Into the Impossible With Brian Keating - Fermilab’s Scott Dodelson on Cosmology’s Crisis

Episode Date: August 4, 2026

Scott Dodelson spent ten years on the Dark Energy Survey testing the standard model of cosmology. It passed and missed by 2.5 sigma. The Director of Fermilab’s Cosmic Physics Division on why the theory he helped build may be wrong, why nobody can find dark matter, and what it takes to change a scientific consensus. Subscribe if you want science with evidence, not speculation. Dodelson is Director of the Cosmic Physics Division at Fermilab, Professor of Astronomy and Astrophysics at the University of Chicago, and author of Modern Cosmology, the textbook a generation of cosmologists learned the field from. Lambda-CDM predicts how the early universe’s tiny fluctuations grew into the structure we see today. The Dark Energy Survey was designed to check that prediction. The answer came back two and a half sigma off. That is both the most precise measurement ever made of how the universe grew, and a crack he cannot stop looking at. His question is not whether the model is close. It is whether close is enough to trust. We get into the Sigma-8 tension and what it actually takes for a scientific community to change its mind, the Dodelson-Widrow mechanism and his 1994 proposal that sterile neutrinos produced in the early universe could constitute dark matter, and the Neptune vs. Vulcan history of getting dark matter right and wrong. Asked which of those two situations we are in now, Dodelson’s answer is: I have no idea. What you’ll hear: Why the April 24, 1992 CMB discovery may have been the only science story ever to lead the New York Times front column The Dodelson-Widrow mechanism: how ordinary neutrinos in the early universe may have oscillated into the dark matter we see today Why Brian told Neil deGrasse Tyson to his face that we have already detected dark matter What it means when a theory can accommodate any result and whether inflation has that problem Why the particle physics community still does not trust cosmology’s neutrino mass measurements What cosmology looks like in 2036 if Lambda-CDM breaks Killing the model is my dream. 0:00 None of it has been found in a lab. How many free passes do we get? 0:44 Dodelson helped build Lambda-CDM. His dream is to kill it. 2:10 10 years. One prediction. Two and a half sigma off. 5:42 "You're in charge" — what mentorship in science actually looks like 6:56 April 24, 1992: cosmology stopped being speculation 9:48 Geoff Burbidge went to his grave a steady-state believer 11:04 What a neutrino is and why it barely interacts with anything 12:36 Brian told Neil deGrasse Tyson we already detected dark matter 18:08 How ordinary neutrinos may have become dark matter 23:24 Lambda-CDM predicts Manhattan's density 13.7 billion years later 26:42 Two and a half sigma: technically a 1% chance the theory is right 28:04 Neptune was dark matter. Vulcan wasn't. Which one are we in now? 31:20 Dark matter, inflation, dark energy: none found in a lab 36:46 If both cracks are the same crack, Lambda-CDM is finished 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 Featured Guest: Scott Dodelson Substack: https://scottdodelson.substack.com/about Scott Dodelson on LinkedIn: https://www.linkedin.com/in/scott-dodelson-b6ba429/ Scott Dodelson on Twitter/X: https://x.com/ScottDodelson Modern Cosmology (book): https://www.sciencedirect.com/book/monograph/9780128159484/modern-cosmology Dark Energy Survey final results: https://www.darkenergysurvey.org/news-and-results/darchives/ Dodelson and Widrow 1994, Sterile neutrinos as dark matter: https://arxiv.org/abs/hep-ph/9303287 Previous ITI episode with Kyle Dawson on DESI: https://youtu.be/LPx4oiwGp2k?si=u_ZJWfJG5AhvBAEv  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 Twitter/X: https://x.com/BrianKeating Substack: https://briankeating.substack.com Blog: https://briankeating.com/blog Audio-only: https://briankeating.com/podcast #intotheimpossible #briankeating #cosmology #darkmatter #darkenergy #physics #LambdaCDM #podcast Learn more about your ad choices. Visit megaphone.fm/adchoices

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introduce inflation, introduce dark energy, and none of this has been found in the lab. How many free passes do we get? Exploring the tension, killing the model is my dream. That's Scott Donaldson. He runs a cosmic physics division at Fermilap. He teaches at the University of Chicago, and if you've ever taken a graduate cosmology class, probably took it from Scott. He also spent 10 years running the sharpest test anyone has ever made of the standard model of cosmology, the model he helped build.

The answer came back, two and a half signal off. Close enough to call it a triumph, not close enough to stop staring at it in disbelief. I'm Brian Keating. This isn't to the impossible. What is new in the field of Dart, Energy? Yeah. Where we go any further. Well, since you've been in the field, we've had this fiducial model of cosmology.

I guess you, well, you helped establish it, really. So it's called Lambda CDM. I've heard you talk about it. And now there you've had Kyle and other people on saying that it's under stress. So the fundamental question that I'm interested in is not. Or one of the questions I'm interested in is how will we change our mind, if we will. That is we have this pretty simple model. And then there are these data points which are saying, oh, this doesn't work.

It doesn't work. Oh, maybe it doesn't work. And so we have to digest that and everyone digests it in their own way. and then how are we going to collectively land on another model? And so I know people have written books about this, Tom O'Cun's structure of scientific revolutions, but we're living through that time now. And maybe in many ways with AI, we'll get to the earlier.

And of course, it's not just in cosmology. So cosmology is the lens through which I can understand stuff. But as you say, in society in general, we're losing faith in institutions, so we don't know which institutions we're going to land on and which to believe, which to trust. So I think it's kind of an important question. And so I've been trying to explore it in this little corner of our world, cosmology,

which is in some ways the simplest thing we do. It's very hard to be a parent, to be a spouse, to be a friend. But cosmology is really easy because there's no people and it's an easier thing. So that's why I've been trying to explore it in that context. For about 10 years, I was heavily involved in this project called DS, the Dark Energy Survey. And that was started taking data in 2012. And as you know, it takes an enormous amount of time to have, and analyze this data, so we only put out our final results a few months ago.

So that's what's been occupying me for the last 10 years or so. Look about the connection between, you know, the type of science that I do, which is the first light in the universe, the cosmic of microbe background. I've talked a lot about that. I've talked less about the kind of science that DES does, though, with the exception of, you know, conversations of people like Kyle and others. But talk about what is DES, what, you know, it's somewhat strange. Not only are you not using, you know, particle detectors and whatnot, but you're using optical telescopes, right? So what does DES do?

What is it comprised of? You mentioned how long it took, but what really went into that? What's the portfolio diversification between theory, which is what you do, experimental hardware, observations, big data, machine learning? What are the different ingredients in DES? And first of all, what does it stand for? DES stands for the Dark Energy Survey. The detectors on the telescope, which was in Chile, are made of silicon.

And so they leveraged a lot of the silicon technology that formerly was already an expert in. already an expert in. So that's one piece of it. The camera itself was built by Fermilab. So that's a tremendous contribution. Then there's the whole data processing thing. And again, in that realm, high energy physics has a huge advantage because they're used to processing tremendous amount of data. So there's those two pieces. And then there's the analysis, which takes us an enormous amount of time. And that piece is ongoing. It's really thinking back to when it was first started, we thought of it in one way. And all our analysis tools have have evolved partially due to AI.

So that's been kind of interesting to be a part of. But getting back to the science part of it, there's a strong connection between what you do and what I do in the sense that one of the things you do is you measure in the cosmic microwave background. The fluctuations, so I don't know people can see this, but there are small, hot, and cold spots in this microwave background. Basically what that means is the temperature, say, on this spot over here, was a little bit about one part in 10,000 higher than the temperature over here.

All that means is there were more photons there. And because there were more photons there, because the photons were also talking to the electrons and protons, there were more electrons and protons there. So if you go back to the early universe, what you have provided us with is a snapshot of the early universe.

And we know what it looks like. It was incredibly homogeneous. So the universe is basically the same everywhere, and with very, very small fluctuations of one part in 10 or 100,000. That's what you've provided us. What this theory that we've landed on Lambda CDM predicts is,

is how those small fluctuations will accrete more and more matter over the course of 13.7 billion years. And we can test that with DES and see whether those fluctuations have grown to the size they were supposed to, according to the theory. So that's, to me, the most fundamental test we're doing. If it hasn't, that means this theory is wrong.

So to me, that's the stress test that's occupied me for 10 years. Why do we tend to kind of just, as I describe it, you know, have the series of descriptions to our students, you know, from Nobel Prize to Nobel Prize without loss of enthusiasm. How do you view the way that we should teach cosmology? I'm not sure there's a direct answer, but this is my experience of it, which is in DES, to me, the most important thing has not been the papers we've written or the, that discovery we made that basically the fluctuations are what they should be in this theory.

But the most important thing has been the people. And I think we as mentors get an enormous amount of satisfaction by working with these young people. So communicating not just facts to them, but also the very little that we know has been, I think it's eye-opening to them. I'll give you an example. I was at a collaboration meeting a couple of years ago, and this first-year grad student from Spain was there, and she was talking about her work. and on this very complex piece of analysis. And I went up to her afterwards and I asked her a question.

I said, well, do you understand this and this? She goes, you don't understand. I've only been working on this for three months. I said, no, you don't understand. You're in charge. So it's like these people, the young people, they're the ones who basically, it's not like there's some threshold above which you become an Einstein.

We're all kind of swimming around in the sea of ignorance trying to figure out things. We have a little bit more experience than younger people, but they have advantages that we don't have, for example. So to me, that's been the best part about DES is working with these young people. It's been great. You talk in your substack, which will link below,

and that you kind of had this really phase change, I think, is the only revolution. April 1992. Talk about the day, you know, the music died. Talk about Kobe. I remember that I was in college at the time and I was in a summer program or starting a summer program.

And I kind of even had a glimpse that this was something important. But I didn't know I would do it for the rest of my career. Talk about 1992, why that was so transformative for you in your career. It really seems that pivoted you in a completely different direction, which is wonderful to say about your flexibility intellectually. But why was it so important to you?

Yeah. I think what happened before 1992 is there were a lot of people who had really fascinating ideas about cosmology. And one of those ideas, what I just described, that there were small fluctuations that in this kind of cosmic microwave background and they grew to be the structure we see today. But there was absolutely no evidence. There was no prediction that had been made that had been verified. There was only post-dictions. People observe stuff and they said, oh yeah, I can explain it this way that way. So any

reasonable person looking at that would say, I don't trust these guys. And in fact, quite a few astronomers and other people didn't trust a few cosmologists working in the field for very good reason. So what happened on April 24th, 1992 is the first detection of antisotropies, in the cosmic microwave background. So the fact that there are slightly spots that are slightly hotter than other spots. So that discovery was a prediction. Stephen Hawking called it the discovery of the century,

if not ever. So I mean, he was one of the driving forces behind the whole field of cosmology. So he recognized how important it was. And another thing that I highlighted there is that in the old days, they used to print, you're probably too young for this, the New York Times on paper,

And so the first column of the New York Times on the right-hand side was the lead article. So that was the first, I think the only time in the history of the New York Times that a science article was on the first column, and I think they got that right. Because that was the discovery that underpins modern cosmology in a thousand years,

that's the story people will be pointing to, I think. For me, actually at the time was kind of depressing, because I was working on all these fanciful things that were fun to work on, and to understand as a theorist what the physics that goes into that required me to learn a whole new set of tools, which I didn't, you know, I wasn't up my alleys. I knew I had colleagues who ended up just staying and not moving forward, but I kind of had

a family of the sports. I figured I got to learn this stuff. So I tried really hard to learn this stuff and ended up writing this book to help me learn it. And so I'm able to stay in the game to some extent. Yeah. And that pivot from, you know, kind of particle theory to, you know, theoretical astroparticle cosmology, which you're one of the major instigators of, I think. you know, to be fair to you, there were also, you know,

separate, you know, physics and cosmology, physics and astronomy, astronomy and cosmos, they were all kind of separate things. Yeah. As I told you, the previous occupants of his office was, you know, Jeff and Margaret Burbage. This plate is one of the Palomar plates that she took with a galaxy. That's cool. I've got some red shift.

So they, and they didn't believe in cosmology, basically until the day they died, you know, big band cosmology, Jeff used to go into paroxysms of rage when a speaker would have the misfortune of mentoring. He was a steady state proponent until he died. Long after Kobe. What do you make of that? Are there modern-day versions of people like Jeff, you know, that are just kind of eminent,

brilliant scientists that just do not accept either inflation or maybe it's string theory or the standard model? Do you see any parallels between the Big Bang deniers of that age, which you live through partially, and today's age? I think so. I think we tend to weight things according to our experiences. So I just wrote a thing about something that is obvious to you, but maybe not obvious to most people, which is the sky, the light from the stars or galaxies comes in different colors.

You can view a given galaxy with one filter and see it as one color, or through it with a different filter, it's seen a different way. And that's a metaphor to me of the way we perceive the world, right? We're all perceiving the world via our own filters. So as one example, there's kind of a raging controversy now about, neutrinos which are these very small very light particles that actually there's about a billion of them that just passed through my hand that were produced in the early universe we don't know their masses you and your colleagues have done

experiments which lead to the conclusion that their masses are smaller than they should be according to experiments that have been done by particle physicists here and when you raise that to the particle physicists they just don't believe the cosmology basically so I think that's an example of a whole class of smart people who don't necessarily buy into this field, really, or all the field, yeah. I mean, it's partially natural, historically. You think, you know, we've never detected a new particle, weighed the mass of a particle,

except in this accelerator or, you know, something like that. So to use the cosmos as your accelerator, which is natural to people like me, but, yeah, it's very sociological in science that it's different. Have you seen examples of that? Yeah. I mean, I've asked people about that. like my particle physics, you know, friends, Don Lincoln I've talked to in your neighborhood, right? Will you believe it, you know, when a particle physicist sees a cosmologist say, here's the mass of the nutrient.

Now, we're going on a paper with Shasha Arani that you meant over lunch, that really seems to suggest that we're going to need all three different types of things, nutrientless double beta decay, we're going to need laboratory experiments, and we're going to need, you know, along baseline, we're going to need cosmology. And that whole spectrum actually will make the case much stronger than only the cosmologists see it and no, give up all hope. Let's talk about the Donaldson Widrow mechanism. That's sort of where I first got exposed to you.

Very intimidated is my second year of grad school, 1994. Where did this come from? This is at Brown. I was at Brown, and I was trying to understand, well, dark matter. I'm still trying to understand that. We're going to talk a lot about dark matter. And these are called sterile neutrino. So first, what the hell is going on here?

What are neutrinos? Let's do a recap. Neutrinos, flavor, oscillation. What does it mean? Where are they oscillating? Are they jiggling around in here? What's a sterile neutrino? What's a Maran part?

It's a direct particle. Let's go through it. I want to get my money's worth. Flew you all the way out of Chicago. There's ten questions. That's my, that's my fortile. A neutrino. People are familiar with electrons because that's what they're made of. And so particle physicists tend to think of neutrinos as being partnered with electrons. So the fundamental theory of nature says that every... Have you ever had an experience you couldn't explain but also couldn't ignore? I'm a professional skeptic and I build telescopes for a living.

And even I run into questions that just sit past the edge of what science can I have to have? science can actually touch. That's why I want to tell you about my friend Myambiolik's breakdown. It's a podcast where science and spirituality stop competing and start talking to one another. It's hosted by neuroscientist and actress Dr. Mayam Biolic and spiritual explorer Jonathan Cohen. Every week they sit down with scientists, experts, and experiencers covering everything from the mind's ability to heal the body, to telepathy, to government alien disclosure. And when I joined them, we went straight for the biggest picture topics imaginable. God, the Big Bang, Consciousness, simulation theory, Mayam, grilled me and wondered whether physics leaves room for the divine.

I gave her the most honest answer our cosmologists can give. So if you spent your life wondering what's really out there, you're not alone. And knowing you, my brilliant audience, I know you're going to love to add Miami Biolix breakdown to your rotation. Listen to new episodes every week, follow the show on Apple Podcasts, Spotify, or wherever you're listening to this. Watch full episodes of Myambiolics breakdown on YouTube.com slash Myambiolic. Thing like an electron has to have a neutrino associated with it. So, for example, you mentioned nuance earlier. Muons are kind of cousins of electrons.

They also have their own neutrino with them. There's another thing like that. All these things are called leptons. There's a tau lepton. It has its own neutrino. So neutrinos are associated with electrons in this case, or electrons or muons or taos. Whereas the electrons are charged, they have a negative charge.

Neutrinos do not have electric charge. That's why they're so hard to detect. Whereas neutrinos have a mass that we know, neutrinos have masses with our at least a million orders of a million, a factor of a million smaller, probably a billion smaller, yeah, than the electron. So that's what they are. They're very, very hard to detect. However, just like the electron, they do participate in what's called the weak force.

And that's important because there are various decay processes that are important to life and everything that produce neutrinos. So for sure, neutrinos exist. We've seen them. But they only interact very weakly, so they're very hard to detect. So that answers your first question. What is a sterone? It was called flavor states.

Right. The partnering, pairing between the subatomic elementary particles, mu, tau, and electron, those are the flavor states, but they are not the mass states, right? Exactly. So what I mentioned is that these neutrinos, the electron gene, it was paired with the electron. So that is, so that's one type of neutrino. But if you imagine the possibility of a given quantum state that is a superposition, probably you've had a lot of quantum computing people on, so people are familiar with superposition,

a superposition of an electron neutrino and a town neutrino. So that's possible. And it turns out that a superposition of those neutrinos are actually eigenstates of mass. That is, they're agenstates of the things that propagate through space. And respond to space-time curvature, for example. Exactly. If there was no mass, then there would only be this one basis, the flavor basis. But because there's mass, things get mixed up.

It's kind of a quantum mechanical effect. And because they get mixed up, it's possible. and this was first detected from neutrinos from the sun, that neutrino could be produced in the sun in one type, one flavor, and be detected as another flavor. So that's the oscillations you mentioned. So I hit two of your questions. Yeah, yeah, steron neutrinos.

Okay, so each of those things are a type of neutral particle. It turns out that it's quite possible that there is another neutral particle associated with them that does not experience the weak force. So neutrinos are not charged, so they interact very weakly, but they do interact. A sterile neutrino is even less weakly interacting than that. It's completely divorced from the weak force. It's not produced in decays, et cetera. Why do we think they exist?

There's kind of a complicated technical reason for it. That is, if you think about electrons, they're made up of left-handed spinning states and right-handed spinning states. The neutrino that we know are all left-hand spinning. So we kind of expect there to be right-hand spinning states. Also, those are sterile neutrinos. That's sterile neutrino. And the sterile neutrino was hypothesized to be via the Donaldson Widrow mechanism,

a potential, but by no means, confirmed dark matter can. Let me say one thing. I tell people we've detected dark matter. We have unequivocally detect. I told Neil deGrasse Tyson this to his face, and he was astonished. And I said their neutrino. They satisfy every possible property, except for the fact that they don't make up all of the mass that is seemingly required

to explain the dark matter gap between the amount of luminous matter and the amount of total matter that we see, right? So that's like saying, you know, they're like, well, it doesn't solve dark matter. That's like saying, well, hydrogen doesn't explain all barionic matter. It's irrelevant. There might be a whole, as George and as past guest on the podcast, Mike Turner and others have said,

there might be a whole periodic dark matter table, the dark periodic table, right? So am I wrong? Should I shut up about this dark matter detection already occurring? Well, let me challenge you with one thing. Have we detected cosmic neutrinos? We've detected, I believe we've detected. Yeah, we have.

You have detected. Not direct detection. Not direct. You have indirectly detect them. So, yes. So you have indirectly detected neutrino dark matter. There's a difference between direct detection where you actually build a detector that tries to see these cosmic neutrinos, a billion of which had just passed through my hand. That, do you know Joe Formaggio from MIT?

So he has told me that every experimentalist in their career goes through a period of two years where they try to detect cosmic neutrinos and then they realize it's impossible. And unfortunately, their startup runs out. Yeah, exactly. Anyway, but you have detected that you and your CNB colleagues have detected them indirectly. If they weren't there, then the pattern of antisotropies that we see would look much different. So kudos, 100% agree. Yeah. So what is the Donaldson withdrawal mechanism?

In the late 80s, early 90s, there was a guy named John Simpson, and there was evidence, he provided evidence for detecting in the lab a neutrino with a mass of 17 kilo electron volts. Do you hear about this? No. people were like astonished. It turned out to be an experimental artifact due to the magnetic fields, but for about three to five years, everyone was talking about it.

As you probably know, such a heavy neutrino would, it makes a lot of problems for cosmology. So I started thinking about what would happen, would they be produced and stuff? And so then Larry and I came up with this idea that putting aside the 17KV neutrino, which turned out to go away,

maybe it's possible that the ordinary trinos in the very early universe oscillate into these sterile neutrinos, and maybe they have a mat, so produce enough of them so that they constitute the dark matter today. I still think it's a good idea because we know neutrinos exist, right?

As opposed to every other dark matter candidate around where we don't know exists. So to some extent, my prior on that is higher than other things, but of course, I'm pretty biased, so yeah. I don't talk about the mechanism. There's something called a mixing angle, which is quite beautiful when you think about it,

that these states, these quantum mechanical eigenstates, which are superpositions, that they have this ability to effectively rotate, just like an ordinary rotation of a ball or any object in two-dimensional. It's the simplest thing you could imagine. I guess one-dimensional would be simpler, but they can basically be thought about as rotating in this abstract space. How do you think about that? How do you visualize this?

Is it purely a mathematical thing in a theorist brain? Yeah, that's a great question. And you're absolutely right, that you don't need flavors or anything like that. it's actually just a two-dimensional space, the regular neutrino and the steronotrino. Actually, this comes back to the way different people perceive and think about different things and how everyone's opinion, everyone's brain see things differently. So my mind works best very linearly and mathematically. I'm not good spatially.

So I just think about a two-by-two matrix, which is a pretty simple mathematical thing. So I just think of these things as if you take two-by-two matrix with off-diagy elements and diagonalize it, that's what I think of as an oscillation. But people like, I'm sure you think of it in a more spatially, in more like a physically intuitive way, I don't think I have that physical intuition. So what's physically happening in the way I think about it? I do think about it mathematically also, but in quantum mechanics, phase is an important quantity. Even though we can't directly measure phase, you can measure phase differences, right?

And so, you know, stop me if I get this wrong. But energy differences lead to phase differences, which then can be imprinted and you can get physical oscillation. I mean, we get in the oscillations of the solution to the solar neutrino problem, right? It was effectively this oscillation both abstractly in the phase space of quantum mechanical rotation space. But physically, these things are oscilling as they travel through, you know, if the distance between the Earth and the sun were different, we would have gotten a different answer, right? We could have been in this weird position where it exactly always came out to be an electron or something. We just didn't happen to live there, right? So how do we

explain this? The neutrinos interact when matter, but the... sterile neutrinos wouldn't interact with matter, say, in the same way? It turns out matter plays a quantitative role in it, but it's not, I don't think it's a qualitative thing. So I think the qualitative thing is exactly what you said, that in the solar neutrino issue, the electron neutrinos that the sun produced convert as they travel into muon neutrinos that Ray Davis, et cetera. Did he win a Nobel Prize?

Yeah. So that is an oscillation in a two-dimensional space. It's the exact same thing without the flavor thing. The two dimensions are that a regular neutrino and the steronotrino. It's the same exact process that we think might have happened in the early universe to produce. We had a lot of regular neutrinos around in the early universe. They oscillated and produced these sereniturnutri that could be the dark manner today. So it's the exact same process.

Now, a year ago, Kyle Dawson sat in that very chair with Dan Green, and we had the spirited conversation about Desi, right? And a lot of the conversation was kind of a little bit, you know, he's very statesman-like and wonderful scientists and just exceptional person. But I detected a little bit of hedging. You know, he's saying, yes, there's a 4.2 sigma tension. And I often say, you know, we got the Hubble tension. We have the Sigma 8 tension.

Now we've got the W tension. So what do you make of these different tensions? First of all, I've had eminent scientists, including a partner of one of your, you know, former partners, Mike Turner, Lawrence Krauss, alleges he sometimes says he came up with dark energy and on his weaker moments or maybe he's being more accurate than I'm giving him credit for, but with Mike Turner, that they sort of independently came up with some ideas that suggested dark energy. It was real. And he doesn't believe it. Lawrence Cross said that he thinks they're wrong. It's not a cosmological constant. Now, Einstein was wrong once

before. It's too bad. He could have had a good career, right? But tell me, Scott, where do you come down on this? Is it a legitimate tension? Where does it rank in the tensions that I mentioned? Sigma 8, which is clubbing a matter, Hubble tension, which is disagreement at early times and late times. And now this new tension between dark energy and a cosmological constant. Just to focus on one thing, this essay tension you mentioned, that's what I've been spending the last 10 years on this idea that, think about Manhattan in 1790. There were 50 people every square mile in Manhattan, very over-dense compared to the rest of the country. Today there are 50,000. So why is it that the inhomogeneities grew like that? That's a fascinating question, right? Could someone in 1790

have predicted that there would be exactly 50,000 people? No way. But we've done that. Lambda CDM predicts that when you measured in the microbe background, evolves would be precise. precisely the in homogeneities we see today, that precisely modulo the fact that it's off by about two sigma or something. So that's what you call the essay tension. So I guess my question back to you is, is it tension or is it, wow, that is unbelievable we're able to get that close. I'm kind of depressed that it's not a five sigma tension because then it would be whatever, like just like kind of Kyle's thing. But on the other hand, we're so close on this incredibly This episode is brought to you by Accenture.

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We've created a story of how we got here. I guess to pushback, I would say, you know, sometimes we, I feel like we got to grow an extra arm to pat ourselves in the back, and you're doing a great job of that with the experimental, you know, kind of kudos to us. But by the same token, I have kids, you have kids, right? And you remember when your kids were young, you'd take them to the doctor at age two, and they'd measure their height and say, basically they're going to double in size, right? And how do they know that?

Well, they've had about 100 billion humans have lived since, you know, time equals zero. when Ag and Magog came out of the cave and somewhere in 250,000 years ago, they estimate about 100 billion people. I'm going to say pretty good statistics on this, right? Now, we only have one universe and we always complain about how hard it is to be a cosmologist, you know, world's smallest mile in. There's only one universe. I always say at least astronomers have, you know, 100 trillion stars, you know, perhaps in the

observable universe, so there's a lot of different, you know, statistics that they can do. You're making my point that because there's so many regions of that size, you and your colleagues are able to measure it at one time in the universe, and you've measured it incredibly precisely to a few percent, right? That's what your point is, that there's so many different regions that you can measure it so precisely. And we were not quite, we haven't measured quite the area you have, but we've measured a lot, and we can measure it at a time 13.7 billion years later, and we get an answer that's 10,000 times bigger, but precisely on what the prediction gives. So it's the precision that, to me, is astonishing and is a

great success. So why do you still work on it? Like what's left to unreal? I mean, I'm not an expert in Zima A. Your time is very valuable, right? So why are you still so deeply investigate it? So you're looking for something that if you found them, these B modes and the cosmic right back one would prove beyond the shadow of the doubt inflation, something that happened and unimaginably small times, it would be incredible. The rest of us are focused on unkinging Lambda CDM, like basically just dethroning it. So there are people like Kyle do it in one way by trying to figure out whether the distance that they measure to distant places agrees with what you guys in Lambda CDM predicts.

And I've been doing it in this other way by seeing whether that precise prediction, which is so precise, is correct. And so the answer is it still is called an essay tension. That is we're not exactly right. So I overstayed it a little bit. But we're, so we're two and a half sigma off, which means there's technically only a 1% chance that theory is right. But we kind of know that that's probably, yeah, yeah. Million to one things happen.

Yeah. So the day here. So, I mean, exploring the tension, killing the model is my dream. Is that because there are certain dependencies on Sigma 8 that goes like, what, Sigma 8 to the 7th power? There's some interesting, either it's in the power spectrum or I seem to recall there, aren't there some. aren't there some very crisp tests that are available because of the deep sensitivity on Sigma 8? Like, in other words, if you nail Sigma 8, you can get other things to extremely 8 times

higher precision. I mean, that's probably true for galaxy formation, the halo mass function and stuff. But that's not what I'm most interested in measuring this very simple statistic. It actually comes back to something you mentioned earlier. We're doing it with the simplest of statistics, what's called the two-point function or the power spectrum, which is some of the same thing that you use. And I wonder if we're going to be supplanted with AI am alt-teachings that use more of the information. So essentially what we're doing is we're compressing the data into 100 numbers, right?

And from those 100 numbers, we're extracting out this one number and comparing to the one number that you guys measure. So that may not be the best way to test the theory. There might be better ways to do it. So people are thinking about that. Higher order functions. Yeah. Yeah. Last time you give a colloquium here, which is, you know, decades ago or now, but it was extremely well-received.

You made this beautiful kind of point about the discovery of dark matter, you know, in our solar system, which goes by the name of Neptune. And Neptune's discovered by Laverier. Famously used, I went back and I looked at his paper from 1843 or whatever. And they basically, you know, it's just this lionizing hero worship. He discovered a planet with a pencil, you know. It's basically just this lone genius that set this thing. And then you made the case that like, okay, so that was discovered. that there was this weird sort of effect

that was happening to the planet George, Uranus, which I think still should have been called George. But anyway, he took us back to that time, you know, when it could have been some Promenon, Isaac Newton's gravity, or it could have been dark matter that was just unseen and turned out to be dark matter. Then you made the analogy that's fast forward to 1911, 1913,

14, Einstein's coming up with GR, and Laverrier in the acolytes of Laverier were still, and actually Laverier was proposing that Mercury was being perturbed by an unseen companion, just like Neptune called Vulcan, right? And then you said, well, that time he was wrong, right? So the hammer theory is everything looks like a nail and all you have is a hammer. Where do you think we are with dark matter? What is your take?

I've had Mordecai Milgram on the podcast. What do you make of the, you know, kind of alternative, you know, history rhyming again and again looking for dark matter, looking for changes to gravity? Where do you think we're going to end up? I mean, you worked on this for a long time. Where do you come? What is dark matter? Yeah, I have no idea. I actually had this debate with Stacey that this is an organization that promotes civil debates.

So we had this debate. And I don't, so just to give context, Stacey McGaugh is one of the leading astronomers who doubts the existence of dark matter. And he and Mordecahneungum and other people, most notably from a theoretical perspective in a Mexican-Israeli physicist Jacob Beckenstein, put forth alternative models of gravity that would not require dark matter. The problem with those is they don't explain what you guys have seen. And to some extent, it's another example of what we've been talking about. It all depends on your filter. What they look at is, oh, I can see that galaxy over there. Let me look at that and I can fit this better with my theory of modified gravity. What we tend to think, what I think is simpler is the stuff that you measure because there's no people there. It's just electrons,

protons, protons, and photons, it's really simple to understand. But from their perspective, that's like a zillion miles away, how can we possibly understand it? So it's two different lenses on the same universe. And from their perspective, we're never going to convince them that this distant stuff has any information about what is guiding stars in a galaxy near us. So I don't think there's much trouble bridging that gap. However, Stacey, who's a very reasonable person, does not believe Mond is correct. And if you look at the generalizations of Monns, such as the ones that Beckenstein set up,

They're actually so close to theories of dark matter. They introduce new fields that it's the same thing, basically. They're just introducing new stuff. And so I actually think another possibility is that we're just looking at everything wrong, that there's someone's going to come around and say, we have to just rethink everything. Because what we've done in order to make the story of the universe work is introduce dark matter, introduce inflation, introduce dark energy, and none of this has been found in the lab. So, you know, how many free passes do we get?

I don't know. No, it's extraordinary that you're, again, candid and you're honest and you're willing to, you know, kind of admit where these lacunae, these gaps, these flaws. But, you know, I often say the most exciting thing, and you hinted at this earlier, is a flaw, right? Because I tell my students, a flaw leads to a law. And we just don't know it yet. Like the Big Bang was, you know, kind of a solution to a lot of problems in the steady

state mall. The inflationary universe is sort of a patchwork of fixes and, you know, kind of home improvements to the Big Bang model, fiducial model, and Lambda CDM, and there's alternatives to it. But one thing I thought about, you know, 61 years ago this month or last month, the CMB was discovered and the announcement was discovered. And many people, including Jeff Burbage, who used to occupy this office and his longtime colleague, Fred Hoyle and the recently deceased and also, I'm very proud to say past gas giant and Arlachar, you know, they went to their graves believing in the steady state, you know, quasi-steady state,

they added some stuff to it. They were eminent scientist. I mean, these are people that were Nobel-worthy adjacent to Willie Fowler, did win the Nobel Prize for the, you know, BBFH and incredibly eminent scientist. Hoyle, of course, calling the term Big Bang as a pejorative. And my question to you is when the C&B was announced, Hoyle said, well, they found it to be, you know, 2.7 Kelvin. But if it had turned out to be 27 Kelvin or, you know, 270 Kelvin,

they would have explained that too. And it reminded me of this quote, that I have in my book from you, from eminent cosmologist Scott Daudelson. During Bicep 2, you said inflation can produce a B-Mode signal as small as you like. And I kind of used that in the rhyming sense with our friend Fred, Hoyle, who said, you know, they would have found any value. So it was almost like anything that we found would be consistent. And you're not the only one who said this, by the way.

And many people said things much, much more hyperbolic, like, Hello Multiverse, Max Tagmark said, and Lawrence Krause said this now means there's proof of that God does not exist. It was an incredible time for me personally, obviously. I talk about a lot of these in losing the Nobel Prize, my first book. But what do you mean about it? If a theory incomminates any result, is it a theory? I mean, what is it?

What are we to make about it? Yeah, I don't want to go too far towards ignorance in the fact that smart people don't believe in the microwave background doesn't mean they're correct. Yes, right? Sure. I think here there's a sidebar. I think the human need for meaning is the most paramount need that humans have. Victor Frankloules, makes first for meaning, right?

I want to just kind of take us counterfactual, hypothetical. You know, 10 years from now, 2036, Desi's done, Simon's observatory is done. So, CNBS4 never happened. But unfortunately, our rest in peace, it should have happened. Lightbird, you know, there's many different projects, hopefully, that will augment and complement and compete with us. And so I think the most healthy thing is have a competitive scientific environment. And so I welcome our, you know, kind of competition from China.

We have a Chinese team that's trying to do us. And there's many other great and brilliant scientists. But we're there. What is a, what is like cosmology being done look like? What does a solve model of cosmology look like? And is it on the horizon in our grand grad student careers? Okay, I have no idea. But what would satisfy you?

Let's agree that cosmology is a millennial old pursuit, maybe more. Maybe those people from 75,000 years ago. Maybe they also. And we've been just incredibly fortunate to live in a time where the, you know, the amount of information we've gotten is, been, you know, extraordinary about the universe. We have very good reason for believing that the universe is not on the back of a turtle, right?

We're not any smarter than the people who made up that theory, right? Because we have data. So we've just been incredibly fortunate, but it's still a thousand-year-old science, and it's probably going to be evolving. So the chance that we're going to identify dark matter in my career is, I would say it's pretty small. The chance, well, you tell me,

I mean, the chance that we find B-mode, so you're going to be extended. the reach by a factor of five to ten, is that right? Yeah. So that, I mean, you have to have a prior on what inflationary models produce that. I mean, it would be amazing, as you probably know, I went all Google Gaga the first time. You guys reported something, so it would be, you know, it would be amazing. We have to be clear about what we know and what we don't know.

If you guys discovered B modes, that would not be the end of our studies of inflation. Then we put up a gravitational wave thing in space to detect gravitational waves at higher frequencies, right? For sure, what they used to call the Big Bang Observatory, right? So we would definitely hone in on those B modes because they would teach us about physics 12 orders of magnitude larger than can be probed at the large hydrogen collider. So that would open things up. In dark matter, if we detected something in the 100 GEV range that pointed to supersymmetry, that would, open up the pathway, I think, to more colliders being built to understand things. So I think the possibility of, especially this marriage of quarks in the cosmos, the possibility

of discovery would be unlikely to shut things down, but as I said, I really don't know. I tell my students a flaw leads to a law, and we just don't know which law yet. Scott's got a decade in the trenches working on one flaw, whether the lumps in the universe grow the way the theory says they should. There's a second way the stress test the same model. A year ago, Kyle Dawson sat in the same chair and told me what Desi found. 4.2 Sigma. If Scott's crack and Kyle's crack are the same crack, Lambda CDM is finished.

That conversation is on screen now. Click it, watch it, subscribe and share it, and then tell me which tension you'd bet the most on. I'm Brian Keating. This isn't to the impossible. Tune in next time.