringy thought causes inflationary cosmologists to lose sleep

One of the most successful descriptions of the early Universe is cosmic inflation. It sets the initial conditions for the hot Big Bang and is in remarkable agreement with observations. But we still don’t know what kind of inflation occurred, what triggered it, or what came before it. One tantalizing possibility, supported by only the scantest circumstantial evidence, is that our inflationary past has a string theory-inspired origin. It’s an idea that justifiably keeps some of us up at night.

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A GALAXY IN DISGUISE

Famous globular cluster Terzan 5: an imposter after all

Globular clusters are among our best windows into the ancient Universe, with many containing stars more than 13 billion years old. But some may not be true globular clusters at all, instead representing the stripped remnants of ancient galaxies swallowed by the Milky Way long ago. After more than 20 years of Hubble observations, combined with new JWST data, astronomers have uncovered compelling evidence that Terzan 5 is one of them. Here’s what it may really be.

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ASK ETHAN

Ask Ethan: What super-galactic structures are we a part of?

Here in our Universe, structure forms on every scale: from subatomic particles and atoms to planets, stars, galaxies, galaxy clusters, and the grand cosmic web. We’ve known for centuries that Earth orbits the Sun within the Milky Way, but it’s only been about 100 years since we discovered other galaxies and the larger structures they inhabit. While not every cosmic structure is gravitationally bound, the Milky Way occupies a specific place within the vast web of the Universe. Here’s where we fit into the biggest picture of all.

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THE OTHER HALF

Einstein’s E = mc² is only half of the full equation

First introduced in 1905, Einstein’s famous equation, E = mc², revealed the relationship between energy and an object’s rest mass. It underpins everything from particle-antiparticle creation and annihilation to nuclear fusion and fission. But E = mc² tells only part of the story: the rest-mass energy of massive particles. Once particles are in motion, or have no rest mass at all, the other half of the equation becomes essential.

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COSMIC ANTIQUE

Yes, interstellar comet 3I/ATLAS is older than the Solar System

Over the past decade, astronomers have identified three interstellar visitors to our Solar System: 1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS. Of the three, 3I/ATLAS entered at the greatest speed and appears to be older than our Solar System itself. But the most commonly cited evidence, based on its reconstructed orbit, is surprisingly weak. Instead, three independent clues — its deep red color, elevated deuterium abundance, and unusually low carbon-13 abundance — paint a much stronger case for its ancient origin. Here’s how they fit together.

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Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.