310 | Marc Kamionkowski on Dark Energy and Cosmic Anomalies

Cosmologists were, let us be honest, pretty stunned in 1998 when observations revealed that the universe is accelerating https://en.wikipedia.org/wiki/Accelerating_expansion_of_the_universe. There was an obvious plausible explanation, the cosmological constant https://en.wikipedia.org/wiki/Cosmological_constant proposed by Einstein, which is equivalent to a constant vacuum energy pervading space. But the cosmological constant was known to be enormously smaller than its "natural" value, and it seems fine-tuned for it to be so small but not yet zero. Once burned, twice shy, and since then we have been looking for evidence that the dark energy might not be strictly constant, even though that's even more fine-tuned. We talk to cosmologist Marc Kamionkowski about recent evidence that dark energy might be changing with time, and what this might have to do with the Hubble tension https://en.wikipedia.org/wiki/Hubble_Tension and other cosmic anomalies. Blog post with transcript: https://www.preposterousuniverse.com/podcast/2025/03/31/310-marc-kamionkowski-on-dark-energy-and-cosmic-anomalies/ Support Mindscape on Patreon https://www.patreon.com/seanmcarroll. Marc Kamionkowski received his Ph. D. in physics from the University of Chicago. He is currently the William R. Kenan Jr. Professor in the Department of Physics and Astronomy at Johns Hopkins University. Among his prizes are the Gruber Cosmology Prize, the Dannie Heineman Prize for Astrophysics, membership in the National Academy of Science, and a Guggenheim Fellowship. Johns Hopkins web page https://sites.krieger.jhu.edu/kamionkowski/ Google Scholar publications https://scholar.google.com/citations?user=xdxuN98AAAAJ&hl=en Wikipedia https://en.wikipedia.org/wiki/Marc_Kamionkowski Kamionkowski and Riess, "The Hubble Tension and Early Dark Energy" https://arxiv.org/abs/2211.04492 Dark Energy Spectroscopic Instrument https://en.wikipedia.org/wiki/Dark_Energy_Spectroscopic_Instrument paper https://arxiv.org/abs/2503.14738 and followup https://arxiv.org/abs/2503.14743 Dark Energy Survey https://en.wikipedia.org/wiki/Dark_Energy_Survey paper https://arxiv.org/abs/2503.06712

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 1:00Sean Carroll?

  Hello, everyone, and welcome to the Mindscape Podcast. I'm your host, Sean Carroll. I'm a working theoretical cosmologist, among other job descriptions. So recently, there's been some news in cosmology that may or may not turn out to be a big deal. This is often how it is in science, right? You get a result, but of course, by the very nature of having gotten a new result, it's a hard result to get. Otherwise, you would have gotten it earlier. So the first indications that something interesting might be happening are typically faint, and you're not sure whether they're on the right track or not. But there's a couple of different things that have indicated that perhaps there are

• 1:40Sean Carroll?

  kinks in the armor of the standard cosmological model, the so-called Lambda-CDM model. Lambda for cosmological constant, CDM for cold, dark matter. Not something that throws away the whole Big Bang scenario or anything like that, but specific details might need to be tweaked. This is something that I could have done a solo episode about, but the data and exactly what the data are telling us really, really matter here. So I thought it would be better to have a true expert on the Podcast. So we're happy to welcome Mark Emiankowski, who is my colleague at Johns Hopkins and someone I've known for a long time. We've written papers together, including suggesting the idea of dark electromagnetism

• 2:23Sean Carroll?

  in addition to dark matter out there in the universe. We don't talk about that in this podcast. Instead, we're talking about these accumulating possible anomalies in cosmology. Most recently, there's a survey called the Dark Energy Spectroscopic Instrument, DESI, that has suggested that perhaps the density of dark energy is changing with time, which is not what you would expect if it was just a cosmological constant. If it were a dynamical field, you might expect something like that. And there was a hint a year ago that that was true. Very recently, the hint has become stronger. And there is another instrument called the Dark Energy Survey,

• 3:07Sean Carroll?

  DES, as opposed to DESI for the Dark Energy Spectroscopic Instrument, that has less firm results, but also pointing in the same direction, that dark energy might be evolving with time. These are both amazing surveys. Interestingly, they both look at galaxies, right? Out there in the universe, and they look at the distribution of galaxies and how they're evolving with time and things like that. They're both ground-based cameras that replaced previous cameras. The Dark Energy Spectroscopic Instrument, DACE, replaced a camera at Kitt Peak in Arizona, and the Dark Energy Survey replaced a camera in Chile, the Victor Blanco Telescope.

• 3:50Sean Carroll?

  Anyway, these hints that dark energy might be changing with time are still tentative. It's not completely clear yet. And indeed, at face value, it would be remarkable if they were really true because of the specific way in which the dark energy is evolving with time. So we're going to get into that. But of course, I have to take advantage of this to also talk about other cosmological anomalies. The Hubble Tension, which we talked about with Adam Rees some time ago. Mark turns out to be one of the world's experts. In thinking about models to explain the Hubble tension. Mark was in on the ground floor in thinking about the cosmic microwave background as a cosmological probe, and also was the author of some interesting ideas about what dark energy could be back

• 4:34Sean Carroll?

  in the day. So he's really the best person to talk to about what the microwave background tells us, what these galaxy surveys tell us, and what the theoretical implications are of all this stuff. I would say that, hmm, right now, I'm still on the fence about whether there really, truly is something dramatic going on, but it's absolutely a legitimate possibility. Sadly, we're still going to have to wait for even better data to come in. That's how science goes sometimes. But if you listen to this episode, you'll be well-prepared to understand what's happening when that data does come in. So, let's go.

• 5:25Sean Carroll?

  Mark Kamionkowski, welcome to the Mindscape Podcast.

• 5:28Speaker 03

  Hello. Pleasure to be here. Nice to be talking to you on this beautiful Wednesday morning.

• 5:33Sean Carroll?

  I know, you're back in sunny Baltimore. I'm here in Santa Fe, but.

• 5:36Speaker 03

  Yeah, it's a reasonably nice day today. A little cooler than yesterday, but probably more oxygen, right? Santa Fe is at.

• 5:44Sean Carroll?

  Very high altitude.

• 5:45Speaker 03

  Yes, gets me.

• 5:47Sean Carroll?

  So, we are here because there's been a couple of, more than a couple, of anomalies, challenges, puzzles, whatever you want to call them, with respect to the standard cosmological model, which is nowadays known as Lambda CDM. So, we're going to talk about that, but let's first explain what is the standard cosmological model and, you know, why do we believe it? You know, you give us a medium-sized intro to where we are before we have any anomalies.

• 6:15Speaker 03

  Okay, medium sized intro to where we are before we have any anomalies. So, we live in a universe that we have been observing for centuries. But I would say over the past hundred years in particular, our understanding of the universe, which is everything that we know, is a given, it's one physical system, has evolved tremendously. And it sort of started, yeah, just under a hundred years ago, really, with Hubble's discovery that the universe was expanding. So, you know, everybody knows that the Earth spins around the Sun. And the Sun is the center of the solar system. Most people know that the Sun is

• 7:01Speaker 03

  one of about 10 million stars in our galaxy, the Milky Way. And the Sun spins around the center of the Milky Way for the same reason that the Earth spins around the Sun.

• 7:11Sean Carroll?

  So I think you said 10 million?

• 7:13Speaker 03

  10 billion, sorry.

• 7:17Speaker 03

  Good to know you're paying attention.

• 7:18Sean Carroll?

  I'm gonna give up the galaxy, yeah, 10 billion, sorry, 10 billion stars.

• 7:24Speaker 03

  And so the Sun spins around the center of the Milky Way for the same reason that the Earth spins around the Sun. And that's because all of the stars in the Milky Way generate a very strong gravitational field. And you might then wonder whether our galaxy is part of some larger structure, you know, whether our galaxy is one of 10 billion galaxies that spin around each other. But it turns out that the hierarchy ends there. And our galaxy, it turns out, is one of, you know, tens of billions of galaxies that are more or less the same that we know about. But the galaxies don't spin around from each other. It turns out that every galaxy is moving away from every other galaxy. And this is what Hubble discovered almost 100 years ago.

• 8:06Speaker 03

  And the relative, the speed at which any two galaxies are moving away from each other is proportional to their distance. And so the interpretation of this is that the entire universe is expanding. This was discovered by Hubble and it was turned, it turned out that it was kind of convenient because Einstein had discovered general relativity 12 years before that. And, um, you know, several people who were studying general relativity realized that equations of general relativity allowed for such a universe, a universe that was filled with a bunch of stuff where everything was expanding, everything was moving away from everything else. So that was sort of the birth of the standard cosmological model. And since then, we've discovered a bunch of other things.

• 8:51Speaker 03

  Perhaps the next big breakthrough was sort of in the mid sixties. There was a discovery of something that we now call the cosmic microwave background. Basically, the idea is that if everything is moving away from everything else today, if we were to make a movie of that expansion, then run it backwards. At some earlier time, everything in the universe would be on top of everything else. So although the universe, you know, is a fairly low density place now, if everything's moving away from everything else, at some time in the past, which we call the big bang, the density of the universe would have been very high. Anybody who puts lots of air in tires and drives them around knows that when densities get high, the pressures get high, the temperatures get high.

• 9:36Speaker 03

  So the early universe, we have good reason to believe was very hot. And, you know, if you look at a fireplace where there was a fire that is now out, the embers still glow for some amount of time afterwards, even though there's no fire, you can still see residual heat. And in 1965, we discovered this residual heat, the cosmic microwave background. So it turns out that we discovered another relic from this big bang, the consistent with this picture of an expanding universe that Hubble sort of gave us 100 years ago. Is this good so far?

• 10:10Sean Carroll?

  This is great. Yeah, I love it.

• 10:12Speaker 03

  Okay, just checking. So, um, and then, so that was 1965. So that was 60 years ago. And since then, we've learned even more about our universe. So we've been able to study the distribution of galaxies in the universe. And we find that the universe on the very largest scales is very, very smooth. So it's like a pond on a clear day, on a calm day. But if you look very carefully, there are some fluctuations. There are some small amplitude ripples, as if there was some light wind.

• 10:49Speaker 03

  We've also been able to look at this cosmic microwave background very, very precisely, very carefully, and we've been able to see that the temperature of this, you know, glow, this afterglow of the Big Bang is not precisely the same everywhere. It's pretty close. It's close, you know, the, the temperature is the same to one part in a hundred thousand. But if you actually look really, really carefully, that are all small, there are small fluctuations.

• 11:17Speaker 03

  And we believe, have very good reason to believe that these small fluctuations that we see in the cosmic microwave background were then the seeds for the larger amplitude fluctuations we see in the galaxy distributed verse today. We believe that those small fluctuations were amplified by gravitational, you know, gravitational forces. So we have all of these very, very detailed measurements of the cosmic microwave background, of the distribution of galaxies. And we have a model that allows us to relate the distribution of galaxies in the universe today to the distribution of the cosmic microwave background that we see of afterglow from the Big Bang. And in order for our model to account for the features that we see both in the cosmic microwave background

• 12:00Speaker 03

  and in galaxies, um, we need to have, um, we need in these models, in addition to the, you know, ordinary stuff that you and I and everything in the solar system are made of, which we call baryonic matter, which jargon for ordinary atomic stuff. In addition to the baryons, we also know that there has to be a lot of dark matter, about five times as much mass in dark matter as in baryons. We don't know what dark matter is, but the models require that, you know, or the dark matter is required in order for the models to work. And then there's also something called the cosmological constant that was inferred in

• 12:40Speaker 03

  the late 1990s, but we now also understand from the models that we have for these fluctuations that it has to be there. And then the cosmological constant is something we don't really know what it is, but some sense, it's some energy density that pervades all of space. So we have this great model, explains the origin of the universe, why, you know, the expansion of the universe. We have some ideas about why it's expanding, although those are not fully formed yet, I would say.

• 13:10Sean Carroll?

  You mean what started it in some sense?

• 13:11Speaker 03

  Yeah, what set it in motion.

• 13:13Sean Carroll?

  The good news for you is that I have a future upcoming podcast about what happened near the big bang, so you don't have to worry about that.

• 13:19Speaker 03

  Oh, really? Near the big bang? What about before the big bang?

• 13:22Sean Carroll?

  Oh, yeah, that's gonna, that's gonna be there.

• 13:24Speaker 03

  Yeah, that should be fun. Okay, so we have this great model that explains all this, this wealth of observations we have of the galaxy distribution. This is, you know, millions and millions of galaxies that we've been able to map. And the temperatures of the cosmic microwave background, we've been able to measure it, you know, about a million different points in the sky. So there's a lot of data. It's not just a hand wavy, squiggly approximate model. It's not like, you know, about 3,000 miles from New York to Los Angeles. It's, you know, 3,118,632. And that's, you know, it's a really good model.

• 13:59Sean Carroll?

  Right.

• 14:01Speaker 03

  And we're really proud of ourselves.

• 14:02Sean Carroll?

  I think you should be. Let me pause though for a second because something sneaked in there that I think is really interesting. A lot of people, I'm sure that you get emails from people who have explained away dark matter without being professional scientists, et cetera. And of course, they always concentrate on the rotation curves of spiral galaxies. So this idea that the amount, the rate at which stars and gas are rotating around the centers of spirals depends on how much mass there is, et cetera, et cetera. Ordinarily, Vera Rubin and her collaborators proved this. We attribute that to dark matter, but it could be something else. But you didn't even mention spiral galaxies. You went right to the microwave background.

• 14:42Speaker 03

  Yeah, that's a good point. So I think I did that because I was trying to give you a capsule summary of

• 14:51Sean Carroll?

  No, I like it.

• 14:52Speaker 03

  Yeah, the model for the universe, but yes. So the measurements of the cosmic microwave background and large scale distribution of galaxies that I told you about that implied the existence, required the existence of dark matter. Those happened about 25, started happening about 25 years ago, but you are correct that even 20 years before that, you know, around 1970, Vera Rubin and her collaborators and a few other people started to realize that the most of the matter in the galaxy has to be dark.

• 15:25Speaker 03

  And so we actually had reason to believe, you know, we had good reasons to believe that there would be dark matter in the universe even before these large scale structure cosmic microwave backgrounds that I told you about. So in some sense, it wasn't a surprise when that happened, but it was a confirmation and it was, you know, gave us much more confidence that what we were, that the anomalies that we were seeing with galactic rotation curves were actually real and due to some new form of matter.

• 15:53Speaker 04

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• 16:24Sean Carroll?

  But the reason why I like to emphasize it is because it does kind of highlight a difference in how the professionals think about this than how we perhaps talk about it to the broader public. We, you know, we tend to be historically quasi-accurate. We want to give the early people credit. So we talk about spiral galaxies, but the real reason we are confident that there's something like dark matter is much more something like some combination of the microwave background radiation, large scale structure, things like that. And so accounting for spiral galaxies doesn't actually get you out of the need for dark matter.

• 16:59Speaker 03

  Accounting for spiral galaxy does not get you out of the need. Dark. Yes, yes, that's right. That if that's right. I'm trying to parse what she said. If there were indeed, if somebody had some other explanation for the the galactic rotation curve that did not involve dark matter, we would still have reason to believe that dark matter exists as of observations of cosmic microwave background and galaxy disk.

• 17:23Sean Carroll?

  Right. Yeah. Sorry to hector you on that, but it is the internet that we're talking to here. And there are people out there who have ideas and we love them and we support their efforts. But we want to be clear about why we believe these things.

• 17:34Speaker 03

  Yeah, it's actually, I mean, it's a it's a good point that you make. And I think it's something that we are becoming, we've always known, but appreciate more in cosmology with time. And that is that, you know, when we do cosmology, it's sort of like archaeology, or, you know, physical anthropology, or paleontology. Yes, paleontology. I was missing the word there. You know, with paleontology, what you do is you find bones somewhere, and they have these funny looking shapes. But you look at them and it's sort of like a puzzle and you sort of try to put the pieces

• 18:15Speaker 03

  of the puzzle together, consistent with what you know about, you know, bones of animals that exist. So it's a puzzle. But it's also informed by, you know, your solution to that puzzle is informed by other solutions to similar puzzles you have. And we do the same thing in cosmology. It's very similar. It's not an experimental science. We don't like in paleontology, you don't build a dinosaur, although some people are trying.

• 18:41Sean Carroll?

  Yet.

• 18:44Speaker 03

  You don't build the dinosaur, you know, we can't alter the system. We just have observations. There are things that we find with telescopes. And so we try to construct a model that's consistent with the observations and consistent with what we know about the laws of physics. And so, you know, if we have a model for galactic rotation curves that involves something other than dark matter, that's a perfectly legitimate thing to try. But then you have to ask, was that is that solution going to be consistent with other things that I?

• 19:15Sean Carroll?

  Right.

• 19:15Speaker 03

  Um, and now with cosmology, you know, we try to make as many different observations we can. Try to study as many different systems as we can in detail. And, you know, in some cases, there are things we can try in the laboratory. But basically, in order to actually have confidence in, you know, conclusions that we make. In order to, you know, increase our confidence, we want to have different measurement and different observation from different systems and different types of observational techniques. That then all.

• 19:46Sean Carroll?

  Yeah.

• 19:46Speaker 03

  Match and give you.

• 19:47Sean Carroll?

  Speaking of which, um, this dark energy business, this cosmological constant business, uh, where did we figure out that?

• 19:54Speaker 03

  Uh, so the cosmological constants, um. I mean, the story is that Einstein had this, you know, realized that there must, there might be a cosmological constant, the Einstein equations, and then called it is just plunder. Whether that's true or not, I don't know. I actually saw the notebook, the pages you know, uh. Diana Bookwald, the Einstein papers.

• 20:18Sean Carroll?

  Yeah.

• 20:19Speaker 03

  So Diana once showed me the actual like notebook pages, pages in Einstein's notebook where he was doing the calculation to let him to think of the. The cosmological constant. And it was kind of interesting what she told me, it's, um. She said it's the only, um, case that they have in all of his papers, the only example in all of his papers where he was actually doing a numerical calculation.

• 20:42Sean Carroll?

  Flugging in numbers.

• 20:43Speaker 03

  Yeah, he actually like had a graph and those graph and those graph paper, he was like trying to calculate the area under the curve.

• 20:49Sean Carroll?

  Oh, wow.

• 20:50Speaker 03

  He did it by counting the, the boxes. Um, so anyway, you know, the cosmological constant sort of existed as a possible, um, theoretical. Addition to the basic theory of general relativity for over a hundred years. Um, but you know, the observational evidence that that thing actually exists came about in the late nineteen nineties. And you can look in the literature even before the late nineteen nineties, there were people who were sort of speculating that various cosmological observations were better fit with a non-zero cosmological constant. But you know, the real smoking gun

• 21:30Speaker 03

  was measurements made by two independent groups, the Supernova Cosmology Project and

• 21:38Sean Carroll?

  High Z Supernova Team.

• 21:41Speaker 03

  Yes, the High Z Supernova Team.

• 21:42Sean Carroll?

  They sound like the same thing, yeah.

• 21:44Speaker 03

  Um, and so I told you, you know, we talked earlier about how every galaxy in the universe is flying apart from every other galaxy. And if you think about, you know, um, a ball that I throw in the air, if I throw a ball in the air, um, it goes up, but then experiences the gravitational attraction to the earth. And so even though I throw it up initially with some large velocity, the velocity slows, eventually goes to zero, becomes negative, and then it falls back down. Um, now, if I had a really, really good arm, um, and I could throw the baseball at a velocity bigger than eleven kilometers per second, I don't know what that is in miles, if I could throw a ball with a velocity greater than eleven

• 22:28Speaker 03

  kilometers per second, it would actually escape the gravitational field of the earth. Would actually, you know, instead of, you know, going up and then flying back, I would actually, that ball would actually fly away from the earth and continue flying away from the earth forever. But since gravity is a long-range interaction, a long-range attractive interaction, even though that baseball was flying away from the earth, um, and would continue to fly away from earth, you know, forever, the speed at which it does so would continually decrease.

• 23:01Speaker 03

  So, you know, ordinary gravity, ordinary Newton's gravity suggests that, you know, if two galaxies are flying apart from each other, the relative speed at which they fly, that they fly apart from each other should be decreasing with time. And that is, in fact, what was in the standard cosmological model, based on Einstein's general relativity with no cosmological constant until the late 1990s. And then what happened is that the Hyzey Supernova Team and the Supernova Cosmology Project independently actually measured how fast galaxies were moving away from each other. And what they found is that the speed at which galaxies are moving away from each other is actually increasing with time rather than decreasing.

• 23:44Speaker 03

  And this was Science Magazine's Breakthrough of the Year in 1998. It was completely and utterly shocking to everybody in physics. We knew that general relativity, you know, could allow for the possibility of a non-zero cosmological constant, but everybody just assumed that it would be zero. Because the actual value is something like, oh, 0.000 with 120 zeros, one. The actual value is extremely, extremely small. And physicists don't like extremely small. We like one, we like pi, we like 2.3.

• 24:27Speaker 03

  We don't like extremely small or extremely large. So everybody was very, very shocked. I remember being very, very skeptical. People tried to explain it away. They tried to suggest that maybe the supernovae themselves were evolving with time. They speculated that maybe light was being absorbed by the more distant supernovae, thus making them look fainter. And, you know, the people in both projects did a really good job, you know, checking all of these things and dispelling, you know, all of these possibilities, ruling out all these possibilities. And then I became really, really convinced when the cosmic microwave background experiments came out in the early 2000s.

• 25:11Speaker 03

  And, you know, from a completely different type of measurement, different type of observation, they also inferred that there had to be a non-zero value of the cosmological constant. So did I answer your question?

• 25:22Sean Carroll?

  You did, you did. And thus Lambda CDM, the Lambda for the cosmological constant, CDM for cold dark matter. That is the standard target fiducial cosmological model.

• 25:34Speaker 03

  That is our standard cosmological model. I don't like the name.

• 25:39Sean Carroll?

  It's not, not the sexiest name, but you know, maybe we'll overturn it. So that's okay. We can come up with a better name. I guess the, the one other piece of cosmological measurement that I wanted to get on the table was the idea of a baryon acoustic oscillation. I think that's probably the trickiest thing for the person on the street to wrap their brains around, but apparently very, very important to modern cosmology.

• 26:02Speaker 03

  Yeah, this is the hardest thing to explain, but I'll try.

• 26:07Speaker 03

  So we know from our observations of the cosmic microwave background that the early universe was very, very smooth.

• 26:16Speaker 03

  So I said it was sort of like a, the surface of a pond on a, on a very calm day. But suppose I threw a pebble into that pond. There will be a splash, but then there will be a wave that propagates out from the place where, you know, a circular wave that propagates out from where the pebble landed in the pond. And so, you know, that wave expands with time and it's moving at some velocity. And at early times, you know, if I were to take a snapshot, just a few seconds afterwards, the circle would be small. That wave would be small. And at later times, that circle, that circular wave would be larger.

• 26:57Speaker 03

  Now, I told you that although the early universe was very, very smooth, it was not perfectly smooth. And so there were sound waves propagating the early universe. The early universe consisted of this, you know, fluid, you know, all the baryons that make up the galaxy and you and me, the sun, all the other stars. All those baryons would have made up a fluid in the early universe. If I have a disturbance in the early universe, if I were to throw a pebble into the early universe, there will be a wave that propagates out at the speed of sound. Now, although we don't see an individual such wave, what we do see, you know, if I have a, a, a pebble in a pond, then I throw it in the pond.

• 27:39Speaker 03

  There will be that circle, which is the, you know, the wave propagating out, but there will also still be some, you know, bubbling right at the center. And so there will actually be a correlation in the surface height of the water at the center and at the wave. Okay, so the surface, you know, the surface height far away from the wave is zero. The surface height inside the wave is pretty small, but there will be, you know, an increase in the surface height at just this right distance. And so when we look at the galaxy distribution, we don't see any individual wave, but we can measure the probability to find one galaxy at some distance from some other galaxy.

• 28:24Speaker 03

  And if you look at that probability to find one galaxy at some distance from some other galaxy, that probability decreases as you go to larger and larger radius, the excess probability. But then it turns out that there's a bump somewhere around 100 megaparsecs. And that bump is actually the essentially a consequence of these sound waves in the early universe.

• 28:47Sean Carroll?

  Okay, so it's roughly speaking, metaphorically, God threw pebbles at the smooth pond of the early universe and ripples went out and there's going to be sort of a natural correlation length between the different galaxies that we see today because of just the time scales of which everything happened.

• 29:03Speaker 03

  That is correct.

• 29:05Sean Carroll?

  Good. And that's the baryon acoustic oscillation, BAO.

• 29:09Speaker 03

  Yep, it's pretty remarkable when you see it in the data.

• 29:12Sean Carroll?

  Yeah.

• 29:12Speaker 03

  I mean, I mean, this is another example of the whole, you know, everything hanging together. I mean, you know, we had, you know, the cosmic, we have the expansion of the universe, we have the cosmic microwave background. We have these cosmic microwave background fluctuations. And I mean, in the, I remember when I was a postdoc and assistant professor in the Mid nineteen nineties, people sort of understood that you should also see a bump in the galaxy distribute. But I remember thinking that there's no way we'd ever be able to like see. It's an interesting theoretical idea, but you know, you'd need to, you know, measure

• 29:55Speaker 03

  the positions of God knows how many millions of galaxies to actually ever see this. And even so, you know, all kinds of complicated things happened between the big bang and now. But it turns out that the, the model actually works and you actually, you know, and we actually do have surveys of millions and millions of galaxies with very well measured positions. And we see this bump in the galaxy distribute and it's not at all subtle now with current measurements.

• 30:23Sean Carroll?

  Yes.

• 30:24Speaker 03

  You know, really in your face. And so, I mean, we're actually, you know, you look at the distribution of galaxies on distant scales of, you know, hundreds of millions of light years. And you see the imprint of this physics that we have in our models to describe the early universe.

• 30:45Sean Carroll?

  Well, and it's important to emphasize that with both the temperature fluctuations in the microwave background and with the baryon acoustic oscillations, you are measuring in principle, all these parameters, like the density of dark matter, the density of the cosmological constant, et cetera. But you're not measuring them directly. You're saying that I have a model for everything at once and I'm going to make all sorts of predictions and those predictions will depend on the parameters. I'm going to measure some things and then ask which values of the parameters give me the best fit.

• 31:15Speaker 03

  That is correct.

• 31:16Sean Carroll?

  So on the one hand, very, very impressive that the model, the standard model works so well. On the other hand, there's a lot of moving parts, right? If something doesn't start fitting, then it's not going to be perfectly obvious where to look.

• 31:29Speaker 03

  Uh, yes, that is correct. Um, so another thing I think that's been surprising about our understanding, our evolution of the understanding of the universe. Um, is that the universe has turned out for reasons we probably don't fully understand to be much, to be a much simpler system than anyone might have surmised. So you're right. We measure the distribution of bazillions of galaxies. We measure the distribution of millions, you know, the, the temperature of the cosmic microwave background over millions of points in the sky. It's a really complicated system because galaxies have gas, they have stars, they have, you know, interaction between outflows from stars that are supernovae that

• 32:13Speaker 03

  blow up and then, you know, pollute the intergalactic medium with heavier elements. There's gravity. There's this cosmological constant. There's dark matter. It turns out to be a very, very complicated, seemingly very, very complicated system. But it turns out that it's much, when we look at it carefully, the system as a whole turns out to be much simpler than anyone might have, you know, our understanding of the origin and evolution of the universe is actually, I think, you know, much more precisely parametrized and specified by the model than is our understanding of the solar system. Yeah.

• 32:53Speaker 03

  Even though we live in the solar system, have, you know, visited parts of it. Right. And it's, you know, much simpler physics in principle, you know, gravity and Kepler's law. So it turns out that you're right. We have all this data. We have all these parameters to turn. It seems like it would be a really complicated system. It would be hard to have any confidence, you know, have much confidence in any individual parameter, given that you're trying to simultaneously fit for these other parameter. But it turns out that a model with five parameters can account for it all. And, um, I mean, that's been one of the things that's been though, you know.

• 33:35Speaker 03

  So surprising and impressive, you know, that one model can explain, we'll see the galaxy distribution and the cosmic microwave background. Well, that's why we were so proud of ourselves.

• 33:46Sean Carroll?

  We should be proud, but we should also be interested to see if there's anything weird going on. I mean, in terms of weird things that could go on, there's like theorists, favorite ideas, then there's what the experimenters actually come back and tell us about. What is it like, very quickly, I think, what are some of the main alternatives in terms of perhaps the physics of dark matter and dark energy that we're trying to test when we do the cosmic experiments?

• 34:16Speaker 03

  Oh, okay. That's a good question. So, so one thing that we do to test the cosmological constant. So as we said, the cosmological constant is a very, very strange thing from the point of view of fundamental, our understanding of fundamental physics. And so one thing that you can wonder is whether the cosmological constant is really constant. So it sort of says that there's some mysterious energy pervading all of space, but it's, you can ask, is that changing with time as the universe expands? Or is it really, really constant? And so if your cosmological constant is not really constant, and you know, people

• 34:58Speaker 03

  have been using the word dark energy in place of cosmological constant, because the cosmological constant isn't constant, then not a good name for it. So you can ask whether the dark energy density is constant in time or evolving in time. And there's been a major effort over the past 25 years to try to address this question, try to figure out whether the energy density is changing with time or not. And that is sort of done with the same types of measurements that we use to determine the expansion rate and, you know, to determine the dark matter density, et cetera. We have models for how the galaxy distribution, the cosmic microwave background should look. Those models have incorporated

• 35:43Speaker 03

  into them as one ingredient, dark energy, you know, in the simplest models, the dark energy, the only parameter that we use to describe the dark energy, its density, which we assume to be constant. But you can also see what happens if you have a model where the dark energy density evolves with time. And so we have parameters now to that we could measure or fit from the model, fit from the data with the model to figure out or see if the dark energy density is evolving with.

• 36:11Sean Carroll?

  Right.

• 36:12Speaker 04

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• 36:42Sean Carroll?

  With dark matter, it seems to harder to have sort of physically plausible modifications, but people still do play around with it.

• 36:49Speaker 03

  Uh, yeah, that's actually a, in some ways, a bigger industry. You know, we have no idea what dark matter is. The models work very well if we make the simplest assumption that dark matter interacts with itself and with everything else only gravitationally. So in other words, dark matter particles don't scatter from themselves. They don't scatter from the ordinary stuff.

• 37:13Speaker 03

  But that's, you know, an assumption. And again, you can construct more complicated models where dark matter has some type of interaction with itself or some type of interaction with ordinary matter. And then you can describe those interactions in terms of parameters that you can then try to fit from the data, um, you know, from the galaxy distribution and cosmic graph background. But with dark matter, it's a little, uh, we've got a few more possibilities. You know, the dark matter is not only out there in the universe. It's presumably also, you know, in the Milky Way and in the solar system. And, you know, presumably passing through us here on earth every single day. So, you know, one of the prevailing ideas for dark matter

• 37:57Speaker 03

  is that it's a elementary particle that has a mass of roughly 100 times the proton. And it turns out that the dark matter density locally is roughly half a proton mass per cc. And so what that means is that, you know, every time you buy a liter of milk.

• 38:15Speaker 03

  At the store, in addition to your, you know, recommended daily allowance of calcium and vitamin D, you are also getting, you know, one dark matter particle.

• 38:27Sean Carroll?

  I mean, if it's axions, you're getting a lot of dark matter particles.

• 38:30Speaker 03

  Yeah, yeah, it could be. Yeah, I mean, the question is, you know, are you buying it by weight or by?

• 38:39Speaker 03

  So, you know, as I said, the the canonical idea for a dark matter is that it interacts with nothing else except gravitationally. So you don't have to worry about it if it winds up in your milk. But, you know, if it does have some very weak interaction with ordinary matter, then we can construct laboratory detectors to try to, you know, see the effects of interactions of these very rare dark matter particles with ordinary matter. So, you know, that's a fairly big industry.

• 39:07Sean Carroll?

  And so far, we have seen zero.

• 39:12Sean Carroll?

  But just emphasize, there's plenty of room for very, very sensible viable dark matter candidates that we would not have seen yet.

• 39:20Speaker 03

  Yes, that is correct. The, well, I mean, it's actually, it's, it's interesting that, I think the, the people first started to think about particle, elementary particle dark matter, seriously about forty, forty-five years ago.

• 39:41Speaker 03

  So in the late nineteen eighties, people started to get serious about actually looking for these dark matter particles in the lab. And so we've been looking for dark matter particles in the lab for forty years. And during that time, you know, forty years ago, we had predictions or, you know, pervade, you know, very, very elegant, attractive predictions for, for what the dark matter should be. And many of those models have been ruled up because we haven't seen them.

• 40:12Sean Carroll?

  Yeah.

• 40:12Speaker 03

  So in some sense, you know, we don't know what dark matter is, in some sense, a shot in the dark. But we have actually had, you know, over the past few decades, a number of really intriguing and interesting and promising theoretical models for dark matter. And, um, you know, it's interesting that we've been able to rule those out. You know, it's dark matter, it's, this is an experimental science. We're not the, you know, just casting a, you know, flailing about completely in the dark.

• 40:40Sean Carroll?

  That's nice to hear, but it brings us smack into the fact that we do have puzzles that we need to deal with. I guess chronologically, the first puzzle that I personally took seriously that is still lingering is the Hubble tension. In fact, we had our mutual colleague Adam Rees on the podcast talking about it a couple of years ago. So update from a couple of years ago, is it still there? Are we still worried about the Hubble tension? What is it?

• 41:07Speaker 03

  Hubble tension is a big problem.

• 41:10Speaker 03

  So discussed, we have these models, we fit for a bunch of parameters to try and explain the measurements in the cosmic microwave background and galaxy surveys. And one of the parameters is the Hubble constant, which is the rate at which galaxies are moving apart from each other. So essentially measures the speeds at which galaxies are moving apart from each other. And so, you know, the galaxy distribution, the cosmic microwave background, we don't actually see the universe expanding, but this expansion rate is a parameter in the models that we describe, the distributions of the galaxy and the cosmic microwave background. But alternatively, you can try to measure the Hubble constant directly, just like Hubble did 100 years ago.

• 41:55Speaker 03

  So you can look at, you know, some galaxies that are not too far away. And you see those galaxies moving away from each other, moving away from us, from our galaxy. And if you can also figure out the distance, then that gives you the Hubble constant. So the Hubble constant is the ratio of the velocity which galaxies are moving away from us to their distance. In principle, it's straightforward. So measuring the velocity at which galaxies are moving away from each other, from us, is actually fairly easy. And the reason is the galaxies emit light and some of that light is either absorbed or emitted by various atomic transitions.

• 42:36Speaker 03

  And so there are spectral lines, there are lines in the spectrum, the frequency spectrum of the light that we see. And if the galaxy that's emitting this light is moving away from us, then those lines are Doppler shifted to different frequencies or a longer wavelength. So the same effect is when an ambulance is moving away from you. It sounds lower pitched than it does when it's towards you. So we measure these Doppler shifts, we can figure out the velocities very, very well. The distances are surprisingly difficult.

• 43:09Speaker 03

  And the reason is that when we look at a galaxy on the sky, it has some angular size. And if we knew what the physical size was, then we could infer the distance. Or if we see a galaxy, it has some brightness that we can measure very, very precisely. And if we knew exactly how luminous the galaxy was, then we could figure out exactly how far away it was. You know, if you, if you give me, if I give you a standard flashlight and you shine it at me, I can figure out how far away you are because I know how bright the flashlight is. And I can, you know, how luminous the flashlight is and I can measure how bright it is.

• 43:47Speaker 03

  But galaxies don't all have the same luminosity. And so we can't infer the distance just by looking at the luminosity. It turns out, though, that there are things called supernovae. And in fact, a very specific type of supernova, type 1A. A type 1A supernova is a white dwarf, an exploding white dwarf. So what happens is when a, when a star uses up all of its nuclear fuel, it evolves to a state where it's a gravitationally bound star in which there's no nuclear fuel being burned. And the star is held up from gravitational collapse by quantum pressure, quantum electron degeneracy pressure.

• 44:30Speaker 03

  But there's a limit as to how massive such a star could be before, you know, the gravitational forces overcome this electron degeneracy pressure. And so, you know, if I have a white dwarf that's in a binary with some other star and it's accreting matter from that other star, as soon as that white dwarf exceeds this limit, which is called the Chandrasekhar limit, it explodes. And since that happens at a very, you know, very specific type of mass, we believe that all of these have, you know, all of these supernovae are exactly the same. So there's a good theoretical reason to believe all type one A supernovae have the same luminosity.

• 45:12Speaker 03

  And it's also been measured empirically. You can look at a bunch of supernovae, you know, the same galaxy, and they do have the same brightness. And so these supernovae are what we call standard candles. They're objects that have a very well determined luminosity. And so if we observe how bright they are, we can actually figure out the distance to the supernova, therefore the galaxy that hosts it. So there has been a project that's been going on for fifteen ish years called the SHOES collaboration, S H zero E S. And there's also been another collaboration called the Caltech Carnegie Chicago Hubble Project, C C H P. And they've been measuring the Hubble constant in this way.

• 45:56Speaker 03

  They've been looking at supernovae and distant galaxies and measuring the brightnesses of the supernovae and the velocities at which the galaxies are moving. And when they do these measurements, especially the SHOES collaboration, they find that the expansion rate is about 10% larger than the expansion rate you infer from the cosmic microwave background in galaxies and our models to account for them.

• 46:23Speaker 03

  And we call it the Hubble tension because when this was first noticed, it was a discrepancy, but it wasn't clear whether it was statistically significant or not. It also wasn't clear whether it was maybe some misunderstanding of how supernovae work or how the observations work. Or perhaps some problem with the calibration of the distances and brightnesses.

• 46:46Speaker 03

  But things have evolved, you know, with time. The measurements have become better. There've been more of them and the error bars have shrunk. And many of the systematic effects that people were concerned about 10 years ago have been shown to be of no concern or not, you know, not the source of the discrepancy. And if anything, things got, you know, the Hubble tension became much more serious just a few years ago with the launch of J W S T. So until I would say three, three years ago, it was reasonable to be skeptical about the brightnesses of the supernovae. And the reason is that the supernovae brightnesses are calibrated to things called Cepheid variables, which are stars, variable stars.

• 47:29Speaker 03

  They're stars that become brighter and dimmer over time scales of, you know, weeks to months. And Cepheid variables are also standard can. And so we have Cepheid variables in the nearby, in the Milky Way, and we can measure their distances very well. And then there's some Cepheid variables in nearby galaxies that also host supernovae. There are about forty such galaxies that host Cepheid variables that we've observed very well. And supernovae. So that's, so the supernova distances are calibrated to the, to the Cepheid variables. And the Cepheid variables then are what we're, you know, calibrated against.

• 48:11Speaker 03

  So when you look at a Cepheid variable in one of these nearby galaxies, if you look at it with the Hubble Space Telescope, which was the best instrument that we had for doing these measurements, the angular resolution of the Hubble Space Telescope is great, but not perfect. And in many cases, when you were looking at one of these Cepheid variables, there'd be lights from nearby stars that would sort of spill over onto the Cepheid variable limit.

• 48:39Speaker 03

  So this was something that you might be reasonably concerned about. Can we actually separate the light from the Cepheid variable from the light from nearby stars well enough to actually tell how bright that Cepheid variable? Yeah. But now we have the James Webb Space Telescope that launched, what, three years ago now? And the James Webb Space Telescope has much better angular resolution than the Hubble Space Telescope does. And so you can go back and look at some of the Cepheid variables and do the measurement with this better telescope. And when you do that, there's no issue of crowding. The Cepheid variables are very, very well separated from the nearby stars. And you can make the measurement. And for the sixteen such supernovae, or sorry, sixteen such Cepheid hosts for which they've

• 49:23Speaker 03

  done this measurement are the sample of forty-five-ish HST hosts. The measurements are spot on from what was inferred from HST. So this crowding issue is no longer a concern. And so the hubble tension is more serious now than it was three years ago because of this.

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• 50:45Sean Carroll?

  Okay, so unless there's some huge mistake that we are really missing, and the experimenters have obviously been very good, there's a mismatch between the sort of direct local measurements of the expansion rate and the inferred expansion rate we need to fit the data of like the CMV and the wider models. So at a very broad strokes, without getting into model building or anything like that, but how would we solve this? Could we just do, what are we looking for? Were you looking for something that makes the universe slow down at later times or speeds it up at later times? Or what is the target here? So.

• 51:25Speaker 03

  The target here is the baryon acoustic oscillation in the cosmic microwave background. So we talked about how you see this peak in the correlation function for galaxies at 100 megaparsecs. But you also see this peak in the cosmic microwave background. But you don't measure the physical size of the peak. You measure the angular size, same thing with a pond. You know, if you're viewing a pond from some distance and you see the ripples going out. You see the waves going out from where you saw the pebble. You can't tell how big those waves are unless you know how far away you are viewing and viewing them from.

• 52:06Speaker 03

  So we measured the angular size of this sound horizon with baryon acoustic oscillation very, very precisely. One part in the 10,000 with cosmic microwave background measurements. And the angular size is the physical size divided by the distance to the cosmic microwave background. That angular size is determined from cosmological models that have as their parameters, the Hubble constant, the dark matter density and the baryon density.

• 52:36Sean Carroll?

  And the dark energy density?

• 52:37Speaker 03

  And the dark energy density and how it evolves with, whether it evolves with time. So the two solutions that people have can be sort of classified into late time solutions and early time solutions. So the late time solutions, we sort of change that angle, the model predictions for that angle by changing the distance to the surface of last scatter. And that would happen if we some, if the expansion rates in the recent past was somehow different than in the standard cosmological. Those tend to not work because we can also get a measurement of the Hubble constant from the baryon acoustic oscillation of the galaxy distribution. And that is sort of agrees with what we get from the cosmic microwave background.

• 53:21Speaker 03

  So these late time solutions, people thought about early on, but they don't really work. So the other possibility are early time solutions where we somehow change the physical size of the sound horizon in the early universe and ways to do that. And in fact, we have to decrease the physical size of the horizon in the universe to account for the Hubble tension. And so one idea that people have spent a lot of time thinking about is called early dark energy. Where we have something that's sort of like a cosmological constant, but has a much larger magnitude. And it's around only in the early universe for the first half million years of the universe, and then somehow decays away.

• 54:03Speaker 03

  And people spend a lot of time thinking about these from about 2018

• 54:10Speaker 03

  until now. And I'd say, you know, initially, we were thought of as a promising idea.

• 54:17Sean Carroll?

  You were one of these people, by the way.

• 54:19Speaker 03

  I was one of these people.

• 54:22Speaker 03

  And in 2021, there was this very interesting result from one of the cosmic microwave background collaborations, the ACT, out of comma cosmology telescope collaboration. They published a paper late 2021 where they said that their new measurements were actually more consistent with the early dark energy model than the standard lambda CDM. And that was very exciting.

• 54:43Sean Carroll?

  At all excited.

• 54:44Speaker 03

  And that got people even more revved up about early dark energy models. There was more a bigger, you know, another round of model building, but then also more scrutiny from experiments. And I think, you know, that was four years ago, three and a half years ago. And over the past three and a half years, we've had more measurements from the cosmic microwave background, from galaxy surveys, more data, more scrutiny of the data. And I think the pendulum is weakening back to lambda CDM away from early dark energy.

• 55:15Sean Carroll?

  So it sounds like there's two options, late time solutions and early time solutions, and neither one of them work.

• 55:21Speaker 03

  Uh, that is a, I would say, a fair summer of the situation.

• 55:25Sean Carroll?

  So it's very different, just again, for the sort of non-experts here. In 1998, when people claimed, when the two teams claimed that the universe is accelerating and that that was an anomaly, et cetera, there was instantly a theory that explained it and everyone could say, oh, okay, so we found this thing. Here we have an anomaly and it's, my impression is it's still not clear what could explain

• 55:48Speaker 03

  That is correct. I mean, early dark energy was a really, a very plausible idea. Until just a few years ago, and I would not say that it's ruled out. Because early dark energy is not a model, it's sort of a class of models or an idea that can go into models. But what's happened is with new data that is increasingly consist, more consistent with Lambda CDM. The wiggle room for constructing early dark energy models has decreased. And it becomes harder and harder to find some model that actually. So, but your summary, yes, is probably the first approximation, correct. The late time, the simplest late time models don't work, the simplest early time models don't work.

• 56:34Speaker 03

  But it's even, it's sort of a. Yeah, I think if you had Adam on your podcast, I mean, what Adam would say if he were here. Is that the evidence for the Hubble tension is now much stronger than the evidence they had for accelerated expansion in the late 90s. But people are much more reluctant to accept this because we don't have a model to explain it. So if, interestingly enough, it was, if it was the other way around. So if the cosmic microwave background was giving us a Hubble constant that was 10% bigger than that from supernovae, then we would just change our dark energy model. Yeah.

• 57:17Speaker 03

  So we'd say it was time evolving dark energy. Given that the Hubble constant from supernovae is larger than that infer from CMB. The same solution could also be tried, but it would require a dark energy density that increases with time rather than decreases. And decreasing with time is okay. Because the energy can go somewhere else. But in order to have a dark energy density that increases with time is sort of equivalent to having energy just appear out of the vacuum. Which is not something that we like. I think about, I think you probably know this better than I. And in the general relativity community, there's, it violates the strong energy.

• 58:03Speaker 03

  It violates the weak energy. There we go, see? Yeah, I think I learned this from you. But I didn't, I really didn't learn it from you.

• 58:11Sean Carroll?

  I'm forgetting myself, but it violates, but but your point is right. It's just sort of so magical and scary to have energy appear out of nothing.

• 58:20Speaker 03

  Yeah, I think that's the weak energy condition is general relative for creating energy out of the vacuum, which makes us physicists uncomfortable. But, you know, the cosmological constant also made us uncomfortable.

• 58:35Sean Carroll?

  Yeah, I mean, to be fair, it's more than uncomfortable. When you try to construct a model in which it happens, other things tend to go disastrous. I mean, our discomfort is not purely emotional and vibes based. Yeah. Okay, I guess I remembering now, and I know that we're running long. So let me know if I'm abusing your kindness here, but there's another tension even before we get to the variable dark energy stuff. There's the S eight tension that cosmologists worry about and is not sunk into the popular imagination. Should we worry about that?

• 59:07Speaker 03

  Yeah, the S eight tension is a little more subtle and I sort of have less confidence in it. And whether it's a tension or not, seems to bounce around a lot more depending on who you ask and which data set. And one of the conclusions from the new results that we've seen from the DESI collaboration and the dark energy survey collaboration just last week is that with new data, the S eight tension is going away. So the S eight tension is a discrepancy between the amplitude of fluctuations in galaxy surveys on small distance scales compared with that expected from the cosmic microwave, the models that best fit the cosmic microwave background.

• 59:53Speaker 03

  And it's a strange tension because it sort of depends on which data set you look at. And in some cases, how you analyze the data set. And there's some measurements that seem to indicate that the there's discrepancy, but then other measurements that indicate that it's not a discrepancy. And it also involves measurements or observations of the galaxy distribution on smaller scales, where the theory becomes more complicated and we have less confidence. So we a lot of people in cosmology worry about the S eight tension. There's probably less consensus about whether it's there or not. And I think it might be going up.

• 1:00:34Sean Carroll?

  Well, that's another reason why the Hubble tension hasn't quite. Been as completely accepted as the accelerating universe because when you find a tension like that, maybe you find other tensions or other signals that kind of go in the same direction. But rather than building up, we're having other things sort of come and go and waffle around and nothing quite definitive yet.

• 1:00:56Speaker 03

  Yeah, it's a, it's a, it's a perfectly reasonable view. And I think 10 years ago, most people would think that, you know, as we analyze more CMB data, as we understand the analyses better, and as we understand the supernova and the analyses better, we'll find, you know, small errors in one or both that sort of have accrued and, you know, together give you some consistency. But that has not happened.

• 1:01:30Sean Carroll?

  Not yet happened, yeah. Okay, you already referred to two new results, which have sadly very similar names, DES and DESI. And they're both attempts to measure dark energy and they're both hinting that it is not the cosmological constant. So if true, I'm very, I have sort of public statements that I'm skeptical that something like that would come to pass, but it would be a big deal if it were true.

• 1:02:00Speaker 03

  I agree. I am also skeptical. I think sometimes when I'm skeptical, I have to try to like dial it back.

• 1:02:10Sean Carroll?

  Yeah.

• 1:02:11Speaker 03

  Because I think often what you find when you study history of science is that discoveries are made not just by, you know, major fundamental discoveries are made not just by people who've made really good measurements and are really good at the analysis, but there are people who have an open mind and will be accepting of the possibility that this might be big.

• 1:02:37Sean Carroll?

  We're not as young as we used to be.

• 1:02:39Speaker 03

  Yeah. I mean, if you're, you know, I think most of us have this attitude that when there's something strange in the data, there must have been something that goes wrong, went wrong, or we missed it, you know, the Hubble tension. I think many people have that attitude, you know, they're obviously missing something in the supernovae, complicated systems, they haven't modeled it correctly. The model, you know, standard cosmological model is fine. And if you always have that attitude, you'll never discover anything.

• 1:03:02Sean Carroll?

  Yeah. So what is the new result?

• 1:03:06Speaker 03

  So the new result is coming from DESI. And then there's sort of consistent information coming from DES and from various supernova measurements. That's show that the dark energy density evolving with time. And in particular, they show that it is, or has in the recent past been increasing with time. So it's a complicated result. It's not hugely statistically significant, but a little more statistically significant than it was a year ago. But it suggests that the dark energy density was smaller early times, became larger with

• 1:03:48Speaker 03

  time, and then fairly recently started to decrease in energy again.

• 1:03:53Sean Carroll?

  Hmm.

• 1:03:55Speaker 03

  So it's a very unusual result. So I would say it's unusual in several ways. The first is that, I mean, if the dark energy density was evolving in time, you know, that is instant Nobel Prize.

• 1:04:09Sean Carroll?

  Yeah.

• 1:04:10Speaker 03

  And we've been looking for this. So, you know, we shouldn't say, you know, can't be right. But as, you know, we, you know, just discussed earlier, the preferred fit suggests that the dark energy density was increasing with time, which I just learned violates the weak energy petition, which I already knew is creating energy out of a vacuum, which is, I'm supposed to keep an open mind to, but it's really very, very, very strange from the point of view of theoretical.

• 1:04:39Sean Carroll?

  One has priors, that's okay. And, you know, your priors are never zero, but they're bigger on some possibilities than others.

• 1:04:45Speaker 03

  Yeah. So, I mean, another, I mean, another way of saying it's like sort of higher order in discovery space, you know?

• 1:04:54Sean Carroll?

  Right.

• 1:04:55Speaker 03

  Learning that the dark energy density evolved in time is like spectacular enough. But then learning that it increases in time, that's like even beyond that. So I think the bar for that is even higher than it would be just for dark evolution.

• 1:05:08Sean Carroll?

  What are these experiments? What are they measuring?

• 1:05:10Speaker 03

  Um, so the principal experiment for this is the DESI collaboration, which stands for Dark Energy Spectroscopic Instrument, I think.

• 1:05:20Sean Carroll?

  It does. That's right. I looked it up.

• 1:05:23Speaker 03

  Okay. So this is a really spectacular project where they measure the redshifts and therefore the distances to millions of galaxies over a huge volume of the universe. And with these measurements, they can determine the, um, the, they can measure the baryon acoustic oscillation feature at a variety of different redshift or distance bin. So they can measure the angular size of this, um, of this, uh, bump in the clustering, the galaxy clustering. They can measure the angular size at a variety of different distances. And in that way, they can figure out the, um, the expansion rate as a function of redshift

• 1:06:07Speaker 03

  or as a function of time. And so they can actually see the expansion rate changing with time in this. Um, so, I mean, some of the issues are that they're splitting all of their galaxy survey into a bunch of different distance bins. And so they have bazillions of galaxies, but you know, they have six or seven different distances and so, and each bin, it's a bazillion divided by six or seven. Um, and then, you know, the other things that you might be concerned about is that, you know, the universe is actually evolving with time. And maybe there's something about the properties of the galaxies that they're looking at that are evolving with time. And you can read the papers. They've got hundreds

• 1:06:49Speaker 03

  of pages. They've spent a huge amount of effort checking for all of these obvious things that you would check for. And none of these obvious things that you would check for has shown up. Um, but you know, with a result like this, um, that's so unusual, you really require, you know, a higher level degree of scrutiny. Um, I mean, the way I look at it, I mean, the other thing I should say is that there are also supernova measurements that are sort of like those, like the shoes.

• 1:07:16Sean Carroll?

  Yeah.

• 1:07:16Speaker 03

  But they, um, look at supernovae out to larger distances. So they're interested not so much in the expansion rate today, but how it evolves with time. So they're doing sort of complimentary measurements. They're sort of doing the same thing that DESI is trying to do with the Baryon Acoustic Oscillation, but in a slightly different way. And then there's the Dark Energy Survey, which doesn't have distances quite as well, but they have tons and tons of galaxies and their measurements are sort of consistent as well.

• 1:07:49Sean Carroll?

  Consistent with the DESI results of the time-dependent dark.

• 1:07:53Speaker 03

  Yeah, they can't really determine the time evolution of the dark energy quite as well, but there are other places where their observations overlap with DESI's observation. And in places where they overlap, there's consistent. So I think the way I look at it is that DESI has shown that these galaxy surveys can be extremely powerful. They can work. And the other thing that's important to note is that DESI is not the last such project. We've got the Rubin Observatory that's going to start taking data any day now, and they're going to do, you know, analogous things over huge volumes.

• 1:08:31Sean Carroll?

  Ground-based telescope.

• 1:08:33Speaker 03

  Ground-based telescope. There's then the European Space Agency last year launched Euclid, which is a space-based, they're going to do a space-based galaxy survey. And there's then NASA's Roman Space Telescope, which will also be launching soon, and that's going to also do a huge galaxy survey from space. And all these projects have some overlap, but they also have complementarities. They check different things. They will be affected by different types of systematic artifacts. They have different ways of observing the same galaxy populations, and they also have access to slightly different galaxy populations. And also, two weeks ago, NASA launched a project called SpheroX, which is going to have some galaxy mapping capabilities. And so the way that I look at it is that, you

• 1:09:18Speaker 03

  know, these projects can work. They do work. The level of precision that we're getting from them is absolutely stunning and was unimaginable just, you know, even 10 years ago. And, and the other thing is, you know, the DESI results are new. And we've always found, you know, with new telescopes, you know, projects in cosmology and astronomy, when you build a new telescope to make new observations, you're learning about the universe and the telescope at the same time. And so I think it's going to be really interesting, important and interesting for us to,

• 1:09:57Speaker 03

  you know, really look at the telescope and the detectors and the analysis pipelines simultaneously with our, you know, our scrutiny of the cosmological implications. And I think that in the process, um, we'll, you know, understand better what's going on. And even if it's not, you know, time evolving dark energy, it's definitely going to feed into our ability to do these measurements even better in the future.

• 1:10:24Sean Carroll?

  I'm sure some people are going to want to know why we need to build so many different telescopes. Why can't JWST do this? But these are experiments designed for different purposes.

• 1:10:34Speaker 03

  Yeah, so JWST is an absolutely phenomenal instrument. Actually got to see it in the high bay at Lockheed Martin in December of 2019. Um, and I was looking at, watching the videos of how it was going to like unfold and unpack. And I was thinking, there's no way it's gonna work. Like, oh my God, it was like crazy. I mean,

• 1:11:00Sean Carroll?

  As you can't fix it, you can't go up and repair it.

• 1:11:03Speaker 03

  I mean, the fact that it worked and actually worked better than they anticipate in many ways. It's actually spectacular. The images, the things we're finding with it, absolutely amazing. But the thing is JWST is a narrow field of view. It's really good for looking at a very small number of objects at very large distances or very faint objects. But if we're trying to do cosmology, we'll want to map the distribution of galaxies over as large a volume as we can, so over as much of the sky as we can. So it's a different type of telescope.

• 1:11:34Sean Carroll?

  And one of the things about the DESI results is it, it, it brings home, at least my very, very casual looking at the papers, brings home the fact that it really does depend on your model that you think you're testing when you come across and say, here is our result, right? Cause if you just fit to, there's a constant dark energy, et cetera. You get one result. If you say, well, I'm going to let it vary linearly with time, you get a different result. If I'm going to let many things happen, you get a different result. Um, is it possible that there's different levels of confidence in the dark energy used to be increasing result and the dark energy is somehow changing result? Or do they go hand in hand?

• 1:12:17Speaker 03

  Um, I'm still trying to understand that. So yes, um, what we do is we construct models and then we fit for the parameters in those models. And one of the things I'm trying to understand is that, um, if you take the DESI results and you model them with the standard Lambda CDM model, um, my understanding is that that actually gives you a pretty good fit. And I don't know whether it's, I mean, if there was something that was desperately wrong with Lambda CDM, then when you try to fit it with Lambda CDM, you would get a result that was not, that you would not get a good result. But my understanding is that they do get a good result with Lambda CDM.

• 1:13:00Speaker 03

  Um, but then if they expand the model parameters, say, so instead of Lambda CDM, they have this time evolving dark energy density.

• 1:13:08Sean Carroll?

  Yeah.

• 1:13:08Speaker 03

  So this is a model that now has two additional parameters. It has the, um, time evolution and then they have a second parameter, which is the time evolution of the time evolution. And when you do that, that model seems to provide a better fit than Lambda CDM. But I have not yet really understood whether it implies that Lambda CDM is not a good.

• 1:13:28Sean Carroll?

  Right.

• 1:13:29Speaker 03

  And my understanding is also that if they just try to fit a model where you have, um, dark energy that is evolving with time, but has a just one parameter model, it's just evolving with time. You don't have any evolving of the evolving, um, that that also, um, gives you a fit that is consistent with the standard Lambda CDM.

• 1:13:50Sean Carroll?

  Okay. Is there any relationship between this result and the Hubble tension?

• 1:13:57Speaker 03

  Yeah, that is a great question. Um, I mean, as I said earlier, we're always looking for consistency. Um, there is no obvious way in which this connects with a Hubble time.

• 1:14:09Sean Carroll?

  Okay.

• 1:14:10Speaker 03

  I mean, I think it would be much more exciting if there was.

• 1:14:13Sean Carroll?

  Yeah.

• 1:14:14Speaker 03

  Um, but it turns out that, um, when you change or expand the model parameter space this particular way, it does not change the Hubble constants inferred from the measurement. Um, it does change the upper limit to the neutrino mass, which is sort of something that has been, I think, one of the most exciting things from these results that people have not been paying a whole lot of attention to.

• 1:14:37Sean Carroll?

  What is that? Where does that come? So does a neutrino have to do with this? You haven't even mentioned neutrinos yet.

• 1:14:42Speaker 03

  I haven't even mentioned neutrinos.

• 1:14:44Sean Carroll?

  Yeah. Yeah.

• 1:14:45Speaker 03

  No one's been mentioning it. So, you know, in the standard model of elementary particle physics, there are three different types of neutrinos: electron, muon, and neutron. And in the standard model, when it was constructed in the early 1970s, the neutrinos were thought to be massless, and in the standard model, they are massless. They have, don't weigh anything. But then, about, you know, twenty something years ago, it was discovered that neutrinos actually have a small, non-zero mass. And so we know now that neutrino masses are not zero, but we don't know what they are. They're very, very small. And so we know that they're bigger than zero, but they're smaller than some upper limit. Those upper limits come from a variety of accelerator experiments,

• 1:15:30Speaker 03

  and laboratory experiments, and beta decay experiments. But it turns out that neutrinos, you know, the standard cosmological model predicts that there should be neutrinos running around the universe, just like there's light and baryons. And if the neutrinos have a mass, then they would actually contribute something to the cosmological energy density and affects our cosmological models. And the measurements that we have now in cosmology are so precise that the fact that neutrino masses are non-zero actually has to be taken into account. And in fact, with our cosmological measurements, we now have upper limits to neutrino masses, which are complementary and in some ways better than those that we have from laboratory experiment.

• 1:16:16Speaker 03

  And one of the things that was really interesting about DESI is that it improves the sensitivity to a non-zero neutrino mass over what we had before.

• 1:16:27Sean Carroll?

  So, to be clear, are we saying that they have detected? Like, if we didn't know that neutrinos had mass, would this tell us that they did?

• 1:16:35Speaker 03

  No, but they have, so what they have are upper limits that are starting to distinguish between the two different neutrino mass scenarios. So there are three neutrino masses. We've got good reason to believe that all three of them are non-zero. We know that two of the masses have some small mass splitting. And then another pair of masses has a larger mass splitting. But we don't know whether, how those masses are assigned to the electron, muon, or tau. So we don't know whether the, there's two lighter states and one heavier state, or two heavier states and one lighter state. And the DESI results are starting to say that the inverted hierarchy,

• 1:17:19Speaker 03

  the, the, the system with two heavier masses is ruled out.

• 1:17:25Sean Carroll?

  Okay.

• 1:17:25Speaker 03

  And this is kind of super useful. Kind of gone under the radar in terms of popular press coverage of the DESI results. But it's, it's a really, really impressive accomplishment and could be very important for elementary particle physics.

• 1:17:39Sean Carroll?

  So it's a tradition late in the podcast. We always get to let our hair down and explore wilder ideas. So you've already said that the straightforward fitting the data implies this dark energy increasing for a while in density and then decreasing. That's already a very, very wild idea. Are there wilder ideas that could fit the data? So I'll, I'll just tell the audience, like back in the day when we were younger than we are now, you were involved in a couple of papers establishing the idea of the big rip as a possible future for the universe, right? With the dark energy density just going crazy upward in the future.

• 1:18:20Sean Carroll?

  And friends of mine and I wrote papers saying first, uh, so it, the, the technical language we use for increasing energy density is W less than minus one. W is a little parameter, right? And if W is less than minus one for dark energy, then the density goes up. So I wrote a paper saying, um, can W be less than minus one? And we argued probably not. But then we wrote a follow up paper saying, could you be tricked into thinking that W is less than minus one? Um, if gravity were changing its strength over cosmological time? And we said, you know, maybe, but it doesn't look very pretty. Are, are people exploring ideas like that? Gravity changing its strength?

• 1:19:01Speaker 03

  Um, I don't know. In the sense that, um, I haven't seen much. So in the dark energy literature, they're sort of like the early, you know, simplest type dark energy models, which we called. I think you called them quintessence.

• 1:19:17Sean Carroll?

  No, that was, uh,

• 1:19:18Speaker 03

  You wrote, all right, you had quintessence in the rest of the whole.

• 1:19:21Sean Carroll?

  I did have that. I helped popularize.

• 1:19:24Speaker 03

  Yeah, so those quintessence, but then there was sort of a wave of alternative gravity models for dark energy. I have not seen many of these dark alternative gravity models showing up in connection with new DESI results. But I don't know if I haven't seen them because they're not there, I just haven't noticed.

• 1:19:41Sean Carroll?

  They haven't had that long. We like to think that people take more than a week to write a good paper.

• 1:19:46Speaker 03

  Well, the DESI results were also around last year.

• 1:19:48Sean Carroll?

  That's true.

• 1:19:50Speaker 03

  So I don't know. I mean, the crazy idea that I like to think about is oscillating dark energy. So there's sort of, I mean, we know that there's a cosmological constant now. We have very good reason to believe that there was a period that we call inflation in the very early universe, which was powered by a non-zero cosmological constant with a very large magnitude that then decayed away. And early dark energy also, the early dark energy models also surmise that there's a period of cosmological constant domination, you know, half a million years after the Big Bang that then dies away.

• 1:20:31Speaker 03

  And so people have, over the years, considered the possibility that, you know, every few, you know, logarithmic times in the history of the universe, for some reason, there's a cosmological constant that shows up for a little while and then disappears again.

• 1:20:48Sean Carroll?

  A cascade of dark energies at different times.

• 1:20:50Speaker 03

  Yes. So there were papers where you just have essentially just one quintessence model, but it had, instead of rolling down a smooth hill, it rolled down a bumpy hill. That would do that. And then there was also this idea called the string axiverse, which was quite popular about 15 years ago. And the basic idea there is that in string theory, there are, in addition to the fundamental fields responsible for electron and muon and quarks and photons, there are many, many, many more fundamental fields. And there could be hundreds of them that they call axion field. And it is conceivable that in these scenarios, you could have different axion fields sort of randomly becoming dynamically important

• 1:21:33Speaker 03

  at different periods in the of the universe. So that's the thing I kind of like to entertain. And it's kind of a it kind of fits in to some extent with these DESI results, because these scenarios suggest there should be some type of cosmological constant domination that then decays away. I mean, in these scenarios, whatever we think is a cosmological constant now will then decay in the near future, you know, several billion years from now. And the universe will then proceed as if there's no cosmological constant once again. And so we're, you know, with DESI, you know, you look at the, I told you expand, the dark energy density is increasing, but now it's decreasing in time. So it kind of fits in with that scenario.

• 1:22:16Speaker 03

  The only thing that doesn't fit in is the increasing density, which we can't fit or has not yet been explained.

• 1:22:22Sean Carroll?

  Yes. But the idea, this is an important one that we should mention. String theorists never liked the idea of a positive cosmological constant. That's hard to fit into string theory, but zero or negative, they could, they could make their peace with. And the, if the dark energy is evolving and if it's decreasing right now, that is back on the table. We could have a big crunch in the future. We could have a negative vacuum energy at the end of the day.

• 1:22:49Speaker 03

  Yep, that is definitely on the table. I don't think though, I don't think that those allow for the increased density.

• 1:22:56Sean Carroll?

  No, no one, no sensible person else for that, which is, so obviously this motivates people to really get that right.

• 1:23:04Speaker 03

  Yeah.

• 1:23:05Sean Carroll?

  And the final thing, then, I will give you a chance to wax eloquent on birefringence. Because there's been a couple of, a couple of, you know, hints that maybe there is something funny going on with the polarization of light from the CMB. That's, that's the last anomaly that I'll, that I'll lay in front of you.

• 1:23:25Speaker 03

  Okay. So cosmic birefringence, I learned about from a 1998 paper by Sean Carroll and collaborators. Is that right?

• 1:23:35Sean Carroll?

  Yeah, maybe.

• 1:23:36Speaker 03

  So your listeners should know that you wrote this spectacular paper in the late 1990s, where you pointed out that there may be some physical models in which

• 1:23:50Speaker 03

  light that has a right circular polarization could travel at a slightly different velocity than light with a left circular polarization. And if so, a light wave that was linearly polarized would have a linear polarization that rotated with time as it propagated. And that is called cosmic by refringence. And I wrote a paper a few years after that, or the year after that, that showed how you could test this scenario by looking at the cosmic microwave background. The cosmic microwave background, we're looking at light that's been propagating for 14 billion years. So if there's any subtle effect, it would have more time to accrue in the cosmic wave background than anything else. And people have been trying to make these measurements with cosmic microwave background experiments since then.

• 1:24:34Speaker 03

  And there has been some hint, some hints in the data, that the rotation, that the linear polarization actually does get rotated by 0.3 degrees over 14 billion years.

• 1:24:49Speaker 03

  I think it's very exciting, very interesting. It's a very, very difficult thing to measure from the data, though. And the primary reason is that it's hard to calibrate the linear polarization. So they can measure differences in linear polarization very well. So if, you know, if I give you two rays of light that are side by side, and ask you, what's the difference in the linear polarization? You can measure that very well, but the absolute linear polarization is harder to get.

• 1:25:20Sean Carroll?

  And that's once again because telescopes are complicated things.

• 1:25:23Speaker 03

  Yes, because telescopes are complicated. I mean, it's not, it's a, it's a fairly. It's just easier to measure the separation often between two points that are nearby than it is to measure the separation of two points that are really far away.

• 1:25:36Sean Carroll?

  Okay, that's fair enough.

• 1:25:37Speaker 03

  So it's the same thing with polarization.

• 1:25:39Sean Carroll?

  But did you notice that ACT, the Adakama Cosmology Telescope, also has a tiny little detection of baryfringence?

• 1:25:48Speaker 03

  No, I had not noticed that that yet.

• 1:25:50Sean Carroll?

  So the the result that you're talking about was from Planck, he had this beautiful all-sky thing. And it's like marginally statistically significant, and like you say, it's very difficult. So people didn't get too excited. But the Adakama Cosmology Telescope, which is a ground-based thing, you know, they had a recent data release where they said, everything fits LAMB to CDM perfectly well, but there is like a two-point something sigma detection of baryfringence.

• 1:26:16Speaker 03

  So I hadn't looked through those because I was studying the DESI papers really, really carefully in preparation for this podcast. I haven't had a chance to to dig down deep into those papers yet.

• 1:26:28Sean Carroll?

  Well, yeah.

• 1:26:29Speaker 03

  I don't know. That's interesting.

• 1:26:31Sean Carroll?

  It's interesting, yes.

• 1:26:33Speaker 03

  I'll have to take a look.

• 1:26:35Sean Carroll?

  What are your feelings? What are your what are your? This is this is where we close out. Your final thoughts, like 20 years from now, what do you think we'll have landed on most probably?

• 1:26:44Speaker 03

  Most probably.

• 1:26:49Speaker 03

  I think 20 years from now, we'll know much more with much more certainty whether the Hubble tension is real. I'm guessing that 20 years from now, there'll be new ideas from elementary particle theory and theoretical physics that make a phantom energy much more palatable.

• 1:27:16Sean Carroll?

  Phantom energy is the increasing density.

• 1:27:17Speaker 03

  Yes, W S, increasing energy density, more palatable to us. And I'm guessing that we will see dark energy evolve.

• 1:27:28Sean Carroll?

  Okay, all right.

• 1:27:29Speaker 03

  Bold.

• 1:27:30Sean Carroll?

  Because 20 years, I picked that because we might both still be around.

• 1:27:34Speaker 03

  Yeah.

• 1:27:36Sean Carroll?

  50 years, it's easy to make crazy predictions.

• 1:27:38Speaker 03

  Well, not only will we still be wrong, but this podcast will still be wrong. If you'll turn it on and say, "Look, it's much less likely."

• 1:27:45Sean Carroll?

  You might have to have a re-vivification if that's the case.

• 1:27:50Sean Carroll?

  All right, well, that's a lot to think about. It's kind of, it's kind of good because, you know, for a while there, it was possible to believe that cosmologists had figured it all out, right? That we had a theory that fit the data too well. But now we're in a more normal science area where there are anomalies and we got to bang our head against them. It feels good.

• 1:28:10Speaker 03

  Yep. It's a, yeah, I think the frustrating thing is I've been, you know, I spent a lot of time working on early dark energy. And then I go and give talks. I was invited to give talks all the time. And at the end, people say, "Well, is it early dark energy?" I was like, "MC," and they said, "Well, the measurements we've done in the next few years, and you know, if it's early dark energy, we'll know." But now, they're like, "So what's going on?" I don't know what to say. I have no idea what's going on.

• 1:28:38Sean Carroll?

  It's good for the young people out there. There's still a room for a really good idea.

• 1:28:41Speaker 03

  Yep.

• 1:28:42Sean Carroll?

  All right, get to work. All right, Mark Kim, Miki Kelsey, thanks so much for being on the Mindscape Podcast.

• 1:28:47Speaker 03

  Thank you very much for inviting me. It's been an honor and a joy to be here.

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