Into the Impossible With Brian Keating - FLAT Universe: Why Scientists Keep Getting This Wrong

Episode Date: May 27, 2026

An experimental cosmologist with 35 years of CMB research breaks down the curvature tension — and why the viral claim that "everything we know about cosmology is wrong" doesn't survive contact with the actual data. Subscribe if you want science with evidence, not speculation. Dr. Brian Keating is Chancellor's Distinguished Professor of Physics at UC San Diego and one of the leading experimental cosmologists working on the cosmic microwave background. He has spent three decades on experiments including BICEP, BICEP2, the Simons Array, and the Simons Observatory — the same data ecosystem at the center of this debate. We cover: why a statistical preference in one dataset is not the same as a discovery, what Planck actually measured and what its curvature signal does and doesn't mean, why combining CMB data with baryon acoustic oscillations changes the picture, the difference between geometry and topology that most explainers skip, and why science communicators who sensationalize real tensions do more damage than they realize. A clickable title and a photogenic host are not the same thing as a careful inference from the data. Key Takeaways: 00:00 A flat universe means the angles of any triangle in space sum to exactly 180° 02:10 Zero curvature is a unique number — it demands explanation, which is part of why inflation matters 04:45 Geometry describes how space behaves at scale; topology is a separate question most explainers conflate 07:30 Planck's curvature preference appears in some analyses — it is real, but it is also model-sensitive 10:00 A statistical preference within one dataset is not a confirmed result 12:20 Parameter degeneracy means changing one cosmological knob shifts others — results are not isolated 14:40 When Planck data is combined with baryon acoustic oscillation data, the case for curvature weakens 17:00 The honest summary: the curvature tension is worth watching, but nowhere near decisive 18:30 Sensationalizing legitimate tensions trains the public to think science only matters when it's exploding ——— 📬 Get the transcript, fascinating bonus content, and my Monday M.A.G.I.C. Message: https://briankeating.com/yt 🌠 Have a .edu email and live in the USA 🇺🇸? You automatically win a meteorite: https://BrianKeating.com/edu 🔔 Subscribe: https://www.youtube.com/DrBrianKeating?sub_confirmation=1 🎯 Support Into the Impossible on Patreon — get my weekly M.A.G.I.C. Message, unfiltered bonus content, and live monthly Office Hours with me: https://www.patreon.com/drbriankeating ⭐ Join this channel for perks, monthly Office Hours, and your name in the Member Roster at the end of every episode: https://www.youtube.com/channel/UCmXH_moPhfkqCk6S3b9RWuw/join 📚 My books: Losing the Nobel Prize (memoir): http://amzn.to/2sa5UpA Think Like a Nobel Prize Winner: https://a.co/d/03ezQFu Focus Like a Nobel Prize Winner: https://a.co/d/hi50U9U Galileo's Dialogue (first-ever audiobook): https://a.co/d/iZPi9Un 🌐 More: 🏄‍♂️ Twitter: https://x.com/BrianKeating 📚 Substack https://briankeating.substack.com/ss ✍️ Blog: https://briankeating.com/blog 🎙️ Audio-only: https://briankeating.com/podcast #intotheimpossible #briankeating #science #physics #astronomy #cosmology #podcast #universe Learn more about your ad choices. Visit megaphone.fm/adchoices

Great news. The federal EV rebate is back. Eligible customers get up to $5,000 with the federal EVAP rebate on select 2027-volt and 26 Equinox EV models. Visit your local Chevrolet dealer today for more details. You may have heard a dramatic claim recently that everything we know about the universe is wrong. Now, I get it, titles like down on a YouTube channel are fantastic hooks. But it's also not a very accurate summary of the evidence. Now, there's a real scientific issue here. It's called the Curveh. curvature tension. And it has to do with one of the deepest questions in cosmology. Is the universe

flatten? Now, if that sounds abstract, stay with me. Because this is not just some technical footnote buried in some arcane appendix of a plank paper somewhere. No, it touches the geometry of the universe itself, the interpretation of the cosmic microwave background, and how confident we really are in the standard model of cosmology. But here's the important part. A real unresolved tension is not the same thing as a scientific collapse. And it's become ever in increasingly popular among social media pundits to sensationalize these things. A statistical preference is not the same thing as a discovery, and a clickable YouTube title is not the same thing as a careful inference of what the data is actually saying. So today I want to do something increasingly rare online.

I want to take this claim seriously without exaggerating it. I'm an experimental cosmologist, and I'm going to ask the question, what did Plank actually find? Why did some people start talking about curvature again? What happens when you combine Plank data with other data sets? And is this really a crisis in cosmology? Or is it just a subtle and fascinating hint that there may be some unresolved problems that deserve more attention? Perhaps more illumination will come with more precision rather than with hype. So let's get into it.

I'm Brian Keating, the Chancellor's Distinguished Professor of Physics at UC San Diego. I'm an experimental cosmologist with over 35 years of experience in this very field of cosmic micrograde background measurements. The question of whether the universe is flattened. Are you one of those media strategy people clicking through slides? scrolling spreadsheets. Yes? Good. This is for you. Because on Spotify, there's an audience that's different. Locked in. Loyal, invested. They're called fans. Fans don't just listen to music. They feel seen by it like it belongs to them. So when your brand shows up on Spotify, that's who you're talking to. And you're right next to artists like me, Lizzo. So, are you ready to talk to fans?

Spotify advertising. You're among fans. Not sounds deceptively simple. It's not like asking whether or not the Earth is flat, and cosmologists don't mean flatness in the context of a tabletop or a sheet of paper. It refers to the geometry of space on the largest scales. It's a statement about curvature. As Euclid claimed thousands of years ago, if you make a triangle on a flat piece of paper or a table, you'll get interior angles that sum up to 180 degrees. If you do it on a sphere, you can get much larger internal angle summation than 180 degrees. It can actually get it up to 900. And in a hyperboloid, like a Pringle's chip or a saddle, you can sum up to angles much, much

less than 180 degrees. So what does that mean? It means any triangle you make in the universe, whether it be from Earth to the Sun to Pluto or from the Earth to Pluto to Vega or from Earth to Vega to the Andromeda Galaxy, those three sets of points in each case will have interior angles that sum up to 180 degrees if and only if the universe's spatial curvature is flat. Any deviation from that going out as far as you can possibly go will reveal the presence of non-flat curvature, non-zero curvature. And as I point out to my students all the time, it's kind of surprising and shocking we live in a universe with zero curvature, apparently,

because there are an infinite number of spheres that have positive radia of curvature, they're an infinite number of saddles or Pringles chips that have negative radia of curvature. But there's only one universe that's flat. Zero is a unique number. It cries out for explanation. We've talked about that in the context of inflation, and we will continue to do so again. It's one of the main thrust in my research

for the last 30 years on experiments like bicep, bicep 2. This spring, denim gets a softer, lighter update. Introducing Old Navy's drapey denim wide leg, a new fit that moves with you. It's everything you want denim to feel like for summer. Easy, breathable, and effortlessly cool. With a fit that creates natural movement and a wide leg that feels modern,

not overwhelming. Plus, that signature, wait, for this price? Moment. Old Navy's drapey denim wide leg. Simon's Array, the Simon's Observatory, and it's the subject of my first book, losing the Nobel Prize as well. The hunt for telltale signs of inflation could not be more important. It would be like finding the spark that ignited the Big Bang. So curvature's woven into the entire architecture of modern cosmology. It's not some minuscule thing. It affects how we interpret the cosmic microwave background signals that we see. It affects distance. measures out to faraway supernovas and galaxies. It affects the way we connect inflation, the expansion history of the universe, and the formation of large-scale structures like galaxy clusters,

how we connect those to the large-scale bulk properties of the universe. So when someone claims there may be evidence that the universe is not flat, that's not trivial. It's worth paying attention to. But paying attention is not the same thing as sensationalizing something or overreacting. There are many, many tensions in science, and especially in cosmology, the oldest science in a certain sense. Tensions and puzzles are flaws, and I tell my students, flaws lead to new laws. They're opportunities. They're not necessarily death knells. As many science populists like to claim they are. But this is not a story about science failing. It's a story about what science looks like when we get to ever-increasingly precise data sets. We are a victim of our own success nowadays in cosmology.

Something I never thought possible. When I started in graduate school, we didn't know if the universe was 10 billion years old or 20 billion years old. Now we know it's exactly 13.8 billion years old with a precision of only a few dozens or hundreds of millions of years on that number. We've become so precise, so good at what we do, my colleagues and I and the teams that we've built, that even small mismatches can be magnified out to disproportionate scales. And these mismatches can become quite meaningful. And this could not be more important as we enter an increasing age of cosmological precision undreamed of hundreds or even 10.

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public discussions about cosmology. Shape is not the same thing as geometry. Topology and geometry are separate things. When cosmologists talk about the universe being flat, open, or close, they're talking about the different types of geometrical features the universe could have, not shape in some casual sense, like our universe is actually shaped like a ball, or it's a flat plane. Geometry tells you how space behaves, how parallel lines evolve, and how angles in enormous triangles, some stretching from Earth all the way back to the formation of the cosmic great background itself, would compare to what you expect in the Euclidean environment. Any deviations from Euclidean nature is sort of a curveometer. It tells you something about the curvature of the universe.

A flat geometry means that on large scales, space obeys the familiar rules that you learned in grade school. It doesn't tell you about the global shape or topology of the universe in some intuitive sense. Topology is an equally important property of surfaces and shapes. For example, to a topologist, a cup like this, without a handle, is topologically identical to a sphere like the Earth. Now, if you add a hole or even just put a handle, you get a topological shape and surface that's quite different from either this cup without a handle or a spherical topology like the Earth or a ball.

A lot of popular concepts blur these stories. intentionally to make them feel more dramatic than they actually are. If you take nothing else from this part of the video, take this. Saying that the universe is geometrically flat is not the same thing as saying we know it's full global shape in some everyday pictorial sense. Precision here is not pedantry, it's the whole ballgame. And experimentalists spend billions of dollars in decades making measurements so precise that we can now test and ask this question to more significant figures than we ever would have

expected just a few decades ago. For many years, the standard cosmological model has strongly favored what's called a flat universe. The precision in which we know the flatness or the curvature has gotten increasingly better. In just 10 years, it's increased from about 1% to better than 0.3%. This is a remarkable testimony to not the failures of the Lambda Cold Dark Matter framework, but actually its successes, especially when married together with simulations, phenomenology, and especially my experimental colleagues who work on Plank. And we're at the threshold of a whole new error as instruments like the Simon's Observatory come online.

So when we have a hypothesis that we test, such as the universe has curvature or not, we can either prove it or falsify it by comparing the observational expectations to observational evidence. That's all we have. In inflation, in one of its simple forms, naturally drives the universe towards flatness. It's like zooming in on the surface of the earth. The closer you are to ground zero, you believe it's flat, even though you know in larger scales, It's obviously curved. So in inflation, the hyper-expansion of space-time,

faster than the speed of light, for incomprehensibly small period of time, a few hundreds of plank times, results in the universe stretching to become flat for all time. Flatness becomes the default, the working assumption, against which we should test. We shouldn't assume that it's true for all time. Just as we know, the Earth is globally spherical.

That doesn't mean that we can detect departures from sphericity on skills of, say, a city, or even a country. But the data matches the prediction extremely well. And this is where the public storytelling often slips. People say something like, scientists always believe the universe was flat. That makes it sound like an article of faith. I don't believe in gravity, as I've said many times. I have evidence for gravity.

We have evidence that the universe is extremely coastal flat. We can never measure with zero uncertainty. That would violate every known principle of not only statistics, but experimental physics, and even at the quantum level, the Heisenberg Uncertainty principle. So it's not faith, it's inference, statistical inference, and a pretty successful one too. As I talked about with Andrew Jaffe, the subject of his new book, how we applied these Bayesian methodologies and Bayesian reasoning to cosmological data.

It's a fascinating story, one that colleagues like Andrew and others have worked on for decades. It's our most important tool at gleaning new information and finding science that's closer to truth. We'll never know something perfectly, but we can get closer and closer to truth. So even successful inference models remain vulnerable to new data and new analyses and new levels of precision that can often reveal cracks in the picture. That doesn't necessarily invalidate the underlying assumption, the base level hypothesis, testing the curvature of the universe and its departures from zero, or testing the consistency of observations with the inflationary paradigm, or with Lambda CDM. So now we get to the heart of the issue. What did Plank actually measure?

Plank was a satellite launched in 2009. It gave us the most precise map of the early universe ever made. And that's not all. Thousands of papers have been written about Planck's data, and the relic radiation from the early universe has given us an astonishingly rich data set. I was working at Cal Tank when Andrew Lang and Jamie Bach and others were building detectors for the high-frequency instrument.

And I've talked recently to George F. Stathieu, who is a Titanic cosmologist at Cambridge, one of the leaders of Plank. And it has allowed cosmologists to constrain a whole range of parameters with astonishing precision. Again, precision I could never have expected when I was a wee lad starting out in graduate school. But now in some analyses of the plank data, something interesting has shown up. As often happens, when cosmologists allowed the curvature parameter to vary freely in the fit, the data sometimes showed a preference for a slightly closed universe rather than a perfectly flat one. That is the sentence that launched a thousand over-excited commentators. Because at first

glance, it not only sounds like an egregious error, it also might hint to you or might give you the impression that inflation is wrong. And maybe all of our assumptions are wrong. Maybe geometry is not flat after all. Maybe inflation is wrong. Maybe this is the next big cosmological tension and revolution. That's true. It could be. But we have to be measured. We have to slow down and ask, what are the limitations of this claim as well? There's no sacrosaned claim in science. So a preference in a fit is not the same thing as a settled conclusion. And this is where statistical literacy matters. And this is what I want to communicate to you on my channel. I want you to think like a physicist. In cosmology, parameters often talk to each other. They're not isolated nouns in some radio that you can tune and not affect one station when you turn into another. If you change one assumption, the others can shift too. This is what people mean by parameter degeneracy. Different combinations of parameters can partially mimic one another in the observable data set. So when Planck shows a mild preference for curvature in an analysis in a given framework, that by no means that the universe has confessed its geometry to us in a courtroom beyond a reasonable doubt.

It means that within that statistical and model context, one region of parameter space fits a little bit better than another. Now, I'm sorry that that's underwhelming, and I agree it's not as exciting as saying inflation's wrong, Lambda CDM is wrong, et cetera. It's interesting, it's important, it deserves attention. But there's another layer here. A result can be internally intriguing without being externally robust.

A data set can prefer one set of parameters, but it doesn't remain persuasive when you bring in other observation. It may be favored in one dataset produced by one instrument, but not agree with instruments that have collected data on the same types of geometrical constraint as well. So what cosmologists don't stop at one dramatic sentence from one analysis. We don't rely on YouTube videos for our final constraints and parameters. Combined datasets matter. This is the part that often disappears from more dramatic tellings of a story.

Cosmology is not built on one data set in isolation. It's a multi-probe science, and that's what makes it so incredibly powerful. If we just had results from one field, one type of telescope, one wavelength regime, that would be one thing. It would be interesting and it might be important, But if a probe is hinted at from many different directions, from C&B, from observations of supernova, the next question has to be, have we overlooked the mundane explanation for the more provocative one? What happens when you combine the plank data with other probes, for example? There are many other probes that can probe the geometry of spacetime as well. In the case of curvature, barion acoustic oscillations can also aid our measurements,

along with complementary evidence from datasets that have generated. been pulled into the overall inference back towards flatness, or at least made the case for genuine curvature much less compelling than the plank-only story might lead you to believe. That doesn't mean plank is irrelevant. And by no means. It doesn't mean that the tension is fake or manufactured. It means that the full evidentiary picture is more constrained and a little bit more sobering than the most viral version of the story might lead you to believe. So this is one of the most basic habits of mind and precision cosmology. You don't ask what is the most exciting result, you ask how stable is it across data sets? How sensitive is it to priors?

How model-dependent is it? Does it survive when the full observational ecosystem is brought to bear? That's not less interesting. It's more scientific. It allows us to argue for more and more precise data, which is exactly what's coming down the line. And that's exactly why the phrase, everything we know about cosmology could be wrong, is an incredibly poor summary of a situation. The combined inference matters more than the most dramatic single dataset might ever. lead you to believe. So where do we go from here? Is there a real tension or not? Yes, there's a real issue worth discussing. And there are many issues and many tensions in cosmology. That's the sign of a healthy field, one that's not scared of controversy, and even sometimes accusation that we

might be wrong. We get it wrong all the time. The tension has to be considered scientifically legitimate in the sense that it could point to a place where inference is not as robust as we previously thought. It invites questions about whether there are hidden systematic errors, whether the model is missing something, whether certain parameters are degenerate or compensating for one another as you twist one knob, you actually do twist other knobs in cosmology, or whether some deeper extension of the framework may eventually be needed. These are real scientific possibility, but there's a huge chasm between this deserves careful scrutiny and the standard cosmological model is collapsing and cosmologists are panicking.

This occurs every few years, and while it's fun to go on places like Joe Rogan and and discuss and debate these claims, the distance between good judgment and sensationalism is a large, large gap. So the honest summary right now is something a little bit less click-baby. Some high-precision analyses have produced an interesting preference that is difficult to ignore, but the overall case for a genuinely curved universe remains far, far from decisive once the broader dataset landscape is taken fully into account. That might not fit on a thumbnail quite as easily. I know I struggle with it. But it's closer to. to the scientific truth. That's what I want you to tend towards. Let me close by saying something

a little uncomfortable. Online science communication is extremely important, and it lives sometimes under incentives that are not always aligned with calibration. I learn a lot from my fellow science YouTubers, but I know that platforms reward surprise, conflict, collapse, reversal, and emotional payoff. The algorithm loves words like crisis, shattered wrong, tension, everything changes. And to be fair, the creators respond to it because it works. I know I've been guilty of that too. statistical tension with a multi-probe caveat is not as clickable as we were wrong about everything in the universe. But nature is rarely that theatrical, even when it comes to something generally surprising and

interesting to a professional cosmologist like me. And one of the problems with over-dramatizing legitimate scientific tensions is that it trains the public to think science only matters when it's exploding. In reality, much of the deepest progress in science comes from learning how to interpret small deviations, ambiguous preferences, fragile mismatches, without losing your mind or your standards of science. scientific accuracy and legitimacy. Working scientists should be able to hold two thoughts in their head at once. This is fascinating, but it's not yet decisive. And that's what makes it so interesting to be a cosmologist. What does my most responsible summary of the situation look like? The geometry of the universe is one of the deepest and most important questions in cosmology.

The standard picture strongly favors the universe that's very close to flat. Some analyses of Planck have suggested a preference for slight positive curvature. That preference is interesting, not trivial, and worth understanding, but it's also model-sensitive, statistically subtle, and weakened by the broader multi-data set, multi-messenger picture. Therefore, the right conclusion is not that cosmology has been overturned. The right conclusion is that precision cosmology is doing exactly what we're supposed to be doing, exposing where our confidence is strongest, where it's conditional, and crucially as an experimentalist, more data is needed. Science doesn't become less beautiful when the answer is nuanced.

it becomes more honest, and that, to me, is the most beautiful way to interpret cosmology. So, the universe doesn't owe a simple answer, and YouTube certainly doesn't reward simple answers. But if you really want to understand cosmology, rather than merely consume its most dramatic headlines, then subtlety is not a bug. It's the whole point. The curvature tension is real. It may yet teach us something important. It may point to new systematic. It may speak to model limitations or even fundamentally new physics, and that will be amazing. But we do owe it to ourselves to not turn every unresolved hint into a revolution before the evidence earns that conclusion. That's not skepticism.

That's theater. And cosmology is the greatest story ever written. It deserves the greatest actors and the greatest script. If you want more videos like this where we separate the hype from data, subscribe, please do leave a comment. And watch my episode with Georgia Staddeu. If you don't come away convinced that Plank and the standard cosmological model is fascinating, quite robust, but also open to new surprises,

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