Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 170 | Priya Natarajan on Galaxies, Black Holes, and Cosmic Anomalies

Episode Date: October 25, 2021

There is so much we don't know about our universe. But our curiosity about the unknown shouldn't blind us to the incredible progress we have made in cosmology over the last century. We know the universe is big, expanding, and accelerating. Modern cosmologists are using unprecedentedly precise datasets to uncover more details about the evolution and structure of galaxies and the distribution and nature of dark matter. Priya Natarajan is a cosmologist working at the interface of data, theory, and simulation. We talk about the state of modern cosmology, and how tools like gravitational lensing are providing us with detailed views of what's happening in the distant universe. Support Mindscape on Patreon. Priya Natarajan received her Ph.D. in astrophysics from the University of Cambridge. She is currently professor of astronomy at Yale University, the Sophie and Tycho Brahe Professor at the Niels Bohr Institute of the University of Copenhagen, and an honorary professor for life at the University of Delhi, India. She is an Affiliate at the Black Hole Initiative at Harvard University and an Associate Member of the Center for Computational Astrophysics at the Flatiron Institute in New York. She is a frequent contributor to the New York Review of Books and other publications. Among her awards are a Guggenheim Fellowship, the India Abroad Foundation's "Face of the Future" Award, and an India Empire NRI award for Achievement in the Sciences. She is the author of Mapping the Heavens: The Radical Scientific Ideas That Reveal the Cosmos. Web site Yale web page Google Scholar publications Articles at the New York Review of Books Wikipedia Twitter

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for over 50 years with the Colgate Women's Games, the nation's longest running indoor track and Field series for girls and women. Colgate, your smile is your strength. Hello everyone and welcome to the Mindscape podcast. I'm your host, Sean Carroll. And as I've often said on podcast and elsewhere, cosmology is a great science because the universe is so simple. It has ingredients in it, right? Our universe has electrons and atoms and photons and neutrinos, dark matter, dark energy. But it's a fairly finite list, right? It's not like there's a hundred different ingredients that really matter for the evolution of the universe. And even better, when you look on sufficiently large scales, the universe looks smooth. If you kind of squint and average out over a scale of, I don't

know, 100 million light years across, it's more or less the same amount of stuff in every part of the universe, the same number of galaxies and stars and so forth. And that helps us. We can put together a picture of what the universe is doing, how old it is, et cetera. But then, of course, if you squint a little bit closer, if you zoom in on what the universe is doing, all that simplicity kind of evaporates. And you see a very rich, complicated universe. There are stars and planets, galaxies,

clusters of galaxies, filaments of galaxies, and what have you. And then the particular way that these structures both exist today and have evolved over time depends on the list of ingredients and the expansion history of the universe and so forth. So the bread and butter of modern cosmology is not measuring the expansion rate or the acceleration rate. Those are important things.

We do them and they matter. But most of observational cosmology and theoretical analytic cosmology is interested in the structures that are there in the universe. So that's what we're going to be talking about today with today's guest is Priya Naderajan, who is an astrophysicist at Yale, and the author of a book called Mapping the Heavens. the radical scientific ideas that reveal the cosmos, which traces both through the history of mapping the heavens, but also the modern version of it. And that's what I'm talking about.

Mapping where the galaxies are, how they get distributed. And in particular, Pria is an expert on gravitational lensing as a technique to study structure in the universe. Einstein predicted a long time ago that gravitational fields will bend light, and we can use the amount of bending on cosmological scales to figure out exactly how much stuff there is in the universe, whether or not we can see it. This is obviously hugely important for

figuring out properties of dark matter and more than that. So we get into some details on modern cosmology. And another thing that this is a wonderful discussion for is the fact that it really involves so many of the different moving parts of modern science more generally. There are, of course, observers collecting data. There are theorists proposing models. But in modern cosmology, it also really matters that we have simulations, that we have large-scale computer programs that are trying to bridge the gap between the theoretical ideas

and the observations of the data. And finally, not to neglect them, we have human beings. We have the fact that human beings respond differently to different scientific results and are predisposed in different ways to different kinds of scientific theories. So Pree and I talk about that,

the existence of anomalies, despite the fact that we have a really, really successful cosmological model. No model is perfectly successful. So where are the gaps? Where are the weak points, which might lead us to a better model in the future? This will bring you up to date on what cosmologists are thinking about right now and what to look for as we learn more stuff in the future. So let's go.

Priyana Rajan, welcome to the Mindscape podcast. Delighted to be here. So we're going to talk about cosmology, and I'm going to take for granted. I'm going to assume the audience knows that the universe is big, that it's expanding, that it's full of galaxies that are mostly smoothly distributed, and that the interesting part comes when we say, but it's not perfectly smoothly distributed. So, you know, as a working cosmologist at the interface of data and theory, explain to us how you think about the large-scale structure of the universe. Like, what are the basic facts that the person out there should have in mind while

we have this conversation? I think you sort of outline sort of the basic picture we need to have. You can just think of the universe as expanding and with its entire contents, like not just expanding, but, you know, the expansion is accelerating and that it is filled with a bizarre kind of matter, dark matter, which is nothing that we are familiar with, nothing on the periodic table, some likely exotic particle made in the early universe. and that much of the action in the universe starts kind of late, like relevant to us, that is, you know, the assembly of ordinary atoms kind of happens a little late.

And I'm particularly interested in this sort of aggregation and clumping of matter, sort of a departure from the simple kind of linear, homogeneous kind of distribution. So early on, we believe the universe was more or less homogeneous, but that over time, gravity, which is the most powerful organizing force in the universe, collects, aggregates mass and matter, both dark and barionic ordinary matter, feel gravity, because we believe this dark matter is actually a particle. And so then they aggregate, they kind of co-evolve, and in fact, the dark matter is sort of a cocoon in place.

which ordinary matter falls in. You can really think of it as like a cradle and, you know, cocoon. And gas basically, you know, for us, all of matter essentially in the universe is kind of hydrogen, really. We're really talking about hydrogen. It's all gas. Gas falls in, cools, condenses, form stars, and then, you know, stars evolve. And I think there are many intriguing mystery.

that have to do with the first set of stars that formed. So the onset, how did it all get started? We kind of know how the universe itself got started, you know, from a very hot, dense state. But I think there are many more open questions as the universe kind of expanded, cooled down, and then these dark matter cocoons lit up for the first time. So, you know, the properties of the first stars.

Well, actually, that leads me to a question because I think that this is something that is often glossed over in the details. But we have ordinary matter, which is, like you say, everything in the periodic table, everything we've ever found here on Earth. Dark matter is something new, something different. And we'll talk about why we think that's there in a second. But there's more dark matter than ordinary matter. But also, let's just get straight what form the ordinary matter is in. You said mostly hydrogen. And so what fraction of it is like stars and planets

versus what fraction of it is just out there as gas in between the stars and planets? Yeah, so much of this hydrogen is actually captured in stars and planets. And some of it, a small portion of it is kind of smeared everywhere as material in between galaxies in space. And, you know, dark matter is distributed similarly in the sense that, you know, there's dark matter, is there everywhere in the universe, very lightly smeared. And then there are places pockets where structure forms. And what I mean by structure is galaxies, the visible part sort of forms.

And in those regions, you have a concentration of dark matter. And so, and sort of, you know, light really traces dark matter, which is why, you know, regions where you have an aggregation of dark matter, you also tend to have an aggregation of ordinary matter. Can I ask you, because back in my day, when I was in graduate school, before we had really mapped out the cosmic microwave background radiation, which you should tell us about, but just to set the stage, we didn't know how structure grew. There was this theory that there were tiny fluctuations in the early universe that grew over

time, but there was a competing theory that there were seeds like cosmic strings or something else that sort of stirred up the universe all along. And I get the feeling, in fact, I know for sure, but the audience doesn't know, that we've more or less discarded this cosmic string theory in favor of the primordial theory. Is that right? Yeah, absolutely. And I think the compelling evidence for this idea of dark matter, sort of cold dark matter, very sluggish material particles, constituting the universe. I should note here, sluggish, you know, everything is relative to the speed of light, right? So these are super slow compared to the speed of light. So the theory in which all of the structure that we see is formed from the gravitational sort of amplification of these

little inhomogeneity. So, you know, you have these little piles. It's almost like, you know, little piles of sand, and then because you already have a little excess in one region, you tend to attract more because of gravity and you sort of amplify and you grow masses in sort of clumps. And this theory has been ratified, and I would say that, you know, it's called the standard cold, dark matter paradigm. It's been ratified and probably the most powerful cosmological probe has been the cosmic microwave background radiation. So this is the, a relic radiation, hot radiation from the Big Bang. The universe was at a very dense, hot, sort of violent beginning. And so this is radiation that comes to us when the universe was about

400,000 years old. And this is the last sort of direct signal that we get from farthest back in the universe. And I think what's remarkable about this signal is not only does it sort of telegraph properties of the early universe, because light that is coming to us from that surface, if you will, and it takes a finite time for things because the speed of light, large as it is, is still finite. So this light encounters the unfolding cosmic saga of all the formation of galaxies, you know, the first stars, the first black holes, the bigger galaxies assembling. So this light encounters everything. and it carries an imprint.

And I think that is the triumph of this model. So this model was not only able to explain what we see, but the evolution over time. And also calculated, was able to quantify, you know, what imprints, what are the imprints you would see on the cosmic microwave background. And all of those imprints have been now detected. And so that really basically was nail,

secure nails in the coffin of alternate theories. And, you know, I think this theory is very secure, partly because there are many independent lines of evidence. And this is one. You know, then there's the properties of galaxies, you know, how they cluster and when they form properties of other structures in the universe, like the ones that I work on called clusters of galaxies, which are sort of larger assemblages,

aggregates of about thousands of galaxies held together again, all by the gravity of dark matter, as it turns out. So I think that this theory has really emerged as, you know, sort of the theory, although it is contested. And, you know, I hope we get a chance to talk later on about how we are kind of stress testing this model, because the data has gotten so much better that we can really hone in on the very detailed predictions and challenge them with observations. Yeah, and that's going to be the exciting thing, is sort of looking for places where the theory doesn't quite match the data. But let's just be super duper clear here because out there on the internet, you can read all sorts of things. And I think you already said this,

but maybe to emphasize it, we're not really open for speculation about the whole Big Bang expanding universe model being wrong, right? We're looking for tiny little variations. Absolutely. And, you know, I think that you're absolutely right, that, you know, it bears a repetition that, you know, this idea of a hot, dense beginning, the Big Bang, and the sort of subsequent description that we've been talking about,

that is very, very securely settled, you know, as I said, many independent lines of evidence. And, you know, in a way, one of, that's why it's been really hard to falsify this theory too or to come up with an alternative. So there are, you know, there are researchers working on sort of an alternate theory, partly because, you know, beautiful as this theory is and concordance with data, you know, there's a couple of big nagging questions. You know, first is this nature of dark matter.

You know, several decades on with searches directly looking for the particle, looking indirectly for a signature of the particle, they've all come up empty. So we are yet to find this dark matter particle. And second, the universe is not just expanding, but the expansion is accelerating. And as we all know from our experience of driving around, you kind of need to, you know, hit the gas pedal if you want to start accelerating, right? So clearly something, we don't know what it is. And so we've called that dark energy is propelling the accelerating expansion.

of the universe. And so these two key elements, and in terms of the overall composition of the universe, these are the principal components of the universe, in terms of the energy contribution to overall energy contribution. And so it's embarrassing, right? So, and the stuff that we are made off that we know quite well, and we've charted, is about 4% of the total inventory. So given that we have these two big gaps, there's obviously room, right? However, wonderful, this theory is, you know, we really don't have an understanding of what dark energy really is. And so there are, there is one sort of alternative attempt to try to explain, you know, is there a theory in which you could just modify the nature of gravity somehow so that you can

sort of do away with the idea of dark matter? And that what you really have done is altered how the force of gravity acts over large distances. I mean, I think it's an, interesting avenue worth exploring. I mean, I'm a scientist. I have to be open-minded. Although, you know, and this model, the cold dark matter model, I have to say, one of the reasons it's very hard to falsify in addition to the overwhelming amount of accumulated evidence is the fact that is the history of how this theory evolved itself and how our understanding evolved. So in many ways, it was a theory that started out with degrees of freedom. And as we made observations, you know, you hone and refine a theory. So this theory has been honed and refined

for several decades, right? When you say this theory, tell us exactly which of the many theories that we're talking about, you mean. It's a cold dark matter theory. Cold dark matter standard theory. Okay. Yeah, standard. And when you say cold dark matter theory, that means not just that there is cold dark matter, but that the universe is expanding and the cold dark matter has been responsible for the growth of galaxies and structure. Like that's the whole theory that we're talking about. Yeah, that the whole panoply of, you know, how the universe has started and evolved to be where it is. All of those elements form. But I mean, we refer to it shorthand as cold dark matter because, you know, we see post facto that cold dark matter is in the driving seat. That's what has driven

everything that we really see. But the people out there listen to, they might think you're talking about, you know, what the dark matter particle is, but when we use the phrase the cold dark matter model, we mean this whole picture of cosmological evolution. Yeah. Science isn't about multiplying and dividing or memorizing the name of the planets or the species. It's about thinking and problem solving and taking a critical attitude toward the world. And at the practical level, according to the Department of Commerce,

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from a live instructor right now. IDTech.com slash Minescape. And so, I mean, maybe say a couple more words because this dark matter versus some alternative like modified gravity is obviously a crucially important question. And I think, like you say, we can imagine gravity being modified. But given all the evidence for dark matter, it's hard these days, especially with the microwave background. Absolutely. I think, you know, what has been really difficult for these alternate theories of gravity, even though they could dispel. So, you know, there's a lot of evidence for the existence of dark matter sort of,

halos of dark matter around every galaxy, pretty much in the universe. And that gets reflected. The presence of that dark matter gets reflected in the motions of the stuff that we do see, which is stars, because the stars are now feeling the gravity of the whole galaxy. And so they're moving around much faster than if they would be moving, if only gravity was provided by the visible gas, the stars that we see. So it's sort of the original way in which, you know, Vera Rubin and her collapse, came up with the idea of dark matter in the first place. And so, you know, to explain the, so these are called, you know, the motions of stars that you

map in galaxies, it's called the rotation curve, basically, you know, how the motion of these stars. And so the evidence for cold dark matter, compelling evidence, came from sort of these rotation curves where basically the, instead of seeing an edge to the galaxy when light just drops off. So you see galaxies are very centrally concentrated light in stars, and then they become fuzzy, fuzzy, they appear to kind of have an edge, at least in light.

But it turns out that if you actually look at the motions of stars, doesn't look like they have an edge. It looks like actually there's something holding up galaxies at the outer parts. And so that's reflected in this rotation curve. So these alternate theories can actually kind of dispense this idea of dark matter holding things up and they can explain that. However, the one effect that has not that has evaded calculation for them, but, you know, and they have been attempts, is the bending of light that Einstein's theory of general relativity predicts and is observed in abundance in the universe.

So just that kind of phenomenology of seeing light bending. And this, is manifest, this light bending is manifest very clearly. You see really misshapen galaxies. You see them kind of stretched out like very unusual shapes in very particular configurations. And that theory cannot quite, the calculation hasn't been done, to be fair, but I think that's, you know, one of the reasons, the fact that you have all this compelling light bending that you see, which is information rich, right? I mean, that's one of the areas that I work in. And, you know, there is so much data that shows you that there's sort of rampant light bending.

That theory has not had, you know, well, we'll see, right, in a few years. I mean, I remain open-minded to see, well, you know, this is how we have to be as scientists, right? As you know, Sean, we are trained to stay nimble. and sort of the provisional nature of science, sort of inherently provisional nature of science, at any time it's best to date. Your understanding is best to date, and you have to remain open to revisions and refinements.

Well, and also, let me just add in the temperature fluctuations in the microwave background radiation as yet another piece of data that fits very comfortably with dark matter and is very hard to explain by modifying. Yeah, so they have current versions of their theory that can account for, you know, the imprint in the cosmic microwave background. But, yeah, it's not easy. But as for the cold dark matter model, it's a very nice fit.

And, you know, all the details that we see. So we see this imprint in terms of wiggles in this background radiation. So this imprinted wiggles. And the wiggles are really small fluctuations in the temperature, like the sixth or seventh decimal place, right? But those details, those very fine details are just exquisitely explained by the cold dark matter theory. And just to get on the table, you know, my vested interest is very much in that modified gravity should be the answer, not dark matter.

I would much prefer if gravity were modified. That'd be much cooler for me. but the data are not letting me believe that. Yeah, not right now anyway, and not this version, right? So it may turn out that there is a version of a theory. I don't know, one of the reasons I remain open-minded, although I'm kind of, you know, obviously my work is very invested in the cold dark matter model, right,

is the fact that, you know, we know that Einstein's theory is incomplete, right? because we believe that all the forces do have to be unified. So this is not the last word on gravity. It's a compelling word on gravity. So Einstein's theory has been tested in many, many regimes, and it comes out tops. I mean, it really comes out really fine. But, you know, you never know what may be lurking there in terms of some tiny gap or mismatch.

I mean, this brings us to my favorite kind of, you know, angle. My angle is that it is, I mean, for my entire career, one of the things that's really driven me is to find these potential tiny gaps between, you know, theory and data. Because I think that there is a lot of information and potential for discovery in these gaps. And, but, you know, not all gaps, right? So obviously, right, there's sort of the standard case from the history of science where Urbane Laverier was very clever and he looked at the, you know, the perturbations in the orbit of Uranus.

And he was able to cleverly predict that, you know, there's Neptune that was causing these perturbations. So, you know, everyone was worried, right? Because at the time, because, you know, Newton's theories were laws were sacrosanct. And, you know, here's the little deviation. But he was able to sort of, you know, explain. in a way within the context. So, you know, Newton's laws remain intact. And similarly, there were anomalous, anomalous procession in the orbit of Mercury. And, you know, economical as Arbenne Laverier was,

he said, hey, same explanation. I think there's another planet somewhere between the sun and Mercury, and he called it Vulcan, and people went around looking for it and so on. It's not there, obviously. And the resolution to that required a brand new theory, right? So it kind of, although Einstein didn't set out to start explaining that anomaly, his theory naturally explains that in his complete reimagining of gravity itself, right? The nature of gravity. So I think the gaps are very enticing because you never know if you're in the case, right? And one of the great things that Einstein bequeathed to us was this idea of the bending of light, of gravitational lensing, which is where, you know, you've already mentioned it, but I really wanted to dig into the

details about that for this discussion because it's what you work on and it's kind of something that doesn't get quite as much popular discussion as maybe it could. So I think we can all intuitively see how a big, massive gravitational field could deflect light. That seems to make sense. But one of the great questions in cosmology is you see a galaxy, an image of a galaxy, or the microwave background, I suppose, you see something out there in the sky and you're saying, well, its light has been deflected. How do you know that? Like compared to what?

How do you know what it should have been if there weren't any gravitational lens? Right. Thankfully for us, most of the universe, as we mentioned earlier on in our conversation, dark matter is very lightly smeared. So most of the universe is kind of empty of matter. But however, there are regions where it's really concentrated.

So when we look out into the night sky, you know, the places where you would get dramatic light bending, the fraction, the patches on the night sky where you could expect extreme distortion, probably 10% max. So most of the time when you're looking at the universe, you're not seeing dramatic distortions, slight distortion perhaps. So when you look through regions where there aren't large clumps of matter, mostly dark matter, then you you know that you are not seeing, you're seeing the undistorted shapes of galaxies. So, yeah, this is a big, tricky issue because, you know, galaxies are born with a range of native shapes.

Yeah. So we need to know the distribution of native shapes for galaxies or birth shapes. And then modulo that, we see something that is distorted. So it is with respect to this distribution. So it's done statistically that, you know, you have the, um, you have the, um, a distribution of the undistorted birth shapes of galaxies. And then you look at a patch of the universe where you know there is something like a cluster of galaxies because you see all the galaxies that are clustered together to make that cluster.

And you know it's a huge repository of dark matter. And then you go measure the shapes of galaxies behind this lens. So this matter acts as a lens, very much like our intuition with glass lenses and optical lenses. And so we are looking at a patch of the universe in the back and the light from there gets really highly distorted. We can then look at the distorted shapes you get from the stuff behind the lump versus the rest of the universe. And that is what gives us a calibration. We are able to then back out how much matter there should have been in this sort of entire cylinder from us, from our eyes, all the way to where these galaxies, distant galaxies are.

So what we are able to do is to sort of kind of invert the distortions, like we can undo the distortions and figure out how much matter you needed to produce that. And in fact, it turns out this is one of the most compelling pieces of evidence for the existence of vast amounts of dark matter. Because when you go and back out and you look at a region that has, for example, a cluster of galaxies, you know, you're able to see all the light. So you can count up all the galaxies, the visible matter that is part of this inferred amount of mass that you need to produce the lensing. And you see it's a deficit of about a factor of 10. So you need almost 10 times more matter than you actually see in stars.

And particularly we now know that clusters of galaxies are the regions of the universe that are sort of the largest, contain the largest amounts of dark matter. Are most galaxies in clusters? No, very small fraction of galaxies are actually bound in clusters. Most galaxies live a quiet life in the field. Clusters, I say so, because clusters are very violent environments, very harsh environments because, you know, things are whizzing around.

Galaxies are falling in, the gravity is so strong that they're getting ripped apart when they fall in. So it's a very dynamic, transformative environment. Is Our Galaxy the Milky Way inside a cluster? No. Thankfully, I guess, for us. So it turns out the way structure is organized in the universe, it's kind of hierarchical by mass.

So there's like small lumps, medium lumps, big lumps. So our galaxy is part of one of these sort of medium-ish lumps, which is called a group. So we are part of something that's called the local group. And but, you know, so it's more than, so it's a few galaxies that are kind of hanging out together.

Okay, so even the lonely galaxies, if they're big enough, we'll have like little satellite galaxies and, you know, we have a friend in Andromeda, but otherwise it's not a very big collection. Is that right? That's right. Okay.

Well, they're very faint. There are lots of faint galaxies that are part of our local group. And do they also, by the way, have dark matter? I mean, isn't that a, if it were modified gravity, then I would think that there'd be a sort of unbreakable relationship between how much visible or ordinary matter there was and the gravitational field. Whereas if it's a combination of dark matter and visible matter, then you might have mixtures where sometimes it's all dark matter, very little visible,

and sometimes it's the other way around. Yeah, that's a great question. So as it turns out in our universe, in this normal sequence of how galaxies form, assemble, and evolve over cosmic time, you actually find that the fraction of dark matter that a galaxy should have is a huge range. There's a lot of stochasticity. So you can have some galaxies that are quite bereft of dark matter, and then you can have some extremely dark matter rich galaxies. And so there's a real range that is allowed. And, you know, you can imagine that, you know, you can see probably how that happens,

Because there's a lot of randomness in terms of physics and dynamics of things getting close to each other, two galaxies smashing in, having a close encounter, modifying each other along the way. So you can imagine that there's a lot of room for variation. Good. And let's go back to the, I want to dig in even more to the gravitational lensing story. So if I understood correctly what you're saying, we can sort of look at regions of the universe where we think there's not a lot of stuff. in the foreground nearby, and so we can see the galaxies far away. And of course, the specific image of those galaxies is not going to be duplicated somewhere else, but the statistical features of that image, you know, how many galaxies, how close they are,

et cetera. And so then we can go and compare where there is a cluster of galaxies. There will also be galaxies in the background. We know because we can look at their red shifts, and we can compare their statistics. Is that what we do? That's right. We compare their shapes. They're shapes.

They literally just compare their shapes. And look at how distorts. We plot out the distribution of shapes, and you see how wide that distribution is. But there's something even cooler, though. So in parts of the universe like clusters where, you know, where I'm sort of repeatedly saying, right, I mean, this is the joke about people like me who work on clusters. Like every grant proposal starts out with clusters are the largest repositories of dark matter. in the universe.

Yeah. And anyway, so in clusters, the inner regions of clusters, there's a huge amount of dark matter, and the light bending is proportional to how concentrated the dark matter is, how closely packed it is, right? So the inner parts of clusters are so closely packed that the light bending is so extreme that you can think of light. I mean, I find this analogy useful. Look, I mean, the thing with analogies is they say.

stop working at some point, but they let you get to a point. So you can think of light as a tube. The bending is so extreme kind of close into the sort of concentrated part of a cluster, for example, or even a galaxy. So there are galaxies that have huge amounts of dark man in the center that you can think of this tube of light cleaving into two. So basically what happens is you end up seeing multiple images of the same. single object. So in reality, there's only one object, but you see multiple copies of its image. And there are very specific configurations that, you know, you can look at those configurations and you can figure out, ah, this one has split, but you can do something even better, right?

Because every galaxy in the universe has its unique fingerprint, the way we know these are multiple copies of the same object is they have the same spectra. You're going to take the spectrum and they're identical, right? So you, you know, you know, you're going to take the spectrum and they're identical, right? So you know that this is in fact just an artifact in the sense that it's a multiple image of an individual source. And this multiple imaging of how dramatic it can be really depends on the concentration of dark matter. So you can split. So there's a curious thing that happens, right? So you split, you have an distant, faint galaxy.

one other thing the lensing does, it kind of magnifies because it changes the surface area. You can think of the tube starting out as a small tube and then becoming thicker, if you will. So the surface area changes. So you actually magnify and you bring things into view. So that's why it's a lens. You bring things into view and you see these faint objects that you wouldn't otherwise see. So not only do you magnify them, you can multiply emphyry. them, you split them up, but then you produce two, for example. So classic splitting is you

have two very bright images and you have one demagnified image in the center. You actually produce an odd number of images. So you have three, five, seven. And we've actually seen all of these configurations now with the Hubble Space Telescope and verified that they are copies of each other by taking their spectra. I mean, you mentioned the space. telescope, which is good because I wanted to sort of take a little side light onto the data and where we get it from. So the space telescope is one place, but I presume there are other places. And also talk a little about the ecosystem of who is observing, who builds a telescope, who are the theorists who analyze the data?

Because you're not out there with your eyeball to the end of the telescope collecting data, right? Right. No, I'm not. And in fact, sadly, now almost nobody is looking through the past literally, right? The people, even the observers are looking at computer screens, right, at this point. But yeah, so I think the sort of the ecosystem of how all of this work gets done is that, you know, you have the skilled teams of people with different kinds of technical expertise. So you have the observers, you have the instrument builders who have built the spectrographs. and the light buckets. So ground-based telescopes, space telescopes,

that are actually collecting all the data. Then you have people who skillfully reduce this data, remove all the noise from the signal. And then there are people like me who then model who have theoretical ideas, theoretical pictures and storylines, and we build our models in such a way. At least that's the aspiration. We build models that can be derived.

directly confronted with the data. So, and then you have people who are actually doing computer simulations. So they're basically doing computer models of the evolution of structure in this sort of cold dark matter paradigm or any sort of, you know, they can change the initial conditions. They can play with the kind of dark matter particle. So these are just sort of, you know, our sort of cosmology where we can't perform controlled experiments. These are our proxy experiments, right?

So then you have the simulator. So then you have these sort of interpolating people like me. So I analyze simulations and then I build a theoretical model. I build a conceptual model that can connect the simulations, enabling us to mock and do observations in the simulation, then confront that with the data and then infer what is going on. I mean, when you say simulations, what exactly are we simulating? Like a galaxy has 100 billion stars.

Do we have like 100 billion conceptual stars in a simulation? Or do we include exploding stars and magnetic fields? What is the state of the art there? The state of the art now is quite remarkable. But we are still not at the level of we are we are simulating aggregates of stars. We are not yet simulating individual stars. However, we have clever techniques to mimic the actual explosion of the star. Because what really matters about a supernova exploding is the energy.

What is happening to that energy? How is it getting distributed? And the metals that are locked in in this old star, how they're getting expelled. So those are processes that you can mimic without actually sort of following the individual star and its life cycle. So, you know, we've abstracted these out. There are simulations are done on multiple scales. The kind that I was talking about are simulations, cosmological simulations, in which we sort of have a sort of a grander, we want a grander view.

So we want to see a piece, a small chunk of the universe with all the stars in galaxies locked in and the dark matter. And we want to see sort of the co-evolution. And now we can even drop in particles that would behave like black holes. and we can see how the whole, because we know, you know, we know all the ingredients in the universe. So you just pop them all in, abstract their behavior depending on what we are interested in. And then we can look at the evolution, turn the clock on, and we can look at the evolution of these components, the combined evolution, which is very hard to do theoretically. You know, it's very hard to do because these all the non-linearities and how things couple on small scales and so on that you can't capture with sort of, you know, theoretical models.

however abstract they are and so on. And then there are simulations where people zoom in. So there are these simulations called, you know, general relativistic magneto-hydrodynamic simulations. So these are simulations where you zoom right into the heart of a galaxy. You focus on the black hole, you focus on the gas flows around the black hole, and, you know, and then you put in,

you have magnetic fields that are threading it and so on. So, you know, we can abstract away portions of problems and focus and numerically tackle them. And I think, you know, this is one of the exciting challenges that has also kind of informed the arc of my work, which is how do you bridge the scales? How do you take a simulation that looks at just the inner part and then a simulation that looks at a much larger scale? How do you put them together?

How do you piece them together in terms of the implications, in terms of, you know, the astrophysical phenomena, motions of particles, you know, are there being, flows kind of pushed out to large radio, how do you couple them? So yes, simulations have been very powerful in cosmology. They've been transformative in cosmology. Tires matter. They're the only part of your vehicle that touches the road. Tread confidently with new tires from Tire rack. Whether you're looking for expert recommendations or know exactly what you want, tire rack makes it easy. Fast free shipping, free road hazard protection, convenient installation options and the best selection of Redisdine tires.

Go to Tyraq.com to see their Redestine test results, tire ratings, and reviews, and be sure to check out all the special offers. Tyraq.com, the way tire buying should be. Well, and just to emphasize the extent to which we've made progress, you know, like back in my graduate school days that were formative for me, all the simulations had just dark matter in them, right? Because we figured, well, most of the matter is dark and it's easy to do,

they don't even collide, just putting in ordinary matter where you could make stars, et cetera, that was a big step. And the state of the art has come a long way, even though we're not done yet. We still have a long way to go and getting even better. Yeah, absolutely. And you mentioned a little bit about the black holes at the centers of galaxies. And this is another place I think that maybe some comparisons are useful. Because I think a lot of people think, well, there's this supermassive black hole at the center of a galaxy,

which is a statement many people have heard, isn't that where, most of the gravity in that galaxy is coming from, but that's nowhere near right. Absolutely. Even though the black hole has all these bizarre properties, it turns out in terms of the mass budget of the overall galaxy, it's kind of teeny-weedy. It's maybe even a millionth, you know, of the total mass including. So, for example, the total mass of the Milky Way is 10 to the 12 times the mass of the sun. and the mass of the black hole is four million times.

So it's like 10 to the six orders of magnitude, right? There's six orders of magnitude difference, right? But they punch more than their weight because they have these bizarre properties and they sit in the centers of galaxies. It turns out you could have some of them wandering around too. I mean, that's some exciting new work that we recently published, that you could have a population of kind of wandering black holes. But regardless,

There's only a very small region around the center of the galaxy where the gravitational effect of just the black hole dominant. The stars take over very quickly. So there's a small region of influence. And basically outside that, it's gravitationally inconsequential. But we now know, so we had dismissed them, right? We thought, okay, they're really tiny. They're important. Right around.

If you're right there, of course, it's consequential, but otherwise not. But it turns out now they were believed to be so marginal, but now we've understood that they actually play an outsized role in modulating how stars form. Stars appear to form inside out in galaxies by and large. So the inner regions are where bulk of the stars are forming. And so black holes are sitting there. So it turns out that black holes can heat the gas and they can, you know, the energy that is falling into. the black holes, a gas particle that is getting gobbled by the black hole gets pulled in, gets heated up, gets fed up, starts to radiate and gives out a lot of heat. That heat can prevent

stars from forming because, you know, for forming stars, you need to cool the gas. So this is actually adding energy. So we realized suddenly that black holes could be sources of energy in galaxies and therefore the sort of intricate sort of feedback between how they either do they promote or do they prevent stars from forming? This whole cycle has been much better understood recently. And now we believe that even though gravitationally they're not as important, they punch much more than their weight in terms of determining, you know, the phenomenology of a gal, how it looks, because the how it looks is determined by the stars.

So on how many stars form. So it modulates, we believe, the efficiency of, you know, formation of stars. One thing that is coming clear from what you're saying is that, you know, again, the whole scale of time and space in cosmology is so different than what we are used to in our everyday lives, right? It's been 14 billion years since the Big Bang. We look at pictures of galaxies and they're not moving.

They're just sitting there, right? Like when we take a picture of them, but on the scale of, let's say, one billion years, a lot happens in the universe. The universe is actually a really dynamic place even on the scales of galaxies and clusters. Yeah, absolutely. You know, it is, it's one sort of analogy that, you know, I can think of is that, you know, when you look at an ant hill from really far away, you don't see all the little ants kind of busily walking around. It seems to be a pretty stable structure that keeps its shape and stuff when you're looking for, when you get closed, you see, oh, wow,

look at the amount of activity these ants are, you know. up to it. So it's somewhat like that, right? So it's a question of scale and perspective. So if, as you said, a billion years is, it's a fraction of the age of the universe. And still, so for example, you know, when two galaxies crash into each other, sort of the finale, you know, of them ending up as one sort of mixed up, messed up product of soup of stars and gas could take a billion years. Right. That whole process, right? And that's our future, right? We're going to crash into Andromeda? Yes, yes. And, you know, we would make Milcomita, as it's called, right? I did not know that. I've often asked about this and, you know, people appear somewhat like worried.

And then I have to say, well, you know, we're talking of billions of years in the future. Also, come on, come on, folks. I think we need to pay attention here and now, right? Like, we seem to be on course to destroy our planet within the next couple of hundred years. So. But it's the galaxies merging that I'm really worried about. Yeah. Right. Exactly. So, I mean, let's get back to these black holes.

Does every galaxy have a massive black hole at the center? It appears so. It looks like it is such an essential and fundamental feature of how structure forms. Just as, you know, stars form in this cocoat of dark matter, it appears that the formation of a black hole is pretty fundamental. So most, if not all galaxies, appear to have a black hole in its center. And in the cases where we don't see one, we see signatures of it having been kicked out. So it might have been there, might have crashed into something, and then it got kicked out. So there are a few cases where we see that kind of signature.

And what is the story of where these black holes came from? Were they there for a long time? Or did they slowly accumulate? So, you know, this is another thread of work of my research, which is trying to understand the origin of the first black holes. So one thing we know, there's a natural way to form black holes, which is just the end states of stars. So if you are a star that is eight times or so more massive

than the sun when you are born, inevitably you finish your life cycle, you know, all the fuel gets exhausted, and then you leave behind a black hole. But those black holes are really, really tiny. And the black hole in the center of our galaxy is, you know, four million times the massive sun. This one is perhaps a few times the massive amount.

the sun. So how do you grow from? What are the intermediate? What do the teenage years look like? What is adulthood look like for little black holes? So it turns out that black holes can grow either by accretion of matter, so just sucking in matter, accretion and fancy way of things, sucking in matter, or by crashing into each other. So when they obviously merge into each other, they shake up all of space time, they produce gravitational waves. We've seen that happening for little black holes, We've not seen that happening yet for supermassive black holes, but that's coming with the Lisa Intraferometer and Space that is going to be put up within 20 years or so.

So we know that black holes can grow in these sort of two different ways. But there is an interesting conundrum that kind of motivated a lot of my work more than, oh gosh, I hate to admit it about 15 years ago. Because it reminds me of how old I am when I think about, oh my God, that paper was 2005 is, you know, so we also know that these growing kind of feeding black holes are quasars. And these are very bright beacons that we can see. So a quasar is basically the heart, the black hole at the heart of a galaxy that is feeding so rapidly that it outshines the entire galaxy. And we see them, you know, peppered everywhere.

They're really peppered. They're not all that common. They are rare objects. but the ones that are lit. So black holes themselves are actually fairly common, but not all of them are actively feeding and growing. So for example, the black hole, the center of our galaxy is actually fasting.

I mean, you really don't see it. It's not a quasar. But did it used to be a quasar? Pardon? Did it used to be a quasar? Did it go through that phase? Probably, yeah. It probably cycled through.

So when, you know, when food was ubiquitous, it was a quasar. And then it depleted everything that it could eat in its vicinity. And in the absence of having new gas, getting smashed into the center with the merger, you're sitting there, you know, fasting. So these feasting black holes you see as quasars. And we are discovering, and from the brightness of those quasars, you can figure out how big the black hole is. So the bigger the black hole, the more rapidly, voraciously, it can feel fuel itself and feed.

and therefore the brighter it will be. I mean, maybe we need to be a little bit more specific about how a black hole can be bright, since after all, they're black. That's right. So a black hole is bright because of the dying gasps of the matter that are actually getting eventually sucked into oblivion by the black hole. So on route, whatever the black hole is feeding on being a star, black holes can also gobble stars.

So, but if it basically matter coming in the form of gas, and, you know, as we said early on, right, everything is basically hydrogen, right? So hydrogen is glowing. And that's what you see. So you, and that's how you see a black hole. And these quasars are basically where you see very, very, you know, very active feeding episodes. And we are detecting these quasars now to very early on in the universe. You know, when the universe was a fraction of, you know, of its current age, 10% of its current age, right? And we are already seeing quasars. And once again, right, we know where the quasars are because we can measure their redshifts. So we know they're very distant. And they already seem to be harboring a black hole that is a billion times the mass of the sun.

Well, but you left us hanging a little bit because you said we know how to make a black hole from a single star and it would be a few times the mass of the sun. And you said that that could grow by either are creating random matter in the universe or by coalescing with other black holes. So which is it? Or is it both? Or, I mean, how do you go from five to a million? It is both. It's both. But it turns out that it is a challenge to grow them regardless. Okay. And you can, you know, they will grow as long as they are very gluttonous for periods of time. So that they're really super actively feeding. Then you can start

from something that's a few times massive the sun to the guy that's sitting in the center of our actually the center of our galaxy like a few million times you can grow with just one or two episodes of gluttony but you know if you have something more massive than that

then you know it starts to become really problematic because then you have to really be overfeeding for a very throughout your lifetime you have to be overfeeding in order to end up and that's hard because to over feed at that rate, you need a huge amount of gas supply. And we know that the gas supply in the universe

kind of drops with time. Which is why there's not much gas in the center of our galaxy. Most of the gas by now is locked up in stars. Or it's fallen into the black hole, right? Or it's fallen into the black hole. Exactly. It's already gone into black holes or it's captured in stars. So I would think that, you know, a galaxy is a big thing and has some extent. and there's stars all over the place. So if those stars, the more massive of them exploding and creating black holes is the origin of the supermassive ones, I would be mystified as to why the supermassive ones are in the middle of all these galaxies. But there must be some reason why.

It's gravity again. It's gravity in action. So because it's very massive, it starts compared to the stars, right? Compared to the mass of a star, it's much more massive as it's gathering mass. As it's gathering mass, it slows down and it becomes, you know, plumper and plumber, and then it gets settled down to the center. So it really is kind of, it is this dynamical kind of thing, once again, where it's not that the center of the galaxy is some special place where black holes form,

but any black holes that are medium-sized would sort of drift down there in a natural sort of segregation. Yeah, and this segregation is quite efficient because of the sort of mass difference, right? black holes are so much more massive than individual stars. So they can really grind past stars and settle in. But, you know, I was coming, I was talking about these bright quizzes at high redshift because that kind of posed a problem that many, including me, wanted to solve. It was exciting because you see these billion solar mass black holes in place when the universe was a fraction of its age.

So, you know, you could not have, there's not enough time to grow from, you know, a little, you know, a few times the mass of the sun to billion times the mass of the sun within that short span of time that is available in the early universe. There's this other fact about the passage and the flow of time in the universe. There's not a whole lot of time in the universe early on. A lot of the time is actually kind of stretches towards later times in the universe. And so then it was pretty clear. The answer was obvious, but the question was, what do we do? How do we get this to happen? Obviously, what you had to do is you had to make seed black holes that were really massive from the get-go. So if you could make a seed black hole somehow from the physics, that is, you know, 10,000 times

the mass of the sun, 100,000 times the mass of the get-go, just from the get-go. Then you have it. Then there's no feeding problem. Then, you know, these quasars that you see that are really bright are very rare. So, you know, it all works out really nicely because, you know, you need that to happen for the rare cases, but enough of the rare cases, not just a freak case, but it's a population, right? So we propose this idea of direct collapse black holes. So these are black holes that form from gas that collapses very rapidly. You bypass the formation of a traditional star. Okay. You don't have to cycle the gas through a star and then you leave a little wimpy black hole, but in fact, you put, you know, the analogy that really works, at least worked for me when I was first doing the

because it was really quite apt. If you're sitting in a bathtub, I love to take baths. When you pull the plug, you see that vortex, the water rapidly fueling down. That is exactly what happens with the gas that forms these direct collapse black holes early on in the universe. So literally what we had to do is to find the analog of taking the plug out and, you know, a region in the universe where you could somehow cause motions, kind of, you know, could you whip up some motions very early on in the gas, you know,

and it turns out that there are places in the universe this can happen because in the sequence of how galaxies and stars form, you first form a huge disk of gas. And then the gas cools and fragments and form stars. So if you had this big disk of force, gas and you prevented fragmentation and ended up creating a vortex that could cycle all the, and you can create a vortex because, you know, you can set up these sort of global kind of disturbances in the gas that could set up this vortex. So you could instigate this vortex with known astrophysical phenomena, nothing, you know, nothing extraordinary. And then the only thing

you have to make sure is that this gas disc does not fragment before the stuff falls in. If it fragments, it forms stars. Then you're done. Then you can't make this big seed. So we also know how to prevent the fragmentation of gas. How do you prevent the gas from cooling? It's really neat. In the early universe, you know, you can just have to get rid of molecular hydrogen.

So, you know, it's a detailed physics. So basically you can form these very massive black hole seeds and that sort of elizabeth. And that sort of alleviates this problem of how these very big quasars, bright quasars with supermassive black holes or even ultra-massive black holes are already in place early on in the universe. But when you say from the get-go already in place, just to be clear to our audience, you don't mean from the Big Bang. The Big Bang was still very smooth. Oh, no, no. So there is a huge gap, as we said, right? There's not a whole lot happening in terms of the assembly of structure.

And, you know, there's a lot of portion of the age of the universe is, you know, just radiation dominated. Then you move on to sort of a matter dominated era, but only very, very towards the very end, you know, it's very recently that we started assembling galaxy. So this is talking very much relative to the assembly, the epoch when the first stars formed. So it's round about the same time. And, you know, then the question is, could you form the picture? that I've given you, it sounds like you could form the black holes before you make the stars. Oh, okay. Yeah, because there's gas in there without stars. Not quite. Because to prevent the fragmentation, you kind of need some stars in place already,

at other places. You need a neighborhood of stars, little galaxies, and then you can have this gas disk, make your vortex and give you your direct collapse. Because we talk about the dark ages in between when the microwave background came around between 400,000 years or so. And then the lighting up of the first stars, which was when, do we know? Yeah, we don't know quite precisely. There's a little bit of uncertainty because we don't know exactly when the first star likely formed, but probably, you know, about six to eight billion years ago. So, yeah, I'm thinking from the Big Bang.

So it's 14 billion year old. It's still six to eight billion years works either direction because it's a 14 billion year old universe. Yeah, that's very good. Okay, yeah. So that's a lot of dark ages, actually. Right. And the final provocative thing that you mentioned there is that you can have not just supermassive black goals at the centers of galaxies, but ones that are just wandering freely through the universe that are not attached to galaxies. Right. And, you know, that is something that, you know, so one of the big pieces of the puzzle, right, as we're talking about these growth history of black holes, is that, you know, we are looking at the infant black holes and then we look at the geriatric ones. And then so what's happening in between? So there's a phase called intermediate mass. black holes and that's been pretty elusive. It's like it's almost like, you know,

looking at a population of people where you just don't see the teenagers and you see either the infants and little kids or you see adults and older people, right? So, but they are there and like rebel teenagers, these intermediate mass black holes are not actually where we believe they might be lurking. So it turns out they are not in the centers. They're not massive enough to have sunk into the centers, we'd been looking for them in fainter galaxies, because, you know, how big a black hole is, is tied to the mass of the stars that formed around it as we saw earlier. And so we would have to look in sort of wimpy galaxies to look for these sort of wimpy intermediate mass black holes, but we had very little success finding them.

So then it turns out that they were, one was found, and we found it was not at the center, it was far out. So that prompted us to start thinking about, you know, in the simulation, And so there was a breakthrough with this simulation that one of my collaborators and their team did, in which most simulations, these models, you had to pin down the black hole to the center of the galaxy. Just a numerical technique. You had to do that. And in this simulation, they didn't have to do that.

They just let it move freely as it wants. It settles down in the center because that's sort of because of its gravity dominating. But it turns out that in the process of galaxies kind of merging and their black hole stumbling, stumbling around that are many that are left wandering in the outskirts of individual galaxies and in clusters. So we detected this population and we've characterized. And that's sort of, you know, the exciting new result. But, you know, the one question that you had earlier that, Sean, I want to come back to,

you know, it was a great question you asked about this sort of the standard cold dark matter model, right? And the sort of the tensions. And, you know, I want to, you know, I'm very excited. excited about this, so I want to share this. So recently, you remember this looking for gaps thing, recently we found something really intriguing. And that is that the strength of lensing.

So now we are back to lensing away from black holes. Still the dark universe, though, still, you know, still the invisible universe, that the strength of lensing that is observed in the universe is incommensurate with the expectations of the cold dark matter model for the. sort of the inner parts of galaxies, right? So you have a cluster, you have all these cluster galaxies that are part of it that all held together by dark matter. So there's like the smooth sea of dark matter holding everything together.

But then each galaxy has its own little halo and, you know, content of dark matter around it. And from the inside out, so there's dark matter everywhere in a galaxy. So we find that there are some new lensing signals because there's deeper data. So you could look and, you know, capture tinier lensing signals. So even smaller distortions on smaller scales, the Hubble was able to detect. It was a long look. And there we found that the strength of lensing, therefore the number of lensing features,

all these strong distortions doesn't match with the predictions of cold dark matter theory. And the reason this is super intriguing, it's a gap. So by a factor of 10, okay, it's huge. Sorry, a factor of 10 between what and what? What is the thing we're measuring? The predictions of the number of these kind of little lenses inside lenses events that the cold dark matter theory predicts from simulations versus the real universe is providing for us. So it's a factor of 10 off. So the real universe, the lenses within lenses, right?

So we have this big lens and then you have these galaxies that act as little lenses. They are much more efficient by a factor of 10. the universe is a much more efficient lensing machine than our cold dark matter theory suggests. I mean, of course, the discrepancy by itself is awesome, right? It's a factor of 10. You can't kind of say, well, you know, maybe you have a slop of factor of two here. Maybe you have something missing here. You can't.

Unless, of course, you know, you have to conjure up like six different things that are all, you know, factors of 1.5 to 2 and kind of really, you know, knit them together to get you that. the reason this is really intriguing is previously. You know, so as I said, you know, the gap finders are around. Many of us are gap finders. Previous gaps that were found actually went in the opposite direction. So the real universe did not have as much matter piled in the centers of galaxies. It appeared that there was less matter piled in the centers of galaxies than the theory would predict. And so here we are finding something.

completely the opposite. It's a different environment, and it's something that you alluded to earlier, right, that, you know, this is the dense, this is the equivalent. Yeah, cluster is the equivalent of like New York City, like a bustling city, tightly packed, densely packed. So, you know, the previous findings were sort of in the suburbs, right? So in a kind of isolated galaxies or galaxies in groups and so on. So, I mean, this is, you know, in my opinion, this is a very intriguing mismatch, and this is going to be kind of hard to incorporate and argue away within the cold dark matter model. I mean, it could be that, you know, there's something. Obviously, it could be that, you know, the simulations are, after all, you know, we are putting in what we are getting out of

the simulation. So, you know, computationally, we may be missing some feature. Maybe we are not modeling something correctly. But it's still pretty hard to see how it's such a big factor. And if it's such a big factor, then it's kind of puzzling why everything else kind of fit so well. I mean, if you're so off, right, like how does everything else fit so well? To be clear about you, I just made this point, but just to super clarify it, when we talk about a discrepancy between the theory and the observations, the theory doesn't speak to us in an unmediated way. Making that prediction is a highly contentious thing. And like you say, there's a lot of simulations involved, et cetera, et cetera.

So even if it's a small likelihood, the option remains open that the theory is fine, but we haven't correctly understood what it predicts. Yeah, absolutely. Spot on. Because, I mean, what we are inferring, even from these simulations are mediated by a model, right? So there is a model.

So there's an intermediate stage. So it could be that there is something in our sort of conceptual model. But, you know, there's, of course, the other exciting possibility that may be something, you know, about one of our assumptions about the nature of the dark matter particle itself, right, is not quite right. I mean, this is super interesting. It's speculative, you know, and I'm not pushing that. I mean, I'm not saying that, you know, we are calling the entire paradigm into question or any such thing. It's just that it's very interesting because, you know, these kinds of stress tests are exciting, because, you know, because. Once again, you never know if you're in the Neptune situation or the Mercury situation, right?

That's right. So let's again put it into context. Again, you already did this, but this is such a rich story and it goes back and forth. So I want to make sure we get it right. For a long time now, there has been a, or there was, it always seems to go away, but there are always these claims that cold dark matter theory predicts too much concentration on small scales, that it predicts that the centers of galaxies should have more dark matter than we observe there, and there should be more little satellite galaxies than we observe.

But the issue always has been that you're trying to compare theory and observation with the dark matter theory in a regime where it's not just the dark matter that matters. You're looking at the regimes where the stars and the gas and everything are important, and those are the hardest things to model. Is that accurate so far? Absolutely, absolutely. And these discrepancies always seem to show up in the parts of galaxies, in the regions of the universe, where normal matter and dark matter are really rubbing up against each other,

like really closely packed. And we have made this assumption that this dark matter does not interact with the normal matter in any way other than gravitationally. So there's no charges, you know, there's no electrical forces, So there's no other kind of force. And I think, you know, absolutely. So that's really where all the tensions are emerging. And in the previous kind of crises in the inner parts of galaxies,

so even though, so one thing you could do, right, you can imagine that if you need, you could just redistribute, if you found a way to redistribute matter. So you have, suppose that, you know, in nature you start out with, know, the inner parts of galaxies being really dominated by dark matter. But then somehow, I don't know, supernova explosion, something happened that pushes out some of the dark matter and so evacuates it, if you will, right? And reduces the amount of, so that kind of redistribution could work. But once again, and that they believed was really sort of what worked to

explain these things away when the original finding was that. that dark matter appears to be heaped much more strongly compared to the real data. But I think the paradox is that now we are finding with better data, you know, what has really changed is much better data, higher resolution data. We're actually finding that in the real universe, it's actually much more heaped in the centers of galaxies that live in clusters. Right.

Okay. It's a very specific environment. But again, I just want to go through this. whole story very slowly because it does go back and forth. So the original problems were that galaxies were smoother in the center than the dark matter theory seemed to predict. And, you know, people will say, well, then why did you cling to your dark matter theory? Why don't you, you know, try to modify it? And, you know, you and I know, well, of course people try to modify it. That's full employment. I've written papers about, you know, modifications of dark matter particles interacting with each other,

interacting with ordinary matter, et cetera, et cetera. My impression, so I'm going to ask you, the expert, because I've not followed this in recent decades, my impression is that largely those attempts failed. It wasn't easy to add new physics to dark matter to explain these anomalies. And on the other hand, you know, the more the data improved and the more the simulations improved, the anomalies did not tend to stick around. We mostly have accounted for them in terms of ordinary non-dark matter physics. Is that fair? Right. Yeah, yeah. And the resolution was found within the cold dark matter theory and our methods of simulation. Right. Okay. Good. Within that context of that theory, absolutely right. That, you know, that the modifications, the kind of modifications that were suggested, you know, self-interacting dark matter, self-anhylating dark matter, all of these options, it turns out, were not really needed because once the simulations improved and our understanding of this physics, the, you know, the complex physics of, you know, kind of, you know, kind of, you know, kind of.

normal matter and dark matter kind of being clumped in the inner regions and their potential impacts on each other. There's sort of an interplay. They don't interact, but there's an interplay, right? And once we could simulate that better, those anomalies kind of went away. Mostly went away, right? And so good, but the new one that you're pointing at, does it have a name? Is there a label for the new anomaly?

It's called GGS. In good astronomy speak, we have an acronym. And it's an acronym that's not particularly imaginative. But, you know, it's telegraphic. It contains information. It's called GGSL. Galaxy Galaxy Strong Lensing. Yeah, you need a better label for this.

I'm sorry. Your branding team needs to get together and really work on this. Okay. Galaxy, Galaxy Strong Lensing. And it's giving us the opposite direction of an anomaly in the sense that there's more structure on these small scales. But as you say, in the particular environment of galaxies inside clusters, yeah? That's right.

Which is why, you know, we are still pushing really hard to find, there's a lot of room for finding resolutions within cold dark matter because these are complex places, right? There's a lot happening. Yeah. So, I mean, this does seem to be, again, the kind of environment that is the hardest for us to understand. So having an anomaly maybe shouldn't be.

too surprising, but then your point is, but this is such a big anomaly, we still got to pay attention to it. Right, and that it goes in the opposite direction. I think crucially, right, that it's, you know, it's big, it's a factor of 10, but it also goes in the opposite direction. So I think this is the problem that I've been really engaged with, trying to see if you could explain this away and account for this factor of 10 mismatch within the current theory. Like what could, I mean, what could we be missing? You know, what process or whatever could we, could we, be missing. Well, but also it does bring us into interesting philosophy of science territory because when you are faced with a discrepancy, you have lots of options, right? Like one is, well, your

prediction isn't very good. Your simulations weren't very good. Another is your theory isn't very good. You need to understand the ordinary matter more. Another is that you eat a better theory. And one of the things we don't often like to admit as scientists, but it's certainly true, is that we're more likely to pay attention to anomalies that have ready-made theoretical explanations, right? When we discovered the accelerating universe in 1998, part of the reason why it was so rapidly accepted was because Albert Einstein gave us a solution to this problem

back in 1917, the cosmological constant. Inadvertently, right? He didn't want to do it, but he did it. But we had an instant theory that explained this. So for your anomaly, for GGSL, Galaxy, Galaxy Strong Lensing, Is there an easy theoretical explanation that actually involves the physics of dark matter or something like that? Or is it more likely to be found in gastrophysics, as we call it? You know, I'm on the fence about this one in terms of where I think the resolution might lie.

I mean, you know, one can speculate, right? One potential, because it's such a particular environment, you know, very specific set of circumstances. circumstances, it could be that there's something that we have, there's some either physical process that we missed that operates much more efficiently in these dense environments, right? And it could be that, like, it could be a physical process that we're completely missing. Or it could be that there's an, you know, incorrect implementation of the physics process in the simulation, as you said, right? And so, you know, I have a sort of a dark history as a sort of historian slash philosopher of science.

I started a PhD which I didn't finish. But, you know, these are the kinds of questions that motivated me then and which have now, you know, caused me to think much more deeply about what I do. So I was very interested in the role of simulations, right? So what are simulations doing? So they are often thought of as tools, right, as ways of visualizing and a way of, propagating forward, even from what I, the kind of description I gave you today when we were talking, is that, well, it allows you to calculate the evolution of time, over time of a small chunk of the universe, all the complexity we can redo it.

You know, the big question is in a fields like ours, right, where you can't perform controlled experiments. You know, simulations are actually occupy a much more important place because there is a way, least now we are seeing, like in this case, right, it could well be that there is something that we are missing in the simulation, right? In which case, you know, and we are able to narrow it down, and in which case, then, you know, the simulation would have been generative of a new discovery, right? And this is not a role that we expect simulations to have, you know, if you look at the OED simulation, the definition of simulation, right? It's pretty dodgy. So, yeah, I think that it's very, very exciting to kind of come up against, you know, I don't have to, you know, it's a pleasure

to be able to talk to you, someone like you was thought about this a lot. You know, the limits of knowledge, right? These are things you think about a lot, right? What are the limits? Is everything knowable? What determines knowability? And I think, I think, you know, I should come clean. I mean, I already, you know, as you know, scientists fall into camps. I mean, I believe there is no reason to think that everything ought to be knowable. All right. You're allowed to think that. I think that my, I'm very happy to imagine that not everything is knowable, but I'm much more impressed with how much we have figured out than we have any right to be. So I'm suspicious that, you know, I don't think we're at the boundaries or the,

the limits of any of those things anytime soon. But I wanted to ask, I mean, since we're past the hour mark of the podcast, this is where we let our hair down a little bit. What about the sociology of the community in the face of anomalies like this? I mean, I had Adam Reese on the podcast. We talked about the Hubble Tension. We've talked in different places about different kinds of contentious scientific issues. And, you know, I think that there is a tendency among the,

non-scientist to think of the scientific community as much more monolithic as it is. Either they're all, you know, pushing the agenda of the establishment or whatever, or they're all lone geniuses with their own theories. But really, there's a lot of heterogeneity, right? Like, when you have a claim like this that there's an anomaly we should be paying attention to, some people are going to be like, oh, yeah, we should pay attention to that. Others are like, eh, it'll go away. What is your feeling about the different camps out there in the community? I mean, it's so acutely observed, Sean, unsurprisingly, right? So, I mean, the community is really quite heterogeneous in terms of their attitudes.

And I think that there is a large portion of the community that is very risky, averse. And that is kind of, you know, all these, this heterogeneity kind of mirrors the practice of science that has evolved. I mean, since we were both students, right, the science has become much. more collaborative, even astrophysics and cosmology that didn't used to be, you know, run by collaborations, hundreds of people large, projects are now that it's become big science. It's transitioned to a big science. And I think with that comes a certain kind of risk-averseness because, you know, you're asking for lots of funding, taxpayer money to do a project. It better be really well articulated. It better, you better find something. It's not going to be hit or miss, you know, that's too much

of a risk to take. So I think, you know, there are people who have that sort of overall risk-averse mentality. And then there are people like me, the floating around, not parts, you know, not really engaged totally with the big science enterprise, but wanting to take creative risks. And I think, you know, they would think, okay, why not? You know, maybe this is worth exploring and so on. But, you know, I think the thing that interests me about the sociology of science is that I bet you that regardless of how significant or insignificant this GGSL tension is, we don't know yet. I mean, we just don't know. We need a lot more people to work on this. I can bet to you that I am unlikely to garner as much attention as Adam and company did for the Hubble

tension, partly because, you know, of course it has to do with how pivot. the Hubble constant is and how it anchors so much of this model and our view, our cosmic view, is shaped by the Hubble constant, right, and its value and so on. And there's a long history of controversy about it. But I also think that, you know, it, you know, this is something that I often think about, you know, who and how new ideas are proposed matters. And, and I think that, you know, we are in a field where, you know, the playing field is not quite level yet, right? And I think that somebody like Adam Rees, who post Nobel Prize, found another sort of intriguing kind of anomaly bringing, you know, the field to a crisis point makes a big difference.

I mean, the fact that he has a Nobel Prize to back him, you know, makes everyone sit up and say, okay, I think we should maybe look at it a little more carefully. A lot more people will. But, you know, on the other hand, he got the Nobel Prize because, you know, he was able to resolve something that was completely, you know, crazy at the time, right? It was believed to be. So I think this idea of how authority is conferred and intellectual authorities conferred and then how a radical scientific idea gets to be accepted. It depends also obviously on the content of the idea, the significance of the idea. But I think, you know, who proposes it, which community it comes from.

You know, as you said, gravitational lensing should probably be more high profile as an area, but it's not. Hopefully that will change in the next decade or two. So some finding from that field could carry more water than some other fields. So I think there are many, many factors. You know, it's a larger sort of ecosystem. And this, you know, it's been very interesting to watch, you know, even in a detach. way before GGSL, this how these radical new ideas in our field are accepted. And you have experience of that, Sean. I mean, you're extremely creative, original, and take risks as well,

right? So you've not joined a thousand-person collaboration as their pet theorist. I would not be good at that. But, but, you know, the other thing, I like everything you just said about, you know, the, the, the humanness for better or for worse, you know, when we're, when we're in this situation where we don't yet know, when there is an anomaly, a gap, whatever you want to call it, there's a discrepancy between theory and observation, science is at its least objective in some sense in those cases. But then the good news is we're going to get more data, right?

I mean, maybe this would be a good final word. Like, what will be the new data coming in that will help us resolve this kind of issue? Yeah. So I think the new data that can really be a game changer in this is, you know, deeper. So at the moment, we have deep data in six clusters, just six, right? And so it would, you know, enlarging that sample of clusters for which we have the depth of data from Hubble, where you could do this kind of analysis. You could actually detect these lenses within lenses.

I mean, that's what is sort of needed. But I think more than that, you know, what you want is engagement. You want other people to get excited. I mean, I think that's what I really want. I mean, I would like many other people to get excited to explore this further and see, you know, is there something about the setting, you know, the cluster that matters or whatever. I think attracting other intellectual capital, people, their attention, getting people to work this, not attention and just in terms of, you know, looking and following it, but actually

engaging in the work. And I think that would be awesome. See, that's where we differ, because I work so slowly, I don't want anyone else working on the same areas that I am, because otherwise they will get all the good answers ahead of me. But looking forward to where your anomaly goes and what we learn more about the universe. So Priya Dada Rajan, thanks so much for being on the Mindscape podcast. Thank you so much, Sean. This was great fun. I love your podcast series. and it's a real honor to be on it. We're very happy to have you. Thanks.