278 - Priyamvada Natarajan: The Mystery of Dark Matter

32:37And he was able to go only so far in distance when he mapped that out. So later on, over the decades, we have been able to reach out and make this measurement, of, so an independent measurement of the distance and the speed. And when we started making that, it was no longer a straight line. So it looked like, you know, it's not directly proportional, something is going on, the shape changes, right? What that tells you, the shape changing tells you that this relation, the velocity of the universe is not fixed. That tells you there's some acceleration. There's some change in the velocity. And in 1998, there were two independent teams that used, and the way they've reached galaxies farther and farther away was to look at a standard light bulb, like a supernova, type 1A, which is, you know, it's like a standard light bulb.

33:34We know the physics of it well enough to know, just like we know for standard light bulbs. You know what 60 watts is, right? I go and buy a 60 watt bulb in New Haven, Connecticut. You go buy it in California, it is giving out the same amount of wattage. That's why it's standardized, right? And however, therefore, if we both put a 60-watt bulb and we are 100 meters away from it, we know we can measure the luminosity, we know its brightness, and then we push it further one kilometer away from us or, you know, one mile away from us, we can once again measure and we will find again the same diminishment, right?

34:11Because it's standard. It's a standard light bulb. So it turns out that these objects are super useful because if you find a supernova in a galaxy, which can outshine a lot of the stars, so they're quite easy to find. They're rare, but they're easy to find when they do erupt. Then you get an independent measurement of the velocity and the distance to a far away galaxy. And then you can put it on this Hubble diagram and they found that there's a deviation and that that deviation suggested that our universe was accelerating. The question is, we all know if something is accelerating, where is it getting the energy from to accelerate?

34:55I mean, you press the pedal of the gas pedal of your car, you have to supply gas to accelerate your car, right? And so the question is, what is supplying the energy that's needed to the universe? And they're like, hmm, how about dark energy, right? So that's really, that's where the concept of dark energy comes from. And we've not had like a, you know, good microphysics explanation for it. We just, again, it's come out of the phenomenon, right? It's come the measurement that it's required. The question is, what is it? When does it get started? When does this acceleration? Does the, over time? And is it uniform in space?

35:42We believe it's uniform in space. Now, very recently, so for about two decades, we believed in the simplest version of the model, which is that the acceleration is constant over time.

35:59And the overall speed of expansion velocity of the universe depends, interestingly, and this is Einstein's brilliant insight, but it depends on the contents of the universe. So as the contents of the universe shift shape, start out with a radiation-dominated universe, and you move into a matter-dominated universe, the universe, you know, at the time when you start making all the galaxies and everything, right, with each of those stages, the belief was that dark energy remained the same, even though these stages were evolving. But very recently, that assumption has been called into question with new data.

36:40Hints, very tiny hints. It's unclear whether that is really pointing to something. It's so cute. You have both a cat and a dog. They're walking around. The podcat is what she's called. Oh, she's called the podcat. Okay. Yeah. And I can see she's very interested in my explanation for dark energy. It showed up right there, you know, all agog. But yeah, so I think that, you know, again, dark energy is another ill-understood component of the universe. But then, again, the intriguing thing, right, and embarrassing thing, if you will, is if you figure out how much dark energy you need to have in the universe to account for what we're seeing, what we're measuring with the supernovae, for example.

37:28and other techniques, there are multiple other techniques, then it dominates dark matter. So it's like every other uncomfortable thing we are finding in the universe seems to be even more dominant, right? So first you had dark matter that dominated over all matter by almost a factor of nine to 10 times. And then now you have this other invisible entity that we don't know when it originates in the universe. Is it part of the future of our universe? Whatever. And that is even more dominant than dark matter. So that's where we are. I'm really enjoying kind of getting to see how the scientific process works in the context of the dark matter problem.

38:17I know we're jumping back to dark matter from dark energy for our listeners. But so you described the anomalous data and then the proposal of dark matter. Where does numerical simulation and then theorizing, where do these get involved in the story? Yeah, so I think this is a very interesting ongoing debate. So I'm interested in the history and philosophy of science because I do have a dark past. when I did start a degree in the history and philosophy of science, and I have an unfinished PhD. Oh, interesting. Yeah, I mean, I was at MIT in the program in science, technology, and society. I was like young and foolish, and I thought I needed two PhDs.

39:05You know, I wanted to write. I always wanted to write books, and, you know, books that were for the popular audience, right, for not a professional scientist. And I always thought that I somehow had to have, you know, the legitimate chops of a humanities PhD for that. I realized that I didn't. But also science really pulled me back into its orb. So I think where the simulations come in are that once the argument has been made for the existence of some other kind of mysterious particle, Simulations allow you to put that in by hand and then see how they could get manifested. So, you know, you could evolve a simulation.

39:55And now the simulations are so sophisticated that, you know, we don't understand star formation in great detail. But to whatever detail we understand, we can add in a recipe. Literally, it's a recipe. So you add in all these recipes. You start out with an assumption about the dark matter particles, their nature, that they're code, they're collisionless, they interact only via gravity. And then we can add an ordinary matter, which we know interacts via all other forces, interacts with light and so on. And we can evolve this to late times. And then we can see, do you form any kind of galaxy that looks like anything that we see?

40:32And it turns out you do. That you can recreate the visible universe. And so you can then go and tune and say, okay, what do I put in? What do I get out? So there's a big open philosophical question about these simulations. Are they generative of discoveries or do they just ratify? Because it's what you get is what you put in, right? And I think the jury is really out because one of the things that a simulation does is it propagates all these phenomena forward in time and space. and it's not all straightforward and linear, right? So there's lots of nonlinearities. There are ways in which things couple that are hard for us to see even when you write out all the equations, right?

41:15And so you have things, you have phenomena and behavior that emerges from that that was not predicted just with the equations. So I think I am on the camp that leans towards the side that simulations can be generative of discoveries in terms of the scientific process, which is sort of what you're interested in. And then there are people who believe that they're mere tools they really cannot generate. And what's exciting, you may imagine, is the same kind of debates we are now having about AI. It mirrors that same debate. We've been having this debate about simulations in our field for a couple of decades now.

41:57And now you can see that we're having exactly the same kinds of debates about, will AI just facilitate our discoveries and accelerate them, obviously? Or is it going to generate something that we as humans could not have? And I think, again, the jury's out. We'll have to see. We'll have to wait and see. for, I just find that it's a very exciting, unsettled question. Very exciting, possibly terrifying, but exciting no matter how it turns out. But, okay, so it seems like, okay, dark matter data, then theory, and then it seems like there are a few different branching pathways. ways so the theoretician might jump straight to modeling just to see what happens without making a prediction or perhaps the theorist could come up with a theory then make a prediction and then look to a model to see if that prediction is confirmed or they might look to observation to see if the prediction i think the dream for someone like me right that they're interpolating the mind that draws connections, right?

43:15The dream is to be able to do this full arc. Start out with a particle, you know, particle physics, small scale, actual particle with attributes, with properties, then build a model with those specific properties of that particle, how it's generated in the early universe, where it's generated, how it could be the dark matter, and then put that into a simulation, run it forward with a lot of the unknown physics that we don't understand very well, but still we have some recipes, then come out with a slice of the universe and then compare that to the data. So being able to do that full arc is the dream for someone like me.

44:02And so far, because of the scale, the physical scale, cosmic scales, and the particle scales, where there's a huge bridging of scales that needs to happen here, that so far where we have reached is we are using astronomical data, large cosmic scales, phenomena that are being played out, large cosmic scales of space and time. and we are able at the moment to only say that it's consistent with a certain kind of particle and its properties or not. We're not able to say, you know what, these things are going to lead precisely to that particle. That's the dream. We're not there yet. And I think there's some, this theory of the cold dark matter, cold collisionless dark matter, is actually quite to mature.

44:57So in the sense that it has huge explanatory power and on large scales, on large scale phenomena, and in the very early universe, it has enormous explanatory power. So, you know, people have been looking for alternate models, right? I mean, you know, physicists are always going to do that. They're like, okay, no one's going to say, okay, this is it. It's done because we've not found the particle. So they're like, you know what? There's room. Let's try to find another model. It's very hard to build other successful models. This model is extremely successful. So what we can do, and which is what me and my research group are doing now, is see, okay, if the data gets better and better in one realm, for example, with lensing, right, does the model break down?

45:39I mean, is there a point where the model really has trouble explaining what we see as the data gets better, right? Higher precision data. Can we stress test this model? And so that's what, you know, I've started doing. I've been trying to do it for a few years because Because it turns out that my pet choice of technique, gravitational lensing, is very well suited to doing the stress testing. And the kind of data that you get out of lensing has become much more sophisticated. Our instruments are able to capture real fine features of gravitational lensing. So, for example, we can now see gravitational lensing is just like the bending of light into a lens like the concave glass lenses we're all used to.

46:23It's just that instead of glass doing the bending, all matter does the bending, including dark matter. And that's how we map dark matter, basically. It kind of captures Ben's light. And the way that you manifest it is galaxies that are behind an aggregate of dark matter, mostly, will look very mangled, stretched out in very specific patterns that we know and can calculate. And we see that already. So that's what is pointing us to the fact that, ah, there's dark matter there. I can see these mangled galaxies. So the data has now gotten so good. When I started in the game, the models were much more sophisticated than the data.

47:05The models we could build for how the dark matter is spatially distributed, you know, the hills and the valleys. They were much more. The models were very sophisticated relative to the data. Completely flipped about 15 years ago. The data is so much better, and we've had to up the game on the sophistication of our models. And so, for example, now we are able to see lenses that are embedded inside lenses. So there's like, you know, meta phenomena, right? And that's how good the instruments are. Like our cameras are on the Hubble Space Telescope, you know, the advanced camera for surveys, James Webb.

47:41You know, it's just like, it's just remarkable what we have now in terms of technology. Yeah, you sent me a paper that recently came out that talks about some of this data, and I'd like to get to that in a minute. But first, I'm glad you brought the collisionless cold matter back into perspective, because I just want to dig back into one more aspect of this theory prediction simulation pipeline. And that's, you mentioned earlier, the standard model of particle physics, or one of us mentioned it. And this is our extremely successful theory that at least can conceivably describe everything in our known universe that we interact with.

48:34And what I'm wondering is, when you start your theorizing, are you looking for, are you trying to exhaust the low hanging fruit that are possible in our current theory of physics? So the standard model, and it's only once we've really exhausted this, that you're going to look for very exotic explanations. Absolutely. Absolutely. You nailed that. I mean, the idea is, you know, it's also Occam's razor, right? You look for the simplest form of self-consistent explanation. So you start out assuming that the standard model holds, and then you look at the stable of particles. So as you know, standard model phenomenology is quite rich.

49:16So there's more than one candidate that could potentially be the dark matter. So you first look at that stable of particles that's predicted. And as you said, the standard model particle was extremely successful. The only thing is that we do know there's like a missing piece, right? We do know that we don't have a microscopic understanding of gravity. So we don't have a quantum theory of gravity. We have, you know, at the quantum level, at the deepest level, we have a pretty good understanding of all the other forces of nature, but we don't of gravity. That's well known, you know, with people I've been looking for and string theorists have been trying to connect and come up with a quantum theory of gravity.

49:56And I think there's been a lot of progress in that field, but it's not a solved problem. But we know, as you said, that otherwise the standard model particle physics is really rich. And we start out with picking particles that are permitted within the standard model. And we go beyond it only if needed. And the reason we've stayed somewhat cautious is because we haven't simultaneously, remember, we are doing experimental searches for this dark matter particle. And we've come up empty, right? So there are these large vats of xenon where basically you have the, you think of it as a lattice, right?

50:33The crystal lattice. And you're waiting for a dark matter article to go through and jiggle that lattice. And, you know, we've been working very hard to remove all other sources of the jiggle, you know, passing by. It's at all these underground experiments, you know, sealing the noise from terrestrial stuff and so on. But nothing, right? We really haven't found anything still. So the approach has now been, okay, let's move on within the stable of standard model particles. And then also open ourselves to the idea of what if there is some slight modification to the standard model. Like a small tweak, nothing fundamental, that would leave everything else intact.

51:16Because everything else works really well. And a small tweak. And might that work? And so we have opened our minds to that possibility. And, you know, and that's a kind of dark matter particle that in addition to gravity might interact with itself very weakly. You know, very weak self-interaction. Because otherwise it was only the mass and gravity that was driving the interactions of the cold dark matter particle, the collision list. So this particle would have some collisions, which means that in regions of high density where they could collide, they're close enough to collide, they would aggregate more than cold dark matter would.

52:01Okay, that gives me some new questions to ask. Awesome. Go for it. Well, first I'll just ask about that. So would that mean, because I know that you have fermions and you have bosons and bosons are the force carrying particles. Would that mean that instead of there being just one dark matter particle, there would be two new dark matter particles? And then in addition, you might refer to that one as a new fundamental force. there's a new way to sweet green meat wraps handheld hearty and made for life on the move with bold chef crafted flavors fresh ingredients and over 40 grams of protein they're built to satisfy without slowing you down try wraps today in the app or at order.sweetgreen.com available at all participating locations

52:58it would it could be yes so i think what what we believe is that there could be um a new class of particle in addition that has these self-interactions and would be like a very weak kind of you know similar to the weak force it would be like the weak force and that because remember cold dark matter just as the standard model of particle physics is so widely successful in so many domains, so many scales. Similarly, you know, the cold dark matter model, the collisionless cold matter model, is widely successful on large scales. So we don't want to lose that. It's pretty clear that the properties needed to explain the large scale successfully is cold collisionless, right?

53:44And the anomalies that are showing up when many of us are stress testing these models is in the smallest scales, in the innermost parts of galaxies. And that's where it looks like it can, it needs, and it likely aggregates more than cold, dark matter would. So yes, a second class of particle that, or, you know, I'm agnostic, right? Or one particle whose interactions switch on and off. it's the same kind of particle that remains collisionless and cold in most places, but in very crowded, dense places, it has a new kind of, it switches on a new force, right? I mean, we've not yet found such a, I mean, this is what I'm dreaming of, right?

54:31I mean, this is the kind of, you know, trying again for simplicity, right? We are so attuned, this idea of, you know, the opening ourselves up to the idea of going from one single dark matter particle to two, is a huge shift in perspective. And because then the question is, oh, then if two, why not three? Why not four? OK, let's start with two, right? But that's a big barrier. And so many of us would like to see, well, maybe it's not quite the particle we thought. Maybe it's the same particle, but it has a new interaction that gets switched on under certain conditions, right? I don't know. We've not been able to converge on such a model.

55:13So at the moment, I think it would be just that we want to be, I want to push people to open their minds to the possibility of two particles. Well, it is certainly a unique problem that you're on the one hand constrained. I mean, I don't want to say you're totally constrained by the standard model. But I mean, it is kind of the arena in which you're trying to work in. But despite it's being so successful, it doesn't explain gravity. and that's like the one force that you really need for dealing with dark matter. And it exists. I mean, you know what I mean? Like there's incontrovertible evidence for gravity, right?

56:03I mean, yeah. No, I think, you know, that's what, so those are the exciting sort of intellectual conundrums in the field. What this makes me wonder, though, is if you are particularly interested in theories of quantum gravity, like string theory, because perhaps doing string theory will help give rise to new candidates for Darth Manor? Yeah, I think so. And I think string theory is really not at the stage where there's like a testable prediction that we can link to. But I think for me, right, what this makes me feel super optimistic is that, you know, the other class of objects that I'm obsessed with are black holes.

56:49You know, black holes and the vicinity of black holes might be where some unique phenomenology, particle phenomenology, could be additionally tested. So for example, I think that if there is a different kind of interaction, right, or there are two kinds of particles or something, maybe there is something that will show up, you know, at the event horizon of a black hole, right? And because black holes sit, these supermassive black holes sit at the centers of galaxies. That's where the dark matter we are finding is very dense and it's heaped and it's there. There's a lot of it, right? And of course, a black hole is a very tiny, tiny, tiny place compared to the distribution of dark matter.

57:36Even these horizons of black holes are really small and compact, but they could be a unique laboratory. So for me, that's what's exciting about the themes of all the objects that I work on, like dark matter, dark energy, and black holes, that it may turn out that black holes might be a very interesting laboratory, hitherto not explored enough. And that's, you know, something that I'm also very actively working on, you know, trying to understand if, you know, even when black holes form or as they grow and evolve, can they serve as, you know, exciting laboratories, even for a period of time, maybe not all the time, but, you know, for some snapshot in time, for example, you know, when they form.

58:20Could that somehow give us a clue to the aggregated dark matter that is very much around where they're forming, right? Which is likely the center of a galaxy. so I think you know there's this other very interesting idea it would be remiss of me to not mention this with all this conversation about dark matter which is you know proposal by Hawking in the 1970s where he suggested the idea of all dark matter actually being primordial black holes so black holes that were made so little kinks in space-time that would naturally result in the very, very, before matter forms, before, I mean, right after inflation, you would have a lot of kinks in space time, essentially black holes that would be in place that would then just sit around, do nothing in a radiation dominated era, just be hanging around.

59:16But then once the universe switches to matter domination, you have all this hydrogen, these black holes would start gobbling up the gas and growing and maybe seed the formation of galaxies. So this whole idea has been reactivated recently. And I'm, you know, I'm a fan of the idea. I don't think we have a workable version yet, but it's totally worth exploring. And some of my work has also been exploring the, you know, primordial black hole idea. And that would be like, you know, I remember I told you at the start of our conversation that in a way the dream would be, so there's dark matter, there's dark energy, there's black holes, sort of the three big invisible pillars of cosmology of our current model.

1:00:03And if we can even link any two of them, that would be just so awesome. Any two, black holes and dark matter or dark matter, dark energy. And of course, we can unify the trifecta. Wow. Yeah, that would really be great. your new paper that you told me about new cold dark matter crisis revealed by multi-scale cluster lensing you mentioned that there are developments in gravitational lensing and i don't think i've heard you use this this term yet in our discussion what is multi-scale cluster lensing right so cluster lensing is just the bending of light by large clusters of galaxy These are the same clusters that Zwicky was studying.

1:00:48And these are more distant clusters. There are about a thousand galaxies that are held together by gravity of dark matter. And they are the biggest, most efficient light deflectors, right? And so we see these very distorted arcs. And we have now figured out a way. And, you know, a lot of my PhD thesis work and work since then was, you know, developing the algorithms that would allow you to look at the mangled shapes of galaxies and back out the spatial distribution of dark matter you need to have there in place to produce that because you see the light already so you're like what more do we need we know we need dark matter and how much dark matter and where spatially do you need it to produce the kind of distortions that we see right so that's the kind of work that i was this sort of cartography was what i was doing but then the data got better and better.

1:01:40And the Hubble Space Telescope looked at six of these clusters. It was called the Frontier Fields Project. Very deep look for a very long time, 160 orbits. It was looking at the same place, right? And what it found is that you see these arcs, large-scale bent shapes that we can all see by eye. When you go to a Hubble image of a cluster lens, you see that by eye, sort of tangentially see these distorted arcs. So when they looked really deeply, they found that there are many versions of the same distortions on very small scales. There are little arcs within arcs. And that is telling you, in order to produce these arcs inside the big arc, it tells you that you need a real heap of dark matter there, concentration of dark matter.

1:02:30So when I said we make a cartographic map, I mean it literally. So we can actually map out the peaks and valleys in the distribution of dark matter. And what the new data was showing is that there are very small, spiky regions, regions where there's a lot of dark matter concentrated. And because they are producing those little lenses within lensing effect on very small scales. So then you can ask, OK, I have a cold dark matter theory that works very well. let me go back to my simulation of cold dark matter and see, can I reproduce what we're seeing? Can I aggregate dark matter, concentrate it, and make really efficient small-scale lenses to sit within the big lenses, right?

1:03:19And it turns out you can't. It's very, very hard. There's an order of magnitude discrepancy. You just, in the cold dark matter universe, the small-scale matter distribution is just not peaky enough. It's kind of fluffier. And so you do not produce the right number that are seen. So the question is, what's the solution, right? So in a very agnostic way with one of my students, Isaac Dutra and Yaron Takeo, both of them, we looked at, okay, what do we need to do to dark matter, the standard dark matter, to make it conform to what is seen? So what we need to do is to heap it more, heap it in the center more.

1:04:03But, you know, gravity alone will not permit that kind of heaping. You have to give it some self-interaction. It has to interact and have some kind of pressure and kind of, you know, collide. And then it becomes peakier. So we saw that a couple of years ago. We saw a hint of that. And we're like, that's interesting. Let's see if this holds up. Maybe it's one peculiar cluster or a handful of them. So it turns out now we've looked at a dozen or so clusters, and this discrepancy remains. Cold dark matter cannot explain the phenomenology that's seen on small scales. So then we said, OK, let's do the data is good enough.

1:04:45Let's do a stress test, and let's see what else we can probe about the nature of dark matter. So we said, oh, we have a couple of handles, right? So first, as I said, we do a cartographic map. We can just say, OK, does core dark matter, collisionless dark matter, give you the same number of clumps of dark matter? Because, you know, dark matter is distributed in a smooth kind of way. But then there are also these clumps, these peaks, right? And yes, the number works out. Then we're like, oh, does it give us the right number in the right place? Do we get the same landscape map? No, we don't. So the inner parts of galaxies have fewer peaks in cold, dark matter, whereas in the real universe, you have a lot of peaks in the center.

1:05:29So the radial distribution is off hugely. But the number of clumps, so the total number of peaks works out fine. But, you know, we are looking at a large region, so it works out. But where those peaks are is off. They're not in the places that the data is suggesting they are. So then we said, okay, let's look at the lenses within lenses. So we came up with these four metrics. We're like, okay, let's see if it checks off, right? Does cold, dark matter work for this? Can it explain this, that? So yes, it works for what we call just the abundance of clumps, small-scale clumps. That it checks out.

1:06:06Cold, dark matter works. For the radial distribution, for those very same clumps, it doesn't work. And, you know, doesn't match at all. Then we said, let's look at the lenses within lenses, because now we have really good data, and it still doesn't match. There was always a discrepancy. And we're like, oh, one other thing we can do is when we do this lensing cartography, we are able to look at the outer extent of the dark matter halo. You know, the stuff you were talking about that, you know, the misconception is you have just the dark matter and the halo. So here you have the dark matter heaped in the center, but it does extend.

1:06:40and in the cluster we have a measure of how much dark matter is sitting within sort of an aperture of a certain size so it's like a proxy for the extent of the dark matter halo and we're like okay let's go look at that metric that diagnostic and see how cold dark matter works out and it works out well so this complete paradox so there's some things that standard cold dark matter still explains fine. And there's a couple of things that it just catastrophically cannot explain. And so the question is, when you are faced with such a paradox, right, we know from history that you are likely standing at the precipice of something very interesting, one of two possibilities.

1:07:26One is, and you know, and you make a really nice analogy, because you know, but you are also sort of a history of ideas buff, right? That, you know, way back when Uranus, the orbit of Uranus within our own solar system was slightly wobbly, Uranus takes a very long time. So it took a while for people to do this kind of test of Newton's theory of gravity, of universal gravitation. So when they mapped out enough of the orbit of Uranus, they said, hmm, it doesn't match with Newton's predictions. It was a slight anomaly. And Urbain Laverrier, a French mathematician, said, aha, the reason for that is there's actually another gravitating body just outside, Neptune, and that is tweaking the orbit.

1:08:06And so he predicted mathematically and said, this is where we should see Neptune. People went out, they saw Neptune, there we go, Newton's laws restored. The only thing is you needed to refine the theory a little bit, right, at work. Similarly, there was an anomaly in the precession of the perihelion of Mercury. And he said same thing. He said, ah, you know what? Actually, there is a planet between the sun and Mercury. He called it a Vulcan. And he said, that's what's perturbing Mercury. Same explanation that he gave. He said, oh, that's what it is. It turns out there is no Vulcan. People looked for it.

1:08:45There's no Vulcan. So the same explanation doesn't work. And as we all know, like 100 years later, 1915 although Einstein did not set out to solve this problem his general theory of relativity showed us that that's the answer a complete reformulation of gravity is what was needed to explain the mercury precession anomaly right so I don't know when you find an anomaly this is what I find super exciting right and I know I'm often teased by my colleagues they're like what do you mean you're looking for gaps I was like yeah exactly I am looking for gaps gaps between theory explanation observation right and because you never know if you're in the Uranus Neptune situation or the door is opening slightly we don't know where this is going but to the Mercury situation right and so either there is something fundamental about cold dark matter itself that we have not understood like maybe you know there's some self-interaction start to pick in at some point or something, or there is a brand new particle or two particles or whatever.

1:09:57I mean, to me, this is, I'm super excited about this paper. It's a little technical, but I think the bottom line is that it brings us, it forces us and puts it at a very interesting precipice of, you know, it could be this or that, only two options, right? So either we refine our current theory or we have to make room for something new. The examples of Vulcan and Neptune are perfect because I was going to ask, is this going to be constitutive of a paradigm shift for your field or is it just going to be challenging certain assumptions of the model that you currently have? I don't know. Do you lean in any particular direction?

1:10:41Well, you know, because of who I am and how my own mind works, I would like it to be opening the door to something brand new. Personally, right? Obviously. It would be so cool to have been the person that forced the field kicking and screaming to give up like a long held version, you know, explanation and to move towards something new. I've not offered the option, though, like I've not come up with a particle. I'm working on it, but it's hard. As I was saying, it's very hard to make an alternative theory, especially as you pointed out, something where you want to have go from soup to nuts. So you want to go from the particle itself all the way across scales to explain everything, its role in galaxy formation, large cosmic scales, late times throughout the age of the year.

1:11:36It's daunting to make, you know, you can make some, you know, you can make a model that's very contrived that, you know, is not looking that great. So I don't know. I am fascinated at the possibility. At the very least, if this work can allow people to start thinking about two particles, that would be great. I don't know. I'm assuming you get these emails, but due to the nature of this endeavor I'm in the midst of, I get emails every day from people who have new scientific theories. No mathematical or scientific training, but they are convinced they have solved quantum gravity or dark matter or dark energy.

1:12:20And I just don't think these people realized how difficult and how sophisticated the mathematical apparatus has to be for you to be convinced you have a workable theory. Yeah. And I think, you know, I actually I have to say, so I have a whole range of people who write to me. I do get, I don't know what it is about the fertile minds of retired engineers. I think a lot of them wanted to be physicists. So, you know, they do have the mathematical capacities. and you know i often get like elaborate sets of equations and new kind of um ideas on uh how how things would work in the very own universe and yeah i mean i haven't found i do have to confess i do i do quickly look at them i do not immediately abandon everything i look at everything including the cookie ideas, right?

1:13:17I do. But I haven't seen anything that triggers something in me, right? As you said, you know, for someone who has all the experience and the intuition and everything, nothing yet. Nobody has suggested something that is sparked either. And yeah, most of them are, you know, unworkable ideas. And one of the things I started doing is that there were some, you know, earlier in my career when I was a junior faculty member, Or, you know, I was already working on this, you know, there would be press releases and people would know what I'm doing and whatever. And I would get a lot of these letters and emails.

1:13:52And at some point I decided the very persistent ones, I would get them talking to each other. So, you know, a lot of the time they're looking for community, right? They're looking for someone to interrogate their ideas. And I felt that, okay, you know, here you go. Here's another person who has another idea. Go for it. Right. And it was one way to deal with the volume. Hmm, that's funny. But, you know, while we're on the topic, though, of these speculative ideas, are there other alternatives beyond super tiny black holes or self -interacting particles that you think are plausible but exotic to account for the data?

1:14:37Yeah, I think there are lots of potential. There's a huge particle zoo to choose from. I think the one effort that does merit mentioning is an idea called MOND, which is Modified Newtonian Dynamics. And it's a very elegant idea, but in my opinion, it's not workable. We've not been able to get it to work. I'll tell you in a minute, it's actually lensing that is sort of the nail in the coffin of that idea. It cannot explain lensing, and we see it. So this is an idea that suggests that perhaps Newton's laws of gravitation, the 1 over r squared, the force falling off of 1 over r squared, maybe that needs modification on cosmic scales.

1:15:30That maybe that's something in our day-to-day life on the scales that we operate seems to work, Although recently there was a very nice paper where they showed that the one over r square works on cosmic scales. And there was a test. But anyway, prior to this, these guys, you know, it was Moti Milgram, Beckenstein and Milgram who came up with these ideas. And what they suggested was that maybe the acceleration, because, you know, the force is F equals ma, and you can think of a force and, you know, equivalently as the acceleration it would generate, maybe that acceleration depends on scale. So that, you know, the force is very different for the stars that are holding a galaxy together than for the galaxies that are being held together by a cluster.

1:16:21So there are different physical scales, right? So cluster is much larger scale, galaxy is a smaller scale, relatively speaking, and maybe that's what's going on. So that was an interesting alternative. and lots of people are still working on it, trying to get it to work. One of the things, one of the phenomenology, so the problem when you have a very mature theory and you want to find an alternative is that the new theory has to explain everything that the current theory explains and more that make predictions that would allow it to become testable, right? And say, okay, this is how you validate a new theory.

1:16:58So the problem with this theory is that they were unable to actually account for lensing, gravitational lensing. Because in order to account for the light bending, they had to say, well, you know, maybe we need some more particles hidden. Hey, but then that's dark matter, right? So I think, but that was a, it's a beautiful exercise, right? It's a valiant attempt, not successful yet. But as for other exotic things, yeah, sure. I mean, speculative things, there's room. But again, as I said, the rub is always having to explain all the phenomenology that we have in hand, the data that we see across wavelengths, right?

1:17:40So we have multi-wavelength data now. And make a prediction that would allow the theory to be really testable, to be either validated or not validated. it. So, but, you know, I think for me, what has been exciting, right, is that in this other area that I work in is the formation of the first black holes. I actually had a model, you know, several other people. I published a first set of papers with a postdoc of mine, Giuseppe Lodato, 2005, 2006, where we said, okay, is there a way in which you could bypass the formation of a conventional star, because we know a massive conventional star would live its life, die out, and leave behind a little black hole.

1:18:24So our idea was, because I'd always been thinking about this, right? Is there a way to circumvent that cycle? Is there another way to make a black hole, right? So whether direct collapse of gas. So this idea was direct collapse of gas. And it took us a long time, but we remember that arc I was telling you that when you come up with a new idea, there's a huge arc that you need to follow through. and make an actual testable prediction that some instrument will be able to detect and say yay or nay, right? Did you find it? Did you not find it, right? And so I was very lucky in this one instance, right, that there's now growing evidence for direct collapse as a mechanism to form black holes, not just in the very early universe, but maybe even in the later universe.

1:19:09So I think that one object, it turns out, there was a first object that gave us the hint and then now we are seeing that phenomenon everywhere. But you have to go, and it took us, in 2017, I led the paper, in which I was able to make concrete predictions. The idea, the speculation was published in 2005, 2006. 2017 is when we could make a concrete prediction and say, if James Webb were to fly, remember at that time, 2017, James Webb was being canceled every few months because it was over budget, over, you know, we didn't know if this was going to fly. Also, there were, you know, even when we launched, we were all waiting with bated breath, right?

1:19:53So in 2017, we were finally able to say, okay, here is a spectrum that James Webb should see if this modality of making a black hole works at all in the early universe. It was very specific, but it took us, it took us more than a decade to get there. So, you know, coming back to this question of the dark matter, right? And of course, I'm thrilled it was validated. You know what? I'm, you know, walking on air for the last few years. But this is what we need. And we don't have this. And as you can imagine, right, I'm addicted to this. I want to do the same thing. But we're not there yet. I hope we get to it in the next few years.

1:20:35I think that's the challenge for dark matter to go, as you said, from the particle model that would explain all of these anomalies and predict something new that could be detected. Presumably in lensing, right? I think lensing is just like, you know, it's the gift that keeps on giving. I am so thrilled to be working, especially strong lensing, because strong lensing is such a clear cut observational phenomenon. There's no ambiguity. There's no, you know, weak lensing is slight distortion in shapes of galaxies that is seen, but it's so slight that there's so many other things that would cause those shapes being, you know, mangled.

1:21:17This is like, you know, real dramatic mangling of the shapes. There's no other explanation that you can give. So I am really hopeful that strong lensing will once again prove to be, you know, a litmus test for some new kind of model. your your excitement about this is really infectious and it makes me loathe to look at the clock and see how much time we have left but but there are a couple things that i still want to make sure that i ask you one that is as we've been speaking about dark matter i mean dark matter it's something that nobody anticipated before i don't know within the last until the last hundred years it's not something that it's something that we still can't explain it's highly non-intuitive it's highly not trivial and yet if i'm correct i think i am if dark matter didn't exist then neither would we and exactly we wouldn't be here absolutely right and i'm wondering how you think about the fine-tuning problem i recognize that it's a totally orthogonal to what we've been discussing but since you work on one of these highly sensitive but highly important variables or constant whatever you want to call in in our world how do you think about it well you know i'm a little bit of um escapist in that sense in the sense that i am perfectly open to the i mean i want to wiggle out of that problem because it's a really hard problem and but i like the wiggle And the wiggle is that, you know what, the multiverse.

1:22:59There could be other universes with, you know, we need exactly the values of the fundamental constants to be like what they are, including dark matter, everything, including black holes, actually. We wouldn't be here if there weren't black holes either, right? So we need all of this to work out, to be here, to be able to ask this question. and my kind of the kind of explanation that appeals to me is that well it's likely that there are multiple other universes which have different combinations of these parameters and that where things have played out differently and that we just happen to be one and in a philosophical and spiritual way it actually appeals to me too so it's not just you know escapism in terms of not being able to explain it so i'm kind of happy to take the easy way out i mean i i think that if working in cosmology has taught me anything it is the sense of you know sort of cosmic humility the idea that you know is this again it's a kind of a a paradoxical thing right so we as humans are kind of super significant because we've you know with the brain the size of a cantaloupe, this jelly in our heads.

1:24:15We figured out all of the stuff about the universe. And, you know, so we are significant in that sense that we've made sense of all of this, but we're ultimately pretty insignificant in the grand scheme of things. Right. And so to me, there's no reason why we should be the only universe. Right. I mean, to me, it's part of that, the openness to have, okay, maybe there are many other universes and that we happen to live in one that has the right conditions that with the properties of dark matter and black holes to make life possible and this kind of life possible and so on. And I think that philosophically, the idea, you know, this idea of where the initial conditions came from is really a big open question.

1:25:06Not just why this universe, but why a universe at all, right? And as Derek Parfitt said very nicely, why is there something rather than nothing, right? I mean, these are sort of, they're the big open philosophical questions. And I think that to me, given that we've not gotten traction on that one, an attractive explanation is that, you know what, this multiplicity of possibilities, is perhaps infinite too, and that we just happen to be one. And, you know, so also, I'm not particularly religious, but I was brought up as a Hindu and one of the philosophical schools of Hinduism. Hinduism has six philosophical schools predominantly.

1:25:52And one of them is an atheist materialist school. And it's called Advaita. And that's one in which, you know, there's a concept of infinity and infinite everything. and it just appeals to me because it doesn't seem kind of too crazy an explanation. I lack the requisite training in cosmology or high energy particle physics to have an opinion that anybody should care about at all because it's not really justified in any rigorous way but I also find the idea of a multiverse where the universes are just like very spatially distant from one another not that they're like different dimensions or slightly causally disconnected like the many worlds multiverse I find this one to be much more palatable and I can go to bed at night thinking that that's a possibility that explains the fine-tuning problem so I'm glad to hear that is where you've ended up.

1:27:01Yeah, I mean, I think it's a very logically comfortable place too, right? I mean, as you said, it lets you sleep at night. Yeah. Well, the last thing that I will ask for today, I know we spent a lot of our time talking about dark matter, but you did bring black holes into this here and there. I'm wondering if in recent years you've had any sort of thoughts or done any research about black holes that you think have been similar or that you think has been similarly, I don't know, paradoxical or groundbreaking to what we've discussed about the crisis in dark matter. So, you know, I think the idea, personally, this idea of the idea of direct collapse and the fact, as I mentioned earlier, right, the having followed through on that idea, starting from, you know, a speculation where basically started out showing that, you know, the physics will permit it.

1:28:07the laws of physics will permit something like this from happening. Then the question of, can it happen in our universe? Do we have those conditions present in our universe for this process to happen? Then the question is, when and where can this happen in the universe? Then the question was, how is it testable? So that whole arc, I mean, I think was super exciting to be sort of part of. and I remember the early days before I was able to make a concrete prediction that would really touch observations that there was a lot of skepticism in the field and even now there's a lot of pushback, right?

1:28:45There's a lot of pushback but that's science. When people are always skeptical they want to interrogate an idea. I mean, look at me, I'm interrogating cold dark matter, right? It's been established. It's been established for 30, 40 years and I'm still interrogating it. So that's part of the course, right? You propose but the dream of every scientist is to be witness in one lifetime to that entire arc. And that's what the pace of discovery in our current, that's what makes it special, right? Makes it very special to be alive now when it's possible, not just for my idea of direct collapse black holes or, you know, how black holes were born in nuclear star clusters or how you can make black hole seas throughout the age of the universe.

1:29:27All of these kind of new ideas, right? What is special about it is at this time, the pace of discovery. So it's this convergence of the sophistication of our understanding of the physics and mathematics, computation, and the instruments that are getting the data. It's a real grand convergence right now. And therefore, it's possible that within a scientist's career and lifetime, you can fill this arc. It can be closed. So I just am really thrilled. And yes, the idea of drug collapse, and there's a couple of other things that are brewing right now that I'm really hoping to kind of push along the arc and make them kind of testable.

1:30:12Because in a way, right, that has been really the motivation for me in my career. I've not particularly cared about, you know, the accolades and recognition are just wonderful. I'm deeply grateful for that. But it's about the ideas, about seeding new ideas and persuading people to think differently, to kind of push people away from what we have done so far or, you know, locked into current paradigms to kind of even push a little bit, right? Even if it doesn't completely break it or whatever. I would, you know, like to think that that's success for me. and it comes at a huge price. You pay a huge price when you come up with something original.

1:31:02People really push back. And also, the reality is it's a competitive field, which is not, sadly, not as well-funded as it should be. And right now, with the complete loss of perspective of how important basic science is and all the cuts, it's kind of sad. But I think for me, the success, intellectual success, is just this legacy of kind of, you know, being the person who pushed everyone to think slightly differently. And I'm thrilled to be alive now to be able to do that. And it's just a great time to do science. And a great note on which to end. I've been looking forward to this conversation for a couple of years now.

1:31:51And maybe in a couple of years when some of these developments and black hole theorizing have come to fruition. We will sit down and I would love to do this again. Absolutely, absolutely. Thank you so much for speaking with me. Would love to come back. And thank you so much, Robinson. I think I have found your questions to be really searching and coming out of a place of great curiosity and as well as understanding. So you really are not as uninitiated in physics as you claim to be. okay thank you so much that's very sweet of you to say