Transcript

Scott Dodelson:
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. How many free passes do we get? Exploring the tension, killing the model is my dream.

Brian Keating:
That’s Scott Dodelson. He runs the Cosmic Physics Division at Fermilab. He teaches at the University of Chicago, and if you’ve ever taken a graduate cosmology class, you 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 2.5 sigma off. Close enough to call it a triumph, not close enough to stop staring at it in disbelief. I’m Brian Keating. This is Into the Impossible.

Brian Keating:
What is new in the field of dark energy?

Scott Dodelson:
Yeah.

Brian Keating:
Before we go any further.

Scott Dodelson:
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.

Scott Dodelson:
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, Thomas Kuhn’s Structure of Scientific Revolutions, but we’re living through that time now. So I’m kind of—

Brian Keating:
And maybe in many ways with AI, we’ll get to that later.

Scott Dodelson:
Yeah, right. So it’s not, 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 gonna 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’s really easy ’cause there’s no people and it’s an easier thing.

Scott Dodelson:
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 DES, the Dark Energy Survey, and that started taking data in 2012, and as you know, It takes an enormous amount of time to process 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.

Brian Keating:
Talk about the connection between, you know, the type of science that I do, which is the first light in the universe, the cosmic microwave 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 with people like Kyle and others. But talk about what was DES? What, you know, again, 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? How much— what’s the portfolio diversification, you know, 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?

Scott Dodelson:
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 Fermilab was always 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 amounts of data.

Scott Dodelson:
So there’s those 2 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 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 if people can see this, but there are small hot and cold spots in this microwave background.

Scott Dodelson:
Basically what that means is the temperature, say on this spot over here, was a little bit about 1 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 was basically the same everywhere, and with very, very small fluctuations of 1 part in 10 or 100,000. So that’s what you’ve provided us. What this theory that we’ve landed on, Lambda CDM, predicts 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.

Scott Dodelson:
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.

Brian Keating:
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?

Scott Dodelson:
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 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. So, 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 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.

Scott Dodelson:
I’ve only been working on this for 3 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 and 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 it’s, to me, that’s been the best part about DS is working with these young people. It’s been great.

Brian Keating:
You talk in your Substack, which we’ll link below, and that you kind of had this really phase change, I think is the only way, 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 1982, 1992, why that was so transformative for you in your career. It really seems to have pivoted you in a completely different direction, which is wonderful to say about your flexibility intellectually. But why was it so important to you?

Scott Dodelson:
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 in this 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 postdictions. People observed 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 the few cosmologists working in the field for for very good reason.

Brian Keating:
Yeah.

Scott Dodelson:
So what happened on April 24th, 1992 is the first detection of anisotropies 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.

Scott Dodelson:
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 that they got that right, because that was the discovery that underpins modern cosmology. In 1,000 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, it wasn’t up my alley. So I knew I had colleagues who ended up just staying and not moving forward, but I kind of had a family to support, so I figured I got to learn this stuff. So I tried really hard to learn this stuff and end up writing this book to help me learn it.

Scott Dodelson:
And so I’m able to stay in the game to some extent. Yeah.

Brian Keating:
And that pivot from, you know, kind of particle theory to part of, you know, theoretical astroparticle cosmology, which you’re one of the major instigators of, I think, you know, to be, to be fair to you, there were also, you know, separate, you know, physics and cosmology, physics and astronomy, astronomy and cosmology. They were all kind of separate things.

Scott Dodelson:
Yeah.

Brian Keating:
As I told you, the previous occupants of this office was, you know, Jeff and Margaret Burbidge. This plate is one of the Palomar plates that she took with the galaxy.

Scott Dodelson:
That’s cool.

Brian Keating:
I’ve got some redshifts. So they— and they didn’t believe in cosmology basically until the day they died. You know, Big Bang cosmology. Jeff used to go into paroxysms of rage when a speaker would have the misfortune of mentioning it. He was a steady-state proponent until he died, long after COBE. 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 lived through partially, Yeah. In today’s age?

Scott Dodelson:
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 view it with a different filter and see it in 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.

Scott Dodelson:
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 of the field. Yeah.

Brian Keating:
And it’s partially natural historically. You think, you know, we’ve never detected a new particle, weighed the mass of a particle except in an accelerator or something like that. So to use the cosmos as your accelerator, which is natural to people like me, But yeah, it’s very— it’s sociological in science that it’s different.

Scott Dodelson:
Have you seen examples of that?

Brian Keating:
Yeah. I mean, I’ve asked people about that, like my particle physics friends. Don Lincoln, I’ve talked to, is in your neighborhood, right? Will you believe it when a particle physicist sees a cosmologist say, here’s the mass of the neutrino? Now we’re working on a paper with Shasha Arani that you met over lunch that really seems to suggest that we’re going to need all 3 different types of things. Neutrinoless double beta decay, we’re going to need laboratory experiments, and we’re going to need long baseline, we’re going to need cosmology. And that spectrum actually will make the case much stronger than only the cosmologists see it and give up all hope. Let’s talk about the Dodgson-Widrow mechanism. That’s sort of where I first got exposed to you. Very intimidated.

Brian Keating:
It was my second year of grad school, 1994. Where did this come from?

Scott Dodelson:
This is at Brown.

Brian Keating:
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 neutrinos. So first, what the hell is going on here? What are neutrinos? Just 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 Majorana particle? What’s a Dirac particle? Let’s go through it. I want to get my money’s worth.

Brian Keating:
Flew you all the way out from Chicago.

Scott Dodelson:
Okay, 10 questions.

Brian Keating:
That’s my forte.

Scott Dodelson:
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 thing like an electron has to have a neutrino associated with it. So for example, you mentioned muons earlier. Muons are kind of cousins of electrons. They also have their own neutrino with them. There’s another thing like that.

Scott Dodelson:
They’re called— 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 taus, 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 which are at least a million orders of magnitude— a million, sorry, a factor of a million smaller, probably a billion smaller.

Brian Keating:
Yeah.

Scott Dodelson:
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.

Scott Dodelson:
What is a sterile neutrino?

Brian Keating:
Those are called flavor states. The partnering, pairing between the subatomic elementary particles, mu, tau, and electron, those are the flavor states, but they are not the mass states.

Scott Dodelson:
Right, exactly. So what I mentioned is that these neutrinos, the electron neutrino is paired with the electron. So that is, um, 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 tau neutrino. So that’s possible. And it turns out that a superposition of those neutrinos are actually eigenstates of mass. That is, they’re eigenstates of mass. That means they— the mass eigenstates are the things that propagate through space.

Brian Keating:
And respond to spacetime curvature, for example.

Scott Dodelson:
Right, exactly. If there was no mass, then there would be— 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 2 of your questions.

Brian Keating:
Yeah, yeah, sterile neutrinos.

Scott Dodelson:
Sterile 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.

Scott Dodelson:
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-handed spinning, so we kind of expect there to be right-handed spinning states also. Those are sterile neutrinos. That’s sterile neutrino.

Brian Keating:
And the sterile neutrino was hypothesized to be, via the Dodgson-Widrow mechanism, a potential but no— by no means confirmed dark matter candidate. Now let me say one thing. I tell people we’ve detected dark matter. We have unequivocally detected— I told Neil deGrasse Tyson this to his face, and he was astonished. And I said, they’re neutrinos. 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, and then they’re like, well, it doesn’t solve dark matter. That’s like saying, well, hydrogen doesn’t explain all baryonic matter. It’s irrelevant.

Brian Keating:
There might be a whole— as George and as past guests 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?

Scott Dodelson:
Well, let me challenge you with one thing. Have we detected cosmic neutrinos?

Brian Keating:
We’ve detected— I believe we’ve detected—

Scott Dodelson:
Oh, we have. Yeah, we have. You have detected.

Brian Keating:
Not direct detection.

Scott Dodelson:
Not direct. You have indirectly detected 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 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 2 years where they try to detect cosmic neutrinos and then they realize it’s impossible.

Brian Keating:
And unfortunately their startup runs out.

Scott Dodelson:
Yeah, exactly. Anyway, but you have detected that, you and your CMB colleagues have detected them indirectly. If they weren’t there, then the pattern of anisotropies that we see would look much different. So kudos, 100% agree. Yeah.

Brian Keating:
So what is the Dodgson-Lidro mechanism?

Scott Dodelson:
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 kiloelectron volts. Did you ever hear about this?

Brian Keating:
No.

Scott Dodelson:
People were like astonished. It turned out to be an experimental artifact due to magnetic fields, but for about 3 to 5 years, everyone was talking about it. As you probably know, such a heavy neutrino would— 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 17-kV neutrino, which turned out to go away, maybe it’s possible that the ordinary neutrinos in the very early universe oscillate into these sterile neutrinos, And maybe they have a mass, 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.

Brian Keating:
Talk about the mechanism. There’s something called a mixing angle, which is quite beautiful when you think about it, that the 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 2-dimensional. It’s the simplest thing you could imagine. I guess 1-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?

Scott Dodelson:
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 2-dimensional space, the regular neutrino and the sterile neutrino. Actually, this comes back to the way different people perceive and think about different things and how everyone’s opinion, everyone’s brain sees things differently. So my mind works best very linearly and mathematically. I’m not good spatially. So I just think about a 2 by 2 matrix, which is a pretty simple mathematical thing.

Scott Dodelson:
So I just think of these things as If you take 2 by 2 matrix with off-diagonal 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 of a physically intuitive way. I don’t think I have that physical intuition.

Brian Keating:
So what’s physically happening, 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 stop me if I get this wrong, but energy differences lead to phase differences, which then can be imprinted And then you can get physical oscillation. I mean, we get oscillations of— and that was 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 oscillating as they travel. If the distance between the Earth and the sun were different, we would’ve 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 like that. We just didn’t happen to live there, right?

Scott Dodelson:
Yeah.

Brian Keating:
So how do we explain this, that the neutrinos interact with matter? But sterile neutrinos wouldn’t interact with matter, say, in the same way?

Scott Dodelson:
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?

Brian Keating:
Yeah.

Scott Dodelson:
So that is an oscillation in a 2-dimensional space. It’s the exact same thing without the flavor thing. The 2 dimensions are that the regular neutrino and the sterile neutrino. 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 sterile neutrinos that could be the dark matter today. So it’s the exact same process.

Brian Keating:
Now, a year ago, Kyle Dawson sat in that very chair with Dan Green, and we had the spirited conversation about DESY, right? And a lot of the conversation was kind of a little bit, you know, he’s very statesmanlike and wonderful scientist 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’ve 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 It was real. And he doesn’t believe it. Lawrence Krauss said he thinks they’re wrong. It’s not a— it’s a cosmological constant.

Brian Keating:
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? Where— what is it, a legitimate tension? Where does it rank in the tensions that I mentioned? Sigma-8, which is clumping of matter, uh, Hubble tension, which is disagreement at early times and late times, and, uh, and now this new tension between dark energy and the cosmological constant.

Scott Dodelson:
Just to focus on one thing, this SA 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 overdense 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 what you measured in the microwave background evolves to be precisely the inhomogeneities we see today, that precisely modulo the fact that it’s off by about 2 sigma or something. So that’s what you call the S8 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 5 sigma tension because then it would be whatever, like just like kind of Kyle is saying.

Scott Dodelson:
But on the other hand, we’re so close on this incredibly amazing story that we’ve created. We’ve created a story of how we got here.

Brian Keating:
I guess the pushback, I would say, you know, sometimes we, I feel like we gotta grow an extra arm to pat ourselves on 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 ’em to the doctor at age 2 and they’d measure their height and they’d say, basically they’re gonna 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 Og and Magog, you know, came out of the cave and somewhere in, uh, 250,000 years ago. They estimate about 100 billion people. Let’s 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 violin. There’s only one universe. I always say at least astronomers have, you know, 100 trillion stars, you know, perhaps in the observable universe.

Brian Keating:
So there’s a lot of different, you know, statistics that they can do.

Scott Dodelson:
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, that’s what your point is, that there’s so many different regions that you can measure it so precisely. And we, we’re 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.

Brian Keating:
So why do you still work on it? Like, what’s left to unravel? I mean, I’m, I’m not an expert in some way. Your time is very valuable, right? So why are you still so deeply invested in it?

Scott Dodelson:
So you’re looking for something that if you found them, these B-modes in the cosmic ray background would prove beyond a shadow of a doubt, inflation, something that happened in 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’s still called an SA tension. That is, we’re not exactly right. So I overstated it a little bit, but we’re— so we’re 2.5 sigma off, which means there’s technically only a 1% chance that theory’s right. But we kind of know that that’s probably— yeah, yeah, yeah.

Brian Keating:
Millions of one things happen.

Scott Dodelson:
Yeah.

Brian Keating:
So today here.

Scott Dodelson:
So I mean, exploring the tension, killing the model is my dream.

Brian Keating:
Is that because there are certain dependencies on sigma 8 that go as 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 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.

Scott Dodelson:
I mean, that’s probably true for galaxy formation, the halo mass function and stuff, but that’s 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 2-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 and ML techniques 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.

Scott Dodelson:
So people are thinking about that.

Brian Keating:
Higher-order functions. Yeah.

Scott Dodelson:
Yeah.

Brian Keating:
Last time you gave a colloquium here, which is, you know, decades ago now, but, 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 was— goes by the name of Neptune. And Neptune was discovered by Le Verrier famously. 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. It’s like he discovered a planet with a pencil, you know?

Scott Dodelson:
Wow.

Brian Keating:
It’s basically just this lone genius that set this thing. And then you made the case that, like, okay, so that was discovery 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, you took us back to that time, you know, when there could have been some problem with Newton, Isaac Newton’s gravity, or it could have been dark matter that was just unseen. And it turned out to be dark matter. Then you made the analogy, let’s fast forward to 1911, 1913, ’14, Einstein’s coming up with GR, and Le Verrier and the acolytes of Le Verrier were still— actually, Le Verrier was proposing that Mercury was being perturbed by an unseen companion, just like Neptune.

Scott Dodelson:
Called Vulcan.

Brian Keating:
Right. And then you said, well, that time he was wrong, right? So the hammer theory is everything looks like a nail when all you have is a hammer. Where do you think we are with dark matter? What is your take? I’ve had Mordecai Milgrom 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? What do you— what— I mean, you worked on this for a long time. Where do you come— what is dark matter?

Scott Dodelson:
Yeah, I have no idea. I actually had this debate with Stacy that this is an organization that promotes civil debates. So we had this debate and I don’t So just to give context, Stacy McGaugh is one of the leading astronomers who doubts the existence of dark matter. And he and Mordechai Milgrom and other people, most notably from a theoretical perspective, an Israel— a Mexican-Israeli physicist, Jacob Bekenstein, 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.

Scott Dodelson:
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, 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 2 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 hope of bridging that gap.

Scott Dodelson:
However, Stacy, who’s a very reasonable person, does not believe MOND is correct. And if you look at the generalizations of MOND, such as the ones that Bekenstein 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 we— 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 How many free passes do we get? I don’t know.

Brian Keating:
No, it’s extraordinary that you’re, again, candid and you’re honest and you’re willing to admit where these lacunae, these gaps, these flaws. But 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. The Big Bang was kind of a solution to a lot of the problems in the steady state model. The inflationary universe is sort of this patchwork of fixes and 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, 61 years ago this month or last month, the CMB was discovered and the announcement was discovered. And many people, including Jeff Burbidge, 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 guest, Jayant Narlikar, they went to their graves believing in the steady state, quasi-steady state. They added some stuff to it.

Brian Keating:
They were eminent scientists. I mean, these are people that were Nobel-worthy adjacent to Willy Fowler, did win the Nobel Prize for the, you know, BBFH, and incredibly eminent scientists. Hoyle, of course, coined the term Big Bang as a pejorative. And my question to you is, when the CMB 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 Dodelson. During BICEP2, 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’ve found any value.

Brian Keating:
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 Tegmark said. And Lawrence Krauss said, this now means there’s proof that God does not exist. It’s an incredible For me personally, obviously. I talk about a lot of these in Losing the Nobel Prize, my first book. But what do you make of that? If a theory accommodates any result, is it a theory? I mean, what is it? What are we to make of it?

Scott Dodelson:
Yeah, I don’t want to go too far towards ignorance, and the fact that smart people don’t believe in the microwave background doesn’t mean they’re correct.

Brian Keating:
Yes.

Scott Dodelson:
Right?

Brian Keating:
Sure. I think here, there’s a sidebar. I think the human need for meaning is the most paramount need that humans have. Viktor Frankl, Yeah. Man’s search for meaning, right? I want to just kind of take this counterfactual, hypothetical, 10 years from now, 2036, DESI’s done, Simon’s Observatory’s done, CMBS4 never happened, but unfortunately, rest in peace, it should have happened. Lightbird, there’s many different projects hopefully that will augment and complement and compete with us. And so I think the most healthy thing is to have a competitive scientific environment. And so I welcome our kind of competition from China.

Brian Keating:
We have a Chinese team that’s trying to do this. And there’s many other great and brilliant scientists, but we’re there. What does cosmology being done look like? What does a solved model of cosmology look like? And is it on the horizon in our grand grad student careers?

Scott Dodelson:
Okay, I have no idea, but—

Brian Keating:
What would satisfy you?

Scott Dodelson:
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 has 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? You know, 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 1,000-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-modes, so you’re gonna be extending the reach by a factor of 5 to 10, is that right?

Brian Keating:
Yeah.

Scott Dodelson:
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 goo goo 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, 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’d 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 Hadron Collider.

Scott Dodelson:
So that would, that would open things up. In dark matter, if we detected something in the, in the 100 GeV range that pointed to supersymmetry, that would open up the, 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, I really don’t know.

Brian Keating:
I tell my students a flaw leads to a law. And we just don’t know which law yet. Scott spent 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 to 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.

Brian Keating:
I’m Brian Keating. This is Into the Impossible. Tune in next time.

Scott Dodelson:
Tune in next time.