96 | Lina Necib on What and Where The Dark Matter Is

The past few centuries of scientific progress have displaced humanity from the center of it all: the Earth is not at the middle of the Solar System, the Sun is but one star in a large galaxy, there are trillions of galaxies, and so on. Now we know that we’re not even made of the same stuff as most of the universe; for every amount of ordinary atoms and other known particles, there is five times as much dark matter, some kind of stuff we haven’t identified in laboratory experiments. But we do know a great deal about the behavior of dark matter. I talk with Lina Necib about why we think there’s dark matter, what it might be, and how it’s distributed in the galaxy. The latter question has seen enormous recent progress, especially from high-precision measurements of the distribution of stars in the Milky Way.

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Lina Necib received her Ph.D. in physics from the Massachusetts Institute of Technology. She is currently a Sherman Fairchild Postdoctoral Scholar in Theoretical Physics at Caltech, and will be an Assistant Professor of Physics at MIT. Her research spans issues in particle physics and astrophysics, especially concerning the nature and distribution of dark matter, as well as techniques for detecting it and constraining its properties.

• Web site
• Caltech web page
• Inspire publications
• Google Scholar publications
• Talk on Dark Matter in the Age of Gaia
• Gaia home page

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0:00:00 Sean Carrol: Hello everyone. Welcome to the Mindscape Podcast. I’m your host, Sean Carroll. As many of you doubtless know, in the 1990s, especially near the end of the decade, the very last years of the 20th century, cosmologist began to put together a picture of the inventory of what our universe is made of. It’s a weird answer that science is giving us whereby only 5% of the stuff in the universe by energy is ordinary matter; by which we mean all of the atoms, all the particles we’ve ever detected in particle accelerators or any other experiment that kind of stuff, which includes all the galaxies that you see, all the light that you see from stars and planets and so forth, it all adds up to only 5% of the stuff in the universe, another 25% is something called dark matter, which we think is some kind of particle that we just don’t see and the other 70% is something called dark energy, which might be the energy of empty space itself.

0:01:00 SC: Now I know to a lot of people who are not professional astronomers or cosmology, this dark stuff, this 95% of the universe seems like some kind of fudge factor, just a recognition that astronomers don’t understand what the universe is doing but in fact, these are testable hypotheses and over the last couple of decades, astronomers have been increasing the precision with which we can test and talk about dark matter in particular, to enormous accuracy.

0:01:28 SC: So in fact, the life of a real astronomer is not spent thinking about, is their dark matter or isn’t there but we can actually map out where the dark matter seems to be, how it’s acting, how it’s interacting with other kinds of particles, both dark matter and ordinary matter. How dark matter can influence the formation of stars and galaxies and even more.

0:01:47 SC: So today’s podcast guest is Lina Necib, she is currently, I think currently a post-doc here at Caltech, my home institution. In the Fall, she’s gonna start a job at MIT as a new faculty member there and Lina specialises in the physics and astrophysics of dark matter, how it’s detected and how we map out where it is in our galaxy. Knowing where the dark matter is, knowing how much of it there is in different parts of space is crucial if we do eventually want to detect it and of course that’s the goal that we build a laboratory experiment here where we can see the dark matter directly. If we’re able to do that, we’ll know what it is but to be able to do that, we need to know how much of it to expect in our experiments.

0:02:32 SC: So it’s fascinating both from the particle physics’ point of view, wondering what the dark matter might be and also from the astro-physics point of view, thinking about the dynamics of stars and gas and dust and how they interact with the dynamics of dark matter.

0:02:45 SC: Remember, you can support the Mindscape podcast on Patreon, there’s a Patreon page you can find a link on the Mindscape webpage and I also like to give the occasional shout-out to people who have found other creative ways to support Mindscape. There’s a PayPal link and I don’t like the PayPal as much, not because it bothers me but because I don’t have any way to give back. If you’re a Patreon supporter, you get ad-free versions of the podcast and you also get to ask questions at a monthly AMA. If you give on PayPal, you get nothing but my eternal gratitude so thank you very much for that.

0:03:16 SC: I also wanna mention, in particular, Adrian Leatherland who made a one-time special donation to help defray the cost of the hosting costs through Mindscape, the actual service that keeps the audio files and sends them to you so you can listen to them, let’s just say it’s not cheap so I’m extremely grateful to Adrian for helping out with that cost, I’m extremely grateful to anyone who listens to the podcast, supports it in any way. It’s very nice when people send a couple of bucks my way but I just like the fact that so many people are listening to it, especially when we’re in the middle of a global mess and we have to stick together in different ways. I think that this episode is one you’re really going to enjoy, let’s go.

[music]

0:04:12 SC: Lina Necib, welcome to the Mindscape podcast.

0:04:14 Lina Necib: Thank you so much for having me.

0:04:16 SC: So we’re here to talk about dark matter and we’ll get into some of the weeds I think but let’s first assuage some of the doubts out there. Probably most listeners of the podcast will be willing to accept the existence of dark matter, even if we don’t know what it is but I know that there are some skeptics. Someone who really devotes their career to studying dark matter, what is the answer you give when someone says, “What is that and why do you think it’s there?”

0:04:43 LN: I would say that it’s a very good question. It’s one of the most important physics questions actually currently and for the past actually about 100 years now. Let’s start with what is dark matter? Basically in the early 1900s and especially in the 1930s, there have been a lot of studies in trying to understand how much mass there is out there based on for example, the dynamics of stars; the motion of galaxies, the motion of stars. You try to figure out what is their mass and then if you kinda do the math, if you calculate, you’re estimating the mass based on the stars or the amount of light that you see versus based on the kinematics. Or the motion of stars and how fast they’re going, you realise that there is a quite bit of a discrepancy there and basically the theory of dark matter is to try to figure out what else is missing or how do we fix this.

0:05:43 LN: I think the simplest way to think about it is that basically the stars in our galaxy and our galaxy is a disk spiral galaxy that you can see in a lot of the pictures but basically the stars are rotating faster than you would expect. You would expect that their rotational velocity is going to drop as a function of they’re a distance away from the center of the galaxy. This is basically another way to say the stars of the edges, you expect them to rotate a lot slower than the stars in the center. If all the mass that was there is just based on the stars that you see but what has been done in a lot of work and in particular, in the work of Vera Rubin in the 1970s is that stars on the edges are rotating pretty much at the same speed to the stars closer to the center, which is very bizarre.

0:06:30 LN: It means that either you have… You have about two ways to go; you either have to change the laws of gravity or you have to add that there is something there that has mass and it is contributing to the gravity of your system and of your galaxy but you just can’t see it and these are like the two major trains of thought here.

0:06:52 SC: Okay so what is to stop me from saying that it’s just something we don’t understand about gravity. After all, gravity is very weak and we’re talking about the size of a galaxy, we’ve never been there, we’ve never visited distances that far away.

0:07:05 LN: That’s right and so the… If you only have the dynamics, right? You can actually build and some people did build, you know, the decent theory of gravity that would explain these kind of distributions and these changes. However, there are a lot of other things that we need to take into account. So your theory has to explain/predict multiple observations or observables. For example, one of them is basically the paraspectrum of the cosmic microwave background, which is a lot of words.

0:07:41 SC: That’s a lot a words.

0:07:42 LN: And this is basically… It is a lot of words but basically what happened is that, if we kinda go back and do a brief history of time, the universe started very, very hot and then as the universe expanded, it was cooling. So we were forming more and more bound states and as we were doing that, at some point, a lot of the electrons or all the electrons actually bounded with protons to become hydrogen and then the universe was kind of like all of a sudden neutral, which means that light can go through without hitting anything. If the electrons and the protons are all over the place, then whatever, if you have light, it will kind of get absorbed… It will get absorbed by all of these particles.

0:08:26 LN: Anyways so basically, at about 300,000 years after the Big Bang, after the beginning of the universe, there was light. [chuckle] And interestingly enough, it is light that we can actually detect now. Interestingly, it’s imprints or it’s properties and you know, the fluctuations in that, what we call the cosmic microwave background or CMB, really tells us about the initial structures and this is one of the most precise measurements that we have in modern astronomy, astrophysics, actually.

0:09:01 SC: These are these famous pictures we see, right? Of like the ellipse with the green and red colors, etcetera of the background.

0:09:06 LN: That’s right, yes. The interesting thing that we can get out of the CMB is basically the origin of matter and galaxies and basically the original size of the galaxies. One thing that, a theory of dark matter does explain quite well is how much amount… What is the amount of dark matter that we can see there that was initially, that is not out of the matter budget of the things that we’re made of.

0:09:38 LN: So definitely, we know because of this very precise measurement, we know that 84% of the matter budget of the universe is part of this dark matter that is different from the state of matter, different from electrons, different from protons, different from what makes the stars and our iPhones and all of the stuff that we already know. It has to be something there. That kind of measurement is a little bit difficult to make up just with a theory and with the new theory of gravity and that’s why I work on dark matter.

[chuckle]

0:10:19 LN: And Alex and I disagree but yeah so what I’m trying to say is that there are a lot of other observables, the CMB is just one of them, that kind of push for more of this theory that there is something else that is there.

0:10:32 SC: Yeah. No, I completely agree and I think that a lot of people who are dark matter skeptics are a little bit stuck in the 1980s thinking that all they have to do is to explain the rotation curves of spiral galaxies and they can declare victory over dark matter but these days, there’s a lot more data from a lot of completely different sources that points in exactly the same direction.

0:10:54 LN: Absolutely. Exactly. I think there are a lot of these wonderful reviews that kind of… And I’ve been to a talk that actually kind of numbered them and there were, I think 14 different observables that the speaker was mentioning. All from these different mechanisms or different scales, from galaxy clusters, to smaller galaxies, to just the Milky Way, etcetera, that kind of all pointing to a direction that there is a new particle, there is something there. Maybe it doesn’t have to be like a particle, particle but there is some new species of matter that we call dark matter now and that can explain all of this. So…

0:11:35 SC: Well, that’s the other thing. Right?

0:11:35 LN: But they haven’t started figuring it out.

0:11:37 SC: Yeah, the point here is, it’s not just some stars that are dark or you know some planets or something like that. We have reason to believe that it’s a different kind of thing, is that right?

0:11:50 LN: That’s right. So whatever it is, it has very small probability to interact with the standard model. It’s kinda made out of different things. There has been a lot of these theories about yes, being exactly that, like being dim stars and these ended up being called MACHOs which stands for Massive…

0:12:15 SC: Massive Compact Halo Objects.

0:12:18 LN: Thank you. [chuckle] MACHOs. There are a lot of these experiments that actually ruled out different scales of MACHOs and that kind of tells us that it’s really… It’s not just some object that is just a star that is just way too dim and we can’t see it that contributes to the mass. Yeah, it’s something else.

0:12:36 SC: But it could be black holes, right?

0:12:39 LN: Yes it could be primordial black holes. There has been a lot of work, especially recently, on trying to understand what is the possibility or what is the parameter space so is the theory of primordial black holes explaining dark matter still possible, there is some disagreement within the community of how much of it is ruled out. The theory is not completely ruled out but quite a few chunks of it are. It depends on the measurements and there are some very small windows for example. I think 15 to 30 solar masses is still possible, something like that so it is not a completely ruled out theory.

0:13:24 LN: It also could be or it is even more possible that primordial black holes would make up a fraction of what we call dark matter and then the rest something else. The way that we think about dark matter of course, you have to think about the simplest thing first. You think it’s the one thing but honestly, it could be a composite set of things for, in the ’80s, we thought it was neutrinos and neutrinos… I know physicist who says like “come on people, we have to say that neutrinos are a part of the dark matter because yes, they are dark… ” [laughter]

0:13:57 SC: They exist and they’re dark.

0:14:00 LN: Exactly, it’s just that they’re a very small fraction, they cannot explain all the dark matter that we have.

0:14:06 SC: Well yeah, we’re working our way backwards from the weirdest theories to the most popular ones. Modifying gravity by itself doesn’t work, gravity might be different on cosmic scales but it’s not enough to explain all of the data that the dark matter does. Black holes, you seem to be saying, if I can summarize it that if there is some mechanism for making black holes early on, they could be some of the dark matter but it’s hard to make them be all of it. Is that fair?

0:14:36 LN: That’s right, yes. It’s much more difficult to make them pure, to make them all completely the dark matter. It could be a small fraction, that has not been ruled out, yes.

0:14:46 SC: And what kind of experiments? How would you know, how do you test that?

0:14:51 LN: A lot of these experiments looking for a primordial black holes are basically using gravitational lensing on different scales. Gravitational lensing is basically that if you have something very, very massive between you and an object that actually emits a lot of light, then the light is gonna get bent in different ways. What you would see in the sky are these beautiful rings, I wanna say ring that tell you that there is a mass so there are a variety of gravitational lensing on different scales, like weak lensing, micro-lensing etcetera.

0:15:34 LN: These or at least part of these methods are used to try to determine if there are a lot of these primordial black holes or these black holes around. For example, you know how much dark matter there should be and so for each mass range of black holes, you would estimate how the number that you would see and then you would kind of scan the sky and try to study the motion of stars and try to see if there is this lensing because statistically based on the number that you would expect, you would have to see some of these rings and some of these evidence for weak lensing etcetera. When you don’t see that, you can put a bound on a specific mass and a specific fraction of these black holes.

0:16:25 SC: So if the dark matter in other words were little tiny subatomic particles, they’d be kind of distributed smoothly and they wouldn’t cause a lot of these lensing events but if you lump a bunch of dark matter into a black hole, it can sort of have a bigger impact now and again and you would notice it.

0:16:41 LN: That’s exactly right. If your dark matter is made out of particles, you have to adjust and have different ways of detecting it versus if it was black holes then you can use these methods of weak lensing. For particles for example, there are three major methods or detection experiments that we’ve been looking into. Colliders, direct detection and indirect detection. I’m happy to go through what these are.

0:17:14 SC: Please do, yeah. Sorry, let’s just catch our breath. The point is that most working astrophysicist or cosmologist think that not only is their dark matter but it’s something that’s not in the standard model of particle physics, something you’d never found here on Earth and it’s some kind of particle, it’s probably not all just black holes. So the question is, how could we test this kind of idea experimentally?

0:17:39 LN: That’s right and the interesting thing is that for different mass regions of whatever this particle is & different directions, you have to have this lucky different set of experiments. Basically, each one of your experiment is going to be sensitive to some part of the parameter space of your dark matter, the dark matter parameter space is huge [chuckle] somewhere between 40 and 72 orders of magnitude, depends on how you count.

0:18:07 SC: Sorry, what do we mean by the parameter space of the dark matter?

0:18:16 LN: Imagine that you’re trying to discover this object, whatever it is and you’re assuming it’s somewhere between a particle and a black hole and when I’m talking about parameter space, you want to understand, one of the bigger question is, what is it’s mass? What is the mass of this object? Because the mass is going to tell you about how much of it there is ’cause you already know the mass density basically and then when you figure out it’s mass, you have to understand how it interacts with everything else.

0:18:50 LN: Does it, what we call talk to the senior model or does it interact with it or is it just going through us and basically we’re completely invisible to it? We know already that it has extremely weak interaction with the standard model but is it weak or is it zero? Both of these are a possibility.

0:19:11 SC: And we know it’s weak, just because we would have noticed it already.

0:19:11 LN: Exactly [chuckle] by process of elimination, we would have seen it already, yep.

0:19:11 SC: So I guess, does it make sense to first… Why don’t we mention some candidates for what the dark matter could be, just so our listeners minds have tuned to well, how would we go look for these particular ideas?

0:19:35 LN: Yes. One of the most common ideas has been what we call WIMPs for Weakly Interacting Massive Particle. WIMPs would end up being in the mass of a one to a hundred GeV. That translates to somewhere between the mass of a proton to a mass of I think gold atoms or something like that and this is just one particle that makes up all of the dark matter. It’s just one very simple particle and it has some small interaction with the Standard Model and it is in that one particular mass range.

0:20:18 LN: The reason that that mass range was particularly appealing from a theoretical point of view is that it would have had some relation with the weak force in the Standard Model but basically, that is kind of the simplest idea really that you could come up with and ergo, it’s very popular because it’s much easier to test. For example, that would be one of these. Of course, the primordial black holes that we discussed earlier is also one of the candidates.

0:20:49 LN: You can also think of axions or axion-like particles. These particles basically can play a role in how the quarks get mass and there is a whole… Basically they are motivated by theoretical needs and then they happen to be naturally very good candidates for dark matter as well and this is very attractive from a physics point of view because if you have a theory that kind of fixes a lot like addresses many problems.

0:21:25 SC: Yeah, solves two problems at once.

0:21:28 LN: Exactly so axions would do that.

0:21:29 SC: So axions do that, yeah.

0:21:31 LN: Yeah and they tend to be a lot lighter than the WIMPs that I was talking about so axions… Well the QCD axions or like the axions that people think about are going to be ending up somewhere in the 10 to the minus six electron volts, which is basically about let’s see.

[chuckle]

0:21:53 LN: It’s kind of… I’m trying to figure it out. Okay 10,000 times lighter than a neutrino which I’m not sure is gonna say anything but…

0:22:03 SC: I think we can say, yeah 10 to the minus six electron volts for an axion whereas a proton is 10 to the plus nine electron volts so there’s 15 orders of magnitude in between.

0:22:16 LN: Exactly. You can see already that the different theories of dark matter have a wild different of masses. Actually, the lightest mass that you can actually think of that is possible to be dark matter is a regime that we call fuzzy dark matter and it is basically the smallest mass is 10 to the minus 22 electron volts. So something even smaller than that.

0:22:40 SC: Right.

0:22:42 LN: Yeah so there are a lot of beautiful theories and then the question is, how can we find them?

0:22:47 SC: Do we take neutrinos seriously? Are there weird versions of neutrinos that could be the dark matter?

0:22:55 LN: There are serious searches for; not our neutrino, our three neutrinos are fine but if there is a sterile neutrino, a fourth neutrino, it could have like a bigger high mass because then you would try to solve the problem of neutrino masses which is a bit different. In brief, we have this theory of Standard Model that kinda tells us about the particles that we’re made of and their interactions and it is a very, very successful theory and it kind of… It’s upsetting how successful it is from a theoretical physicist point of view because we want to fail so you can figure out what else is missing but one of the things that it doesn’t address is neutrino masses. Trying to figure out how you can give neutrinos masses and maybe sort that out into the whole world of dark matter that is also a possibility.

0:23:51 SC: Okay so we have, besides the black holes, we have WIMPs which someone just invented, right? They’re not in the Standard Model. They are extra particles that would be fun to have.

0:24:00 LN: Yes, yep.

0:24:01 SC: Axions which were also invented for another reason to solve other problems in the Standard Model. Neutrinos which we actually know about but the ones that we know about can’t be the dark matter ’cause we know how heavy they are and they don’t have the right masses so you need to invent a different kind of neutrino.

0:24:16 LN: Right, exactly and the thing with neutrinos is they’re a bit too hot, which means that they are going way too fast for their mass and having them being the that hot… Basically from their mass and their interactions, if they were all the dark matter, they would have destroyed a lot of the structure that we see is just because you can imagine that you have a clump of something and then the neutrinos are going so fast through it, they just break it apart. That would not make a good theory of dark matter because the world is definitely not broken apart yet. [chuckle]

0:24:52 SC: But this is also just a crucial thing to appreciate in terms of how the science of dark matter works ’cause there are a tremendous number of constraints. It’s not just like “Oh, there’s some dark stuff. It’s mysterious. We don’t know what it is.” Maybe it’s quantum mechanics or something like that. There’s very, very specific properties this better have and if you do something simple like make it’s mass too low so that it’s too high temperature and fast, then everything breaks and it’s not a good theory anymore.

0:25:23 LN: Absolutely and it’s funny because as a grad student, a couple of my earlier projects were to kinda build a new theory of dark matter and trust me, it’s a lot harder than it sounds.

[laughter]

0:25:35 LN: There are a lot of constraints and whatever theory you can come up with has to actually… Has to not be ruled out by existing experiments which actually were quite a lot. So, yes and we don’t know enough, that’s true but we know what it’s not in a lot of different ways and I think that it’s kind of… It’s more of a detective work which I think is really fun and you try to kind of piece it up together because whatever dark matter is at the end of the day is going to satisfy all of these observables and getting there is going to be absolutely amazing.

0:25:35 SC: And you also said something that I think is very provocative so let’s circle back to it. That you want the Standard Model of particle physics to fail and this surprises me ’cause on the internet, I read that establishment scientists are not open to new ideas and they just want to prop up the ideas they already have but you’re telling me you want your theory to fail, that sounds weird.

[chuckle]

0:26:35 LN: Yes so we know that the standard model that we have is incomplete. Well, I’ve already mentioned for example, it does not have a theory of neutrino masses.

0:26:47 SC: There you go.

0:26:50 LN: And we know that it’s what we call an effective field theory so it addresses a small… It is valid up to a certain energy but not beyond. So for example, we know that it doesn’t address quantum gravity in any way but even simpler than neutrino masses. So the theory is incomplete and then when it breaks, you would have a hint of what it is that you need to address and what it is that you need to fix. I don’t think physicists are not open to new ideas if anything, they come up with way too many ideas.

[laughter]

0:27:27 LN: Very hard to keep track of so yeah everybody wants… I think when it fails, it’s going to be the most exciting for the field because that’s when we know that’s the hint, these are the things that go wrong, that’s what we have missing in our theory and this is what we need to address…

0:27:48 SC: To be perfectly honest, we were kind of hopeful that would happen at the Large Hadron Collider and so far, it has not.

0:27:57 LN: Absolutely so now every time our experimentalist friends would show up new plots and they’re like, “and The Standard Model is right again.” and you think like, “Oh, no!”

[laughter]

0:28:09 LN: But it is an amazing theory and it’s extremely successful and it’s incredible that it can actually make so many predictions of so many observables and interactions that it can explain so beautifully but we know for a fact it’s definitely not complete and it hasn’t happened in the Large Hadron Collider but it will happen at some experiment at some energy scale, at some point. It will definitely fail and I think that would be quite an interesting hint for physics and what to do next.

0:28:36 SC: Yeah no, it’s a very strange situation to be in where your theory fits all the data and you’re sure that it’s wrong, right? [laughter] It’s a little bit frustrating. You don’t have any clues as to how to move on. But speaking of moving on, we have some evidence that dark matter exists, we have some candidates for what it could be and then I interrupted you but you were going to tell us the different ways we can actually experimentally probe what the dark matter is.

0:29:00 LN: That’s right. Since we were just talking about the Large Hadron Collider or what we call collider searches, that is one of the ways that we can detect dark matter and basically, you would just… In these colliders, what you do is actually collide protons together and that kind of energy build or colliding them at very, very high energies is going to produce a lot of different particles and at some point, you might produce a dark matter so there are a lot of these searches that looking for the dark matter then you might say “Well okay, how am I gonna see this because it doesn’t interact very much with the standard model particles, with the particles that I know?” And the way that you would see it is actually, would be beautiful because you would see what we call missing energy in your detector.

0:29:56 LN: So basically, we know of our conservation of energy and conservation of momentum so what you get at the end of the day is a stream of particles going one way and then nothing to balance it out on the other side, it means that you have something that is dark but it has to be there because of conservation of momentum. Of course, the catch there is that neutrinos do that too. [chuckle] So you have to eliminate the neutrinos part but other than that, that would be an interesting way of looking for dark matter.

0:30:29 SC: I mean, it does… You can see why people on the street become a little dubious ’cause you’re saying that you’re going to detect the existence of dark matter by looking for missing energy. You’re looking for something that you can’t see, by not seeing something else, somewhere else.

0:30:41 LN: That’s right. I think it is interesting but it also balances out. It’s kind of like… Okay so if you were walking by a park and then you see somebody sitting on a seesaw and then the other side is completely empty but the seesaw is not really completely tipped, you’re like “Huh, something is sketchy there.” [chuckle] Either the seesaw is broken or there is something else that I just can’t see sitting on the other side, this is a little bit of the same logic here, where there is either your interactions are broken and the theory is broken. Or there is something else coming out on the other side that you just didn’t see and this is what you’re trying to figure out what else is missing.

0:31:28 SC: And the idea of there being missing energy in events at a particle accelerator, that sounds like something which we might creep up on gradually rather than discovering something and there’s just a big plot and you see the particle that you found. This is something we’re just slugging through and collecting more data and doing this for years and years might be the way to get it. Is that the right way to think about it?

0:31:53 LN: That’s true. Basically what you would make is try to… A lot of these methods are based on statistics and large statistics so what you would try to do is actually put all of these events that looks somewhat the same together and try to see if that missing energy has a common mass or common properties that keep creeping up. It’s not just a failure of your detector that is just missing a spot there. So yes, a lot of it is built on statistics, especially because there are so many events in the Large Hadron Collider and they’re going to increase even more in the next generation, which is called LHC high luminosity Large Hadron Collider.

0:32:37 LN: It is an amazing piece of experimental physics in general to be able to reconstruct all of that, especially so fast but yes, you need statistics and that’s why a lot of these experiments, we run them for quite a long time to be able to disentangle the interesting events from what we call background which is standard model and it’s also interesting in its own right but it’s different.

0:33:00 SC: Okay, if we’re not lucky enough to get evidence directly in an experiment like we build like the Large Hadron Collider, what else can we do to look for the dark matter?

0:33:12 LN: Then we can go into what we call direct detection so you wanna detect it directly. How does this work? You have huge tanks of, it’s actually different material but the most common one is really Xenon. So you we have a huge tank of Xenon, it’s actually a few tons of Xenon, that you put in a huge tank and then you put it very, very deep underground. For example, there’s one… Some of the experiments are usually deep in mine shafts and everything. Then you have… So then your Xenon is just sitting there and you’re waiting for one Dark Matter particle, out of the many that might be going through your experiment, one of them to hopefully interact by basically… When it interacts with that Xenon and it knocks it off just a little, that it will knock out one of it’s electrons and you put the whole thing in an electric field and that electron is going to kind of get pulled to the top of the experiment and that’s when you know that, “Woah! Something hit there.”

0:34:18 LN: So there are few things here. First, why do you need this to be deep underground? Well, because there are always background, there are always something else that might hit your experiment and in this particular case, we call these cosmic rays or cosmic muons. There are a lot of particles that hit the atmosphere all the time and they make a lot of other particles like pions and muons and everything and these muons are actually going through us all the time because they’re coming out of our atmosphere and they usually don’t do anything, even though I recently… Well, I recently discovered that there is actually a whole set of research about cosmic muons flipping bits in computer clusters and that they need to kind of correct for that, which is absolutely amazing, anyways.

0:35:07 SC: I think they also do sometimes cause DNA mutations.

0:35:11 LN: Oh they go… Oh, yeah.

0:35:13 SC: I think.

0:35:14 LN: That is really cool. I think…

0:35:15 SC: I’m not completely sure about this but cosmic rays do help your DNA mutate. So there you go, the muons…

0:35:21 LN: Yeah, muons.

0:35:22 SC: They seem so innocuous and yet…

0:35:26 LN: But they do a lot of things.

0:35:27 SC: Yeah.

0:35:27 LN: That is pretty cool but yeah, they could also hit your experiment and then you’re like “Oh, I discovered Dark Matter. Oh, nevermind, these are the atmospheric muons.” and that’s why you put your experiment very, very deep underground, where all that dirt and… Usually you put them sometimes under mountaintops and everything, all these mountains are going to absorb a lot of these cosmic muons and they would not interfere with your experiment.

0:35:54 SC: Right, okay and is this good for sort of all kinds of Dark Matter detection or are we looking for some kinds of Dark Matter but not others?

0:36:04 LN: This would actually work mainly for WIMPs, although there are new different… New techniques that instead of the Xenon, they would choose different materials like for example, Helium in a lot of semiconductors and super-conductor material and that would help them kind of explore masses below the WIMPs but the standard, large experiment like this is going to be mainly addressing the WIMPs, which is the 1 to 100 GeV or gigaelectronvolt mass range. So yeah.

0:36:40 SC: Right and my impression is, these experiments have started, they’ve been going on and they’ve looked… We could have gotten lucky by now, right? We could have actually seen the WIMPs by now but we’re nowhere near finished looking for where they might be.

0:36:54 LN: That’s right. A lot of people are actually kind of getting stressed/thinking that maybe WIMPs are not the way to go because yes, direct detection experiments have been going on for a while and we’ve been building bigger and better experiments. I mean it’s ruling out huge swaths of parameter space and the Dark Matter is not there yet and interestingly enough actually, soon these experiments are going to be so good, that they are going to be detecting… They’re hitting something that we call the Neutrino Floor and they’re basically going to see neutrinos instead of the Dark Matter and this is incredible, because these are very very small cross-sections that you wouldn’t see with this type of set up actually yet. So yeah, if anything, we’ll see solar neutrinos again, in a different experiment.

0:37:51 SC: So the neutrino… It’s like neutrino static in your radio. [chuckle]

0:37:55 LN: That’s right. It’s just like that background, that when you get… Yeah, you get to it and then all of a sudden that’s… It’s going to dominate your background because direct detection experiments so far have been running pretty much background free…

0:38:08 SC: Yeah.

0:38:09 LN: ’cause nothing is was gonna get through a mile of Earth to get to your experiment and actually hit it. You have to be a bit careful about the material that you’re using for your experiment ’cause sometimes some material radiates a little bit so these have to be very, very clean and again, my hat to experimentalists ’cause they build amazing things but other than that, it’s just… Yeah, we haven’t seen Dark Matter yet, unfortunately.

0:38:35 SC: And you know, to be honest ’cause full disclosure here, that’s the other big thing looming over us, right? We haven’t seen any new particles at the Large Hadron Collider, we also haven’t detected the WIMPs at these underground experiments and in some sense, they went hand-in-hand, right? I mean, Physics arranges itself by what the masses of the particles are and the sort of mass range the LHC is looking at is the same as the mass range that the WIMP detectors are looking at and everyone expected to see something there and we haven’t so it’s a little bit of back to the drawing board time.

0:39:12 LN: Absolutely, I think it has filled kind of the field with a bit of disappointment in trying to figure out exactly where to go from there and what are the new strategies? I think it is kind of… I think it’s a good thing to do in the sense that, it’s kind of more of a call for creativity and a lot of people are kind of adopting that like from both the theoretical and the experimental sides where you try to think of definitely new theories that could explain all of these observables because you know when you see nothing that is an observable because that means it’s ruling out something. It’s not really just that it was waste. No, it’s really not.

0:39:53 LN: So whatever your theory is has to actually explain all this these null results. So you really have to be very, very creative there but also from an experimental point of view, you need to figure out new technologies and new experiments to probe different masses and think of experiments that nobody else has built before and I don’t know I think all of this kind of makes physics really, really fun, even though of course, you would want something to work in the end and you would want to discover new things of course but really looking at physics from a new way. I think it’s very exciting but, this is why I do what I do.

0:40:33 SC: Yeah, of course but and also just to be fair on the other side, if the dark matter is axions, we don’t have a hope of seeing them at these underground experiments. We have to use some completely different technique, right?

0:40:46 LN: That’s right and there have been quite a few of these experiments as well. With axions for example, what you would wanna do is build a Helioscope so basically you’re trying to, how to do I put this? You try to kind of have this experiment where an axion is going to kind of make some resonance. Actually, okay differently, these are very different scale experiments. So remember we’re talking about like multiple tons of Xenon, these are actually a lot smaller and the axioms are going to get into your experiment and actually make a magnetic… Electromagnetic field that you would detect so something like that, yeah.

0:41:39 LN: Which is a completely different set of experiment and this is a different completely different mechanism and they have been kind of doing quite a good job at ruling out different parameter space of course again, different masses of possible axions but that space is very, very large.

0:41:54 SC: Yeah. No, my impression is that unlike the WIMP case where we really have ruled out a noticeable fraction of the parameter space, in the axion case, there’s still a lot of room to be explored.

0:42:05 LN: Absolutely. Because these experiments are very difficult and you’re kind of tracking very, very small mass, basically mass windows, very very thin ones at every time that you’re running your experiment unlike, these WIMP indirect detection where you can pull out a huge part of the parameter space all at once, yes.

0:42:26 SC: And so the last thing that you mentioned was indirect detection. So if you can’t detect them directly, why not try indirectly?

0:42:35 LN: That’s right. Indirect detection has a funny name but basically it means that dark matter is going to either annihilate or decay into particles that we already know and then we see those particles. So said another way you can imagine that the dark matter just annihilates at the center of the Galaxy into electrons, positrons, neutrinos, gamma rays all of the above and then what you would see is from your gamma-ray telescope for example is that there is an excess of gamma rays coming from the center of the Galaxy that you just can’t explain. You’re like wait, I expected a lot lower number of these gamma rays. Why am I seeing that many? That could be a dark matter. Of course, it also could be astrophysics as we’ve learned and so…

0:43:28 SC: Wait, wait, wait. I think for the people on the street, you have to explain what that means, of course, it could be astrophysics. This is all astrophysics.

0:43:34 LN: It is all astrophysics, that’s true. So when we… I think this is physics slang for it could be something that we already know but not really. Basically back in… Well it started in 2009 where so there is this telescopes, it is called Fermi telescope and it basically measures the gamma rays in the sky and in 2009 was the first evidence that there was a little bit more gamma rays for in the range of masses from like one to three GeV or so in the center of the Galaxy. Then with more data and as we talked about earlier you gather more statistics in 2014…

0:44:15 SC: Sorry. I’m sorry. I need to interrupt here too because I think we’re doing another physicist shortcut because you said gamma rays in the range of masses around one GeV or whatever but of course the gamma rays are photons and they’re massless but you’re using E = MC squared, right?

0:44:33 LN: That’s right. Sorry. Thank you for pointing that out. Yes. [chuckle]

0:44:37 SC: They are like energies that you would get by annihilating a particle with a mass of that much.

0:44:43 LN: Yes, that’s exactly right. In physics, I think your first day at grad school, they tell you E = MC squared and C equals one and you just go from there [laughter] and you kind of never go back but yes, the photons are indeed massless but these are kind of equivalent energy that they would have absolutely based on that relation. Yeah and then in 2014 with gathering more and more statistics, it was even more evident that there is some access there that ended up being called the Galactic Center access. So why is the Galactic Center actually interesting at all?

0:45:20 LN: Well, you would expect within what are current theories of dark matter, you would expect that there is a much higher density of dark matter in the center of the Galaxy. You can think of it as like a deep potential well that your gas and stars are kind of like falling into and so if you were… If the dark matter was to annihilate or decay or do something, it would be where there is the most of it which is at the center of the Galaxy so this so far was consistent.

0:45:25 SC: Right.

0:45:52 LN: Then there is this excess of gamma rays that we try to figure out and we were like, “Okay ooh, that would be interesting. Is that really dark matter?” The first thing that you would check for example, is what is the spatial distribution of these gamma rays, which means; is my signal coming from everywhere in the Galactic Center or is it correlated with the stars and in the disc? Does it have like a shape of some sort ’cause you wouldn’t expect the dark matter to have some weird shape.

0:46:23 LN: The first test that ended up that the signal was Isotropic, which means that it’s really uniform coming from the sky. So that’s cool. Then you were like, “Oh that might really be dark matter, that’s interesting” But then you try to think of what else could it be, which is usually… And this is the toughest part with indirect detection ’cause you always have to say what else could it be?

0:46:45 LN: Then the other competing theory, it could be that you have a lot of pulsars, which are just these neutral stars that just turn really, really fast and they’re emitting a lot of these gamma rays that you don’t see otherwise, that you wouldn’t have seen detected previously and then the question is, “Okay, is it pulsars or is it that dark matter” and you have to estimate how many pulsars you have but it’s in the center of the galaxy, which is pretty far and then based on a lot of things that you don’t know. I think that was… The status of it right now is that it might be… It’s probably mostly pulsars, all those that are confirmed.

[chuckle]

0:47:28 LN: Yeah. We need to at least see some of these pulsars so the way you would do it is by seeing it in radio telescopes but you can’t really see huge parts of the sky all at once in radio telescopes so yeah, this is the fun of physics, trying to figure out, it’s investigative work basically.

0:47:45 SC: It does make sense in retrospect. You are hoping to see signals of dark matter at the center of the galaxy because that’s where you should have a high density of dark matter but also there’s a high density of other weird stuff at the center of the galaxy and we’re realizing, we were not quite as careful as we could have been, figuring out what the signal should be from that other weird stuff that is weird but not nearly as weird as dark matter.

0:48:08 LN: That’s exactly right and so then you can go for Option B. Okay, if the Galactic Center is too messy, where else can I see it? And then you might be able to say something about dwarf galaxies. So what are these? We have our galaxy, the Milky Way and we are sitting pretty much at the edge of a disc of the Milky Way but the Milky Way is actually much larger and we’re swimming in what we call the dark matter halo. Basically, a sphere of dark matter more or less, that you can think about.

0:48:42 LN: Anyway, it has a lot of gravitational pull because it is a pretty big galaxy and because of this gravitational pull, it also pulled smaller galaxies, very close to it and these are kind of satellite galaxies. It’s the same way that the moon is the satellite of the Earth more or less.

0:49:01 LN: We have that, this Milky Way has it’s own small satellites, it’s own… That we called call dwarf galaxies, which are galaxies but a lot smaller. Anyways, these smaller galaxies have a lot less Baryons or a lot less stars and gas than the center of the galaxy, which means that if you see signal from them, it’s probably actually coming out of the dark matter. So they have more… What we call the mass to light ratio. They have a lot of mass coming from dark matter and very little light coming from stars, unlike the Milky Way that has a pretty high mass to light ratio, pretty low mass to light ratio.

0:49:45 SC: So we think that in dwarf galaxies, there’s less chance we’d be confused by pulsars or other crazy things.

0:49:51 LN: That’s exactly right. The problem is that they’re much smaller so they have a lot lesser matter, right?

0:49:55 SC: Yeah.

0:49:55 LN: So what we can do and what we do, is we can stack them. So basically trying to get, for example, the gamma rays from a lot of these dwarf galaxies and put them together on top of each other, stack them and try to see, is there something significant there? Do we really see an access of dark matter? And then the status of things is that there was one dwarf galaxy that might have had an access. There was nothing confirmed and a lot of my current work is trying to better understand how much dark matter should we expect in these galaxies because it’s a very difficult measurement that we have to do. So yeah, there is a lot to be done in physics, basically.

0:50:43 SC: So okay but this is good for the people listening. You have some data. Is the constraint that you don’t have enough data of gamma rays from these dwarf galaxies yet or that we haven’t analyzed them carefully enough or that we don’t understand the background astrophysics?

0:51:01 LN: We have a lot of data here, it’s just that… It is not about the background of astrophysics ’cause you’d expect that to be small. It’s basically how much dark matter do you expect in dwarf galaxies all together and that’s a very tough measurement or “measurement” between quotes that you have to make. The same way how much dark matter there is at the center of the galaxy is still a big question.

0:51:27 LN: So let’s focus on dwarf galaxies because they are bit simpler. How would you know, how much dark matter there is in a galaxy like that? Well, the only measurement that we have is really based on the motion of stars and this is not 3D motion so I don’t really see all the directions of motion of these stars, all I see is what we call the line-of-sight velocity that basically is the… It’s basically the Doppler Shift. Is that star going away from me or is it coming towards me and with what velocity? I can make those measurements about few stars and usually just the bright ones so not all of them, of these objects and then have some understanding about okay, these stars are moving within a certain speed, what is the amount of mass that would give me… That would be consistent with such a speed?

0:52:23 LN: So the thing is there are a lot of things that could go wrong there. Basically, you don’t know since you’re only seeing one direction of the motion of stars. Basically, is it coming towards me or away from me? You don’t know the sideways motions and it’s hard to tell. You have missing information, like it’s motion could be very large or very small and you wouldn’t know so you don’t really know the full-velocity of that so it’s hard to get the full mass.

0:52:53 LN: You of course, have to assume that the system is in equilibrium, basically that means that there is nothing weird happening to your stars. They’re not getting pulled or pushed by something else. In particular for example, the gravitational field of the Milky Way is not doing anything to them. You have to assume that the whole thing is a sphere, it doesn’t have to be. This kinda goes back to the joke of the spherical cow in a vacuum. [chuckle]

0:53:21 SC: The spherical cow galaxy, okay, good, yes.

0:53:23 LN: That’s right. There are a lot of… We know some, something about the mass of these objects but we need to know them much much better to be able to rule things out or rule things in with much more confidence.

0:53:37 SC: And so what are the prospects? What’s gonna happen? For one thing you mentioned the Fermi Gamma-ray telescope, tell us about that. It’s in space, right? It’s hard to build a gamma-ray telescope here on earth but it is possible.

0:53:51 LN: That’s right. This is a gamma-ray telescope that is in space and the reason that it’s much more difficult to have these things on earth is because the atmosphere is really, really painful to get through. For earth-like experiments, you can actually do get some gamma rays but are very, very high energies and they are these, what we call Cherenkov telescopes so basically these telescopes in the desert of… I think some of them are in the desert in Africa and basically what they have is a gamma ray that is very, very energetic comes in and then, of course, it interacts and makes a shower of particles, they get those through the experiment and they end up moving in the medium of the experiment, usually water, faster than the speed of light in water. Nothing goes faster than the speed of light in a vacuum but in media, it actually can happen.

0:54:49 SC: That’s right.

0:54:53 LN: And that’s how you would see these but yes, the prospect of… So Fermi has gathered a lot of amazing data and actually it has given us, for example, catalogues where we felt like the… The gamma ray catalogues of a lot of things that are in the Milky Way and even a bit further that actually emit gamma rays. It is definitely a great way to understand the astrophysics of the Milky Way, another experiment and the Milky Way and its surroundings but the reason that I think the world of astrophysics or astro-particle physics is really, really interesting and in it for a ride is of all the experiments and all the telescopes that are gonna come in online and have come in online.

0:55:39 LN: There is the Vera Rubin telescope called a lot of LSST’s coming in 2023, there are other upgrades of current experiments but the one I’m most excited about and a lot of my work is on is called Gaia and Gaia is this space telescope that was launched in December 2013 with a goal of giving us the positional measurements so the motion and distances of one billion stars and this is one percent of the Milky way. Yes, that’s how it works.

0:56:14 SC: So yeah, since you’re younger than me, you forget but when I was your age we were excited about the Hipparcos satellite and this was a satellite that was gonna get, I guess, thousands of stars, their distances and their sideways motions.

0:56:29 LN: That’s right. Hipparcos ran from ’89 to ’93 and it was very exciting but it’s amazing when you see the map of how much Gaia covers now compared to Hipparcos and yeah, a lot of people get offended they’re like, “Hipparcos was a great experiment,” I’m like, “I’m not saying anything else but Gaia is amazing too.”

0:56:49 SC: Yeah but just to see how amazing it is, it’s really hard to know how far away stars are. You can see where they are in the sky but it’s this amazing technological achievement to also measure the distances to a billion stars.

0:57:03 LN: Absolutely and their distances and their motion really, now you can… For the first time, you can actually have 3D maps of, to be fair now, it’s still just the closest stars it’s absolutely incredible the amount of data that we’re getting. The second data release of Gaia was back in April 2018, almost two years ago from April 25th and we got… So one billion stars, their distances and their motion that we call proper motion but a subset of them, we also had the line of sight velocity that I talked about earlier. So for a subset, a very small subset of seven million stars, we actually have 6D kinematics. It’s absolutely incredible and it’s an amazing dataset that you can get so much out of and indeed we did, back in 2018, a new merger of stars has been discovered based on the motion of stars. Let me explain what mergers are.

0:58:13 SC: Yeah.

0:58:13 LN: Basically, remember earlier I said that we have these satellites because the Milky Way is kind of pulling a lot of these small satellite into it?

0:58:22 SC: Uh-huh.

0:58:22 LN: Well, the satellites they have stars and dark matter but sometimes they crash into us and we pull them quite fast so they would just get completely disrupted. They’re completely destroyed and get mixed up with the Milky Way. So the discovery of 2018 is that there is such a merger that has happened somewhere between six and 10 billion years ago and smashed into our galaxy, brought in so many stars with it and it’s interesting because it’s a pretty big object, it’s mass was somewhere between 1% and 10% the mass of the Milky Way and we never knew it was there until we actually finally had the kinematic data for it. It has a very unfortunate name because of the person who first saw it called it The Gaia Sausage.

0:58:22 SC: Because it is roughly Sausage shaped after being…

0:58:22 LN: Because it is extended, yes but we still give that person help every time.

0:59:23 SC: Okay, the Gaia Sausage. Yeah I mean, Gaia just seems like, Greek mythology, very highbrow and then it was like making sausage, okay.

0:59:31 LN: That’s right. So Vasily Belokurov is the person you should blame. [chuckle] He’s an incredible physicist and he saw it and he saw the shape, it was a bit sausagey so he went with that name. After the fact there were attempts at calling it the Gaia Enceladus and the Enceladus is a Greek god, the son of Gaia. Which is amazing but it’s unfortunate that everybody will remember only the sausage so it sticks so I think maybe who knows there was something good there.

1:00:03 SC: So what could we learn about dark matter from seeing this sausage-shaped collection of stars that has been cannibalized by the Milky Way?

1:00:10 LN: That’s right. That’s where what I do comes in. [chuckle] A lot of the things that I’ve been doing is trying to understand how much dark matter would come from these mergers and answer the question, “can I understand how fast the dark matter is going based on the speed of these stars when they merge in?” So what I do is actually use simulations. A lot of them, I in part actually developed here at Caltech, from Phil Hopkins Group. These simulations are called FIRE, which actually is an acronym for Feedback In Realistic Environment but the reason that it’s really cool is because we can make a lot of fire puns for the titles of our paper. [chuckle]

1:00:56 SC: Physicists are just the worse, there’s no pun they cannot possibly resist.

1:01:01 LN: I know we just really can’t help it. [chuckle]

1:01:04 SC: Yeah, right. [chuckle]

1:01:06 LN: So yeah, what I did is actually, I used a lot of these simulations. These simulations are actually simulate what building a galaxy just like the Milky Way. It’s really, really cool. What you do is like you start the really, really early times you have your dark matter particles and you teach them gravity and then you also add in beauty of the FIRE Group is… The physics that has been brought in by the FIRE Group is to add a lot of these physics of the gas interactions and the stars, etcetera, on top of that and then you let that… You just like teach it different interactions and you just let that evolve from very early on and at the end of the day, 13 billion years later, you end up with a galaxy that looks very much like the Milky Way. It’s not exactly the same, obviously but it has the same properties in the sense that you end up with a nice… This galaxy, it has a proper mass, etcetera and this is absolutely incredible.

1:02:02 LN: The cool thing about these simulations and the reason that I really love using them is that I know that I can track the stars and I can track the dark matter, basically, particle by particle to figure out exactly what’s going to land in the sun or the… The position of where the sun would be in these simulations and then figure out if there are correlations between… If there are any relationship between the dark matter and the stars and where they came from, etcetera and what would I expect to see in my experiments today? There was a lot of…

1:02:38 SC: And when you say… Just to be clear, when you say you can track every particle this is not like elementary particles like proton. This is the particle in this simulation that is used to model a whole bunch of mass?

1:02:49 LN: That’s right. Unfortunately, we cannot just like simulate the whole galaxy up to it’s, you know, electrons. We don’t have…

[overlapping conversation]

1:02:57 SC: That’s not really realistic, yes.

1:03:00 LN: So what we do is that we call these particles in our simulation but these are actually like huge clumps of dark matter and clumps of stars. For example, the stars and the… What we call one star in the simulation, it really has 7000 solar masses so it’s basically, about 7000 stars clumped up together because that’s where the resolution is going to be.

1:03:24 SC: Yeah, it’s a low resolution version of the galaxy. I think that makes perfect sense in our modern video gaming era. People know what that means.

[chuckle]

1:03:31 LN: Exactly, yes.

1:03:33 SC: But the nice thing then is we can really appreciate the fact that since we do believe in dark matter, since it’s not just modified gravity, the dark matter is located somewhere and the distribution is kind of lumpy and has structure and is interesting for better and for worse, right?

1:03:49 LN: Absolutely and I think this is… I think, it’s really, really fun because it plays into… The reason that I do this is because it plays into the experiments that we were discussing earlier. So the lumpiness of dark matter, it means that you might have an excess of gamma rays coming from those lumpiness or if the dark matter is faster or slower that would affect how many of them you would see in direct detection experiments in the tanks of Xenon underground, because the faster the dark matter the more energy it might deposit in the xenons. So it all is very much correlated at these very different scales, which I find absolutely fascinating. Because the theory of dark matter, really at it’s essence is a theory of astrophysics in very large scales but also particle physics in the smaller scales and everything has to make sense and I don’t know…

[chuckle]

1:04:41 LN: I can keep talking about dark matter all day but I find this absolutely fascinating.

1:04:48 SC: So I mean, what have we learned? By knowing that there is a sausage there, there’s other structures or whatever, what are the implications for trying and detect the dark matter and finally finding it?

1:05:00 LN: That’s right. Basically, what… Trying to extrapolate the dark matter velocity for example, based on the velocity of the stars from the Gaia Sausage, one thing, we found out that it might actually… The stars and then consequently the dark matter might be going a bit slower than we would expect. Which means that we might have been ruling out more parameter space and saying that “Oh, this dark matter cannot be this.” More than we should have. So we kind of have to go back and really address all of these differences and making sure that our initial models that are not really empirical, they are just based on assuming that the Milky Way is relaxed and everything is fine…

1:05:52 SC: Yeah.

1:05:53 LN: We realized that no, the Milky Way has quite more interesting structure than we thought before and we need to take that into account in our experiments. One thing that is quite interesting that I found last year with my amazing collaborator, Brian Ostic, who is a post doc at Harvard, basically he was trying to… He was building this machine learning methods, trying to figure out which stars did not come from the Milky Way and this is absolutely amazing. Then he built this wonderful catalog and then he sent it to me and he was like “I don’t know what I’m looking at. This is the catalog star. The stars. Just have fun.” I was like “Okay. I can do that.”

[laughter]

1:06:41 LN: This is gonna be great. Then the first thing that you do really when you get a catalog or any data set is that you start plotting it and you just make random plots and figure out, does it look like I expect it to, does it not?

1:06:55 SC: I’ll take your word for it.

[laughter]

1:07:00 LN: Yes, I might be a little bit less theory. I do use data. Anyway, I started plotting these and the Gaia Sausage was there, just there, without me having to look for it and I was like, “This is amazing.” But then there was this other cluttered clump of stars that was not supposed to be there and when you’re young and you’re watching movies and everything, you just think that, “Oh yeah, if I see something amazing, I’m gonna have my eureka moment and I’m like, oh this is incredible.” But then you go to grad school and you realize that every moment like that is just a bug in your code and you’re like…

1:07:37 SC: Yeah, I know. So many eureka moments that didn’t quite see the light of day. It’s true.

1:07:40 LN: Exactly. So of course, as a well-trained failed grad student, [laughter] that was the first thing that kinda came to mind. So I sit on it for three weeks and I didn’t tell any of my collaborators. I was plotting it every which way but then I realized, “No, it’s actually… It might not be a bug, it was this actually might be something which is really cool.” And then you have to actually check against literature and you’re like, “Oh, it might be something that somebody else has already found, in which case, that’s pointless.”

1:08:08 SC: Sure.

1:08:09 LN: Thankfully, nobody else has found that one. So, I got to name it, which is really cool so…

1:08:13 SC: That’s the best.

1:08:16 LN: This is cool. It’s called Nyx, N-Y-X for Greek goddess of the night, that I thought was particularly fitting here but what can we learn from this? So Nyx is that… It’s a clump of stars that are rotating with the Sun like… These are stars that are very close, they’re co-rotating but they also kind of have this wave of movement towards the center of the galaxy, which is very bizarre because you don’t expect them to do that. Then there are these two theories that we could think of that would explain this. Either something hit the Milky Way and kinda caused these waves and these stars are kind of getting pushed just because… Imagine you just… If you have a lake and you’re throwing a rock in it and then it’s gonna cause waves and the stars are just moving that way.

1:09:05 LN: Or it’s a merger, just like the Sausage so it’s another object that just fell into the Milky Way and these stars that I see here, are actually remnant of this merger. The interesting thing is that if it is indeed a merger, it might have also brought in with it a lot of dark matter that would have formed something called the dark disk. It’s something that is kind of co-rotating with us and that would change our estimates of direct interjection, quite a lot actually. So we’re still trying to figure out what it is but if any… But it’s definitely really fun so…

1:09:44 SC: So this is something that I get confused about so maybe help the audience with… In some sense, we hope that there is less dark matter near us than we think because we have a certain threshold that we’ve ruled out, right? Of a number of events in our detectors, etcetera and if there’s more dark matter than we think, that means that the parameter space is even lower. Right? The parameter space is even more constrained by our experiment. There’s more wiggle room if there’s less dark matter around. Is that the way it goes? Did I get it right?

1:10:20 LN: That’s right. If dark matter is a lot less, then a lot of more of our theories are still possible.

1:10:26 SC: Yeah.

1:10:27 LN: However, if the dark matter… For example, if the dark matter is coming out of a dark disk if it has a non-trivial velocity distribution, then the shape of those limits that we see in the literature is actually quite different, because this new structure is going to affect very high masses but it’s not going to affect low-mass dark matter. Usually, these flaws that we are ruling out might actually look quite different. It’s not just like an overall scaling, it’s not just… Oh, more of it or less of it is going to be ruled out, it’s just that the shape, the mass versus the probability that would detect it might look quite different, which would be very interesting.

1:11:13 SC: Right. So Gaia because it’s given us this 3D map of a billion stars, is helping us figure out this interplay between the location of dark matter in the galaxy and our ability to constrain its properties here on earth.

1:11:27 LN: That’s right. So the three… It only has a billion stars basically, with proper motion so the side way motion.

1:11:34 SC: Right.

1:11:35 LN: And the Parallax and it’s only a subset of seven million of them that has 3D velocities but for the third data release of Gaia, which is scheduled for next year, with some delays for now but we’re going to go from that seven million up to 100 to 150 million. It’s going to be amazing, whatever we’re gonna get.

[laughter]

1:11:36 LN: I’m very excited for that. So yeah, we can do that. We can also do… It’s not the only thing that we can get out of Gaia. There are what we call in the process of pulling the satellites they end up forming streams in the sky so basically stars are kind of almost aligned and using Gaia for example, we can actually see gaps in these streams and there is one of these streams, it’s called GD 1, that has a couple of gaps and then the question is, how do you explain gaps in these streams?

1:12:31 SC: Yeah.

1:12:32 LN: And one of these theories is that if you have a clump of dark matter that just goes right through your stream and pulls a lot of stars with it, then that means that’s how you would get one of these gaps in your stream, which we can see with Gaia and the question is how big is that clump? What is it? How fast it was going? Etcetera. Really helps you narrow down the theory of dark matter that you have. Because some theories of dark matter for example, would not have very low mass clumps and if you see a very low mass clump, it means that that theory is not right.

1:13:07 SC: Is that… Sorry. What kind of theories of dark matter would not have a low mass clumps?

1:13:12 LN: Something that we call warm dark matter and even theories of self-interacting dark matter but let’s focus on the warm dark matter. Remember earlier when I said that neutrinos are going too fast so they destroy a lot of the structures that they have. Instead of having something as hot as neutrinos, if you have your dark matter that is a bit warm, which means that it’s going a little bit too fast but not extremely fast, then it will destroy very, very small galaxies or sound like galaxies just by going through them and puncturing them and basically heating up the system is what we call it in astrophysics but it’s not gonna make much of a difference to larger objects. Which means that these theories are not going to allow small enough clumps.

1:14:03 SC: Right, got it.

1:14:04 LN: That’s what we call cutting the power spectrum. Then the question is, the gaps that you see, are they consistent with very, very small clumps and if so, they would rule out models of warm dark matter, for example.

1:14:19 SC: Got it. Okay. Good. Yeah. Alright so there’s clearly this very exciting frontier set of prospects about the new data coming in and teaching us about dark matter, even before we can directly detect it but therefore I feel like we’ve done our duty and we can let our hair down a little bit now. You already mentioned that the idea of self-interacting dark matter, all of this dark matter stuff that you’ve been talking about, all the ideas for it had roughly speaking been that the dark matter is just there. It might be in different places, it might be different densities, it might be different velocities but it doesn’t do anything other than move under the force of gravity but now you’re introducing the possibility that the dark matter could be more interesting, it could interact with itself or dark energy or something or ordinary matter in interesting ways.

1:15:11 LN: That’s right. It could have it’s own self-interactions along with any interaction that it might have for… With the Cini Model without particles. So one thing that is a particle physicist nightmare is that what if dark matter does not interact with us at all, how can we possibly see it?

1:15:33 LN: Well, if it has it’s own self-interaction, which means that, yes it interacts with gravity, that’s a given with dark matter but what if it interacts with each other, like two dark matter particles, they bounce of each other, they scatter of each other, annihilate into more dark matter etcetera. What would we see? Well interestingly, if that happens, it means that the higher density point so the center of dwarf galaxies or the center of the Milky way is going to be a lot less dense and the reason for it is because if there is a high density spot and they’re interacting too much, they will kind of kick each other out and that would drop the density of that part and this is what we call the core versus cusp.

1:16:23 LN: So from simulations, you would expect to have a cusp in the density of dark matter, which means that the profile is very, very steep, that there is a lot of dark matter in the middle of galaxies, in particular dwarf galaxies but if you add in self-interaction, that cusp becomes a core, which means that instead of just going sharply very, very high as you go through to smaller and smaller distances from the center, it’s going to just become more or less a constant or stable.

1:16:55 SC: Okay. So a little smushed out, a little bit more fluffy.

1:17:00 LN: Exactly, a bit smushed out. That would be kind of a probe of seeing that dark matter is indeed there and has some kind of properties, even though it doesn’t interact with the electrons and the protons etcetera. So you would see it from, what we call from astrophysics or astrophysical probes.

1:17:17 SC: Do we have some pre-existing feeling for whether or not we should expect the dark matter to have interesting interactions with itself or is it just easier? I know this is sort of the only quasi-scientific question. It’s like our feelings rather than what we can observe but we do have Bayesian priors on what we expect. Do you think that the dark matter interacts with itself in interesting ways?

1:17:41 LN: I think so. I think it’s very unlikely, it was just… It’s not ruled out but it’s very unlikely that the dark matter is just that one particle that has about one-on-one interaction with the Cini model, that’s way too simple, especially giving them how complex our standard model is. I would expect that the dark matter sector and it’s a whole sector, it’s not just one particle for example, is going to be complex, it’s going to have a very interesting interactions etcetera. So yeah, I think I would not be surprised if the dark matter has some kind of self-interaction and if it’s a very rich sector in general than what we have. I would be very surprised if it was just one. [chuckle] I think it’s very simple.

1:18:26 SC: Yeah. I ask in part because I honestly don’t know myself. I’ve written papers about interesting ideas for dark matter, including one on dark electro-magnetism, where we can have dark magnetic fields and maybe even dark atoms and dark chemistry and things like that but I just don’t know if it’s more likely that it should be that way ’cause there’s a million different ways you could have interesting dark matter sectors or less likely because it’s just an ugly complication that doesn’t actually solve any puzzles.

1:18:58 LN: Yeah. I think it’s a very interesting question and I think it’s one of those things that it’s good if people think differently because they will spend time doing different things but yeah, rationally there is no out prior information about this at all. So it’s something that we need to kind of track. I think I would be very surprised if it’s very simple but that’s just me. [chuckle]

1:19:28 SC: Well no, like you said, it’s good that different people have different intuitions about this because ultimately we’re gonna find out by doing the experiment and we’re gonna figure it out and I think that you’ve given us today a lot more reason to be optimistic that we will figure out I think, don’t you? And correct me if I’m wrong but your enthusiasm is contagious. I think that I’m excited about the prospect for learning more and more about dark matter in the near future.

1:19:53 LN: I tend to be very optimistic about this. Well and I also love my job. [chuckle] it’s one of those things but yeah, I know people were a little bit less optimistic. This is the most optimistic view you can get. [chuckle]

1:20:05 SC: Good. No. I asked the right person to be on the podcast. Lina Necib, thanks so much for being on the Mindscape Podcast.

1:20:11 LN: Thank you. Thank you, this is fun, thank you.[/accordion-item][/accordion]