Daniel and Kelly’s Extraordinary Universe - Is dark matter hot?

Episode Date: July 28, 2020

Scientists don't know what dark matter is, but they know if it's hot (or not!) Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

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This technology is already solving so many cases. Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. I mean, I know that we don't know what it is. But what is it like? I mean, is it squishy? We don't know. What does it taste like? Well, you know, our tongues can't taste it.

So again, we don't really. really know. But is it fuzzy? Maybe, we don't know. Or scratchy? Probably not, but again, we just don't know. You know, for such a hot topic, you would think you guys would know more about it. Well, that's one thing we do know, whether dark matter is hot or not.

Hi, I'm Jorge. I'm a cartoonist and the creator of Ph.D. Comics. Hi, I'm Daniel. I'm a particle physicist and I have no opinion about the attractiveness of dark matter. Well, it's definitely attractive, right? Gravitationally speaking. On a cosmological level. That's right. It is the great attractor from that point of view. But welcome to our podcast, Daniel and Jorge, Explain the Universe, a production of I-Hard Radio. In which we talk about all the amazing and crazy things in our universe.

the things that scientists have understood and the things that scientists are now working to understand. We break down all the crazy for you and explain it in a way that hopefully makes you smile. Let's write all the things that are hot in this universe and all the things that are not hot or cold or super cold because the universe has a broad range, right? Things can be as hot as a million degrees

or as cold as zero degrees. That's right. Everything has a temperature, even black holes. We all have a rating. That's right. Most of the universe out there is at a very cold 2.73 degrees Kelvin, but there are a few hot spots,

a place like Earth where hot little bits of temperature collude to make life and interesting podcasts. And so we like to talk about in this podcast about dark matter a lot. And I feel like we talk about it a lot because it's such a huge mystery. I mean, it's 27% of the universe and we don't know what it's made out of.

I think it's one of the biggest open questions in science. You know, the person or the group that figures out like what is dark matter? anyway. That will be a historic moment. That will be an understanding, an achievement, a breakthrough that will go down in history for sure. Do you think a Nobel Prize would be enough for that discovery? Or do you need to like stack him up or something? Or maybe make up like a special Nobel Prize. The Dark Nobel Prize. You know, they should have already given a Nobel Prize to Viro Rubin for the discovery that Dark Matter was out there. Even if we don't know what it is,

we know it's there. We know it's matter. And Nobel Prize Committee overlooked Vera Rubin. Some say because she's a woman. That's terrible. That's the dark history of the Nobel Prize. It's the dark history of dark matter. But we know some things a little bit about dark matter, that it's there and that it's affecting things gravitationally and keeping galaxies together.

But the question is, how much more do we know about it? What else do we know about this mysterious thing, if it even is a thing? That's right. We would love to know what dark matter is made out of, and particle physicists like me scratch their heads all day wondering what kind of particle isn't made out of or many particles or is it a particle

at all? But along the way while we're looking for its particle nature we have other ways to try to get clues as to what it might be. By looking at how it moves and how it clumps and how it squishes and how it fuzzes, we can try to get a handle on what it is or isn't. Yeah, and so to the end of the program

we'll be asking the question is dark matter hot? Or not? Well, for those of you who are a little bit older, you might remember a popular website a few decades ago called Hot or Not, which was probably inappropriate these days. Totally inappropriate, exactly. Yeah, rated people based on their hotness and I'm guessing it was not the temperature. No, it was not the temperature. Although maybe we should revive it in a physics version, like is the top cork hot or not? Our

neutrinos hot or not. That might be interesting. Or cold, maybe based on how much funding you can get for it. That's right. And it's a weird combination of ideas, you know, dark matter, mysterious, blobs of stuff out there in the universe and temperature. But it turns out to be very important. And it's one of the most powerful handles we have on the nature of dark matter and one of the most valuable clues we have that tells us what it is and what it can't be. Yeah. So as usual, we were wondering how many people out there had thought about this question of whether dark matter is hot or cold. And so as usual, Daniel went out there into the wilds of the internet to ask people this question. That's right. So thank you.

to everybody who was willing to participate in our random person on the internet questions. And if you'd like to answer random questions from me in preparation for a future podcast, please write to us to questions at danielanhorpe.com. To think about it for a second, what would you answer if someone

asked you, is dark matter hot or cold? Here's what people have to say. I guess it seems most natural to me that dark matter would interact with itself. So in doing so, it's reasonable to think that it could have a temperature. I guess, relative to other dark matter.

So I guess it would be hot. What I think is that are parts of the dark matter that can be hot and parts that are going to be colder. I think dark matter is cold or at least cooler than normal matter on average. The average temperature of the universe is a few calvings above zero. And since we have no idea what is what are the constituent? particles of dark material? I think the answer is we have no idea. I don't know a whole lot about dark matter, but I don't usually think of matter having a specific temperature. I'd say we don't

know because we don't even know what it is. I would say that it's probably not hot. Well, hot and cold are relative terms. So if what you mean is does dark matter have a temperature, then I would say probably not because everything with a temperature gives off infrared radiation. had to consult my 11-year-old, who is the cosmologist in our family. So we think that dark matter is cold. The only reason we know it exists is because it reacts with gravity. I don't think it reacts with anything on the electromagnetic spectrum. So it wouldn't be hot or cold. But knowing scientists, they found some intrinsic property of dark matter and named it hot and cold, even though it doesn't mean anything like hot or cold.

Right. I like how people evaded the question very expertly. You're impressed by that or are you disappointed? I'm impressed. They're like, oh, they're thinking like physicists. Avoid answering the question. It's like, hmm, what is hot and cold? Let's divert into that discussion.

Well, we do this a lot in physics. We apply weird sounding characteristics to things. You know, like when we're talking about particles, we're talking about their spin that's not really spin. And we're talking about their mass, but they don't have any stuff to them. And so I understand what people are a little wary of interpreting, like, the temperature of dark matter. Like, what does that actually mean? What are we really talking about?

Yeah. Like, we don't even know if it's a thing. So how can a not thing have temperature? That's right. It feels like a detail. Like, are you worried about what color it is? You don't even know if it exists. Why do you care if it's purple or brown? Right, yeah. Yeah, what color is dark matter, Daniel? It's dark. All right, so let's break it down for folks. First of all, I guess the question is, how can dark matter even have a temperature if we don't know what it is. Right. Well, let's remember what temperature really means. For us, temperature is a macroscopic quantity, right? You touch something, it feels hot or it feels

cold. And that's really actually about the heat difference. Like if something has more energy in it than you do, then the heat flows from it into your finger, like when you touch a hot burner. And that's what you're feeling. So you don't actually measure temperature with your finger. You measure like a relative heat. But when we think about temperature like microscopically, we try to understand how that experience of feeling things being hot or cold translates to like the motion of the particles inside it. And so most loosely we think about temperatures relating to how fast those particles inside something are moving. Like a gas, if it's a hot gas, then the particles in it are moving really fast. That's right. And that's in fact what's happening. But also for

liquids and for solids. In fact, that's why liquids and solids are more solid than gases, right? Because their particles are not moving as much. They're more easily trapped by all the bonds and solid has various temperatures because the atoms in it can wiggle more or less. They can shake and vibrate in that kind of stuff. So it's all about the energy stored in those particles. It's like the motion of the particles inside, like the speed almost. Yeah. If you're talking about a gas, then it's mostly about the speed. And I think this is really interesting stuff to like take something that's macroscopic and kind of qualitative, you know, this feeling of temperature and try to understand it on the microscopic scale.

And it sometimes works and it doesn't always work. And we had a whole podcast where we talked about like the hottest things in the universe. And some of these things are counterintuitive. Like some of the hottest stuff in the universe is the interstellar plasma, which is like some crazy high temperature like 300,000 degrees Kelvin. But if we dropped you in it, you would freeze to death immediately. Right. And that feels counterintuitive.

Because there isn't much of it out there. That's right. of this plasma, hot plasma. It's very hot, but it's very dilute. So it doesn't contain a lot of heat. And so you're a much denser blob of heat. If we drop you in it, most of your heat would leak out. But the particles of that plasma individually are moving super duper fast. And so you can still call it hot. Right. So it's related to the speed or the vibration or like the kinetic energy of the molecules and particles in something. But how does that apply to dark matter? Because we don't really know if dark matter is made out of particles or not. we don't really know. Well, we know that something is out there creating gravity. We know that it's a

kind of matter. And that's really about it. We know sort of where it is in the universe. But you're right, we don't know that it's a particle that could turn out to be something else. And, you know, all the matter that we've ever seen in the universe so far has been made out of particles. So it seems tempting to say, well, then dark matter must also be made out of particles. But, you know, remember that dark matter is most of the stuff in the universe. We've only seen a little slice. We've seen five percent of the universe. So it's dangerous to extrapolate to like a full 25 percent and say the rest of it must also be made out of particles. But we don't really have better ideas. And so we typically just assume dark matters made out of particles. That's kind of like the working hypothesis.

Yeah. It's like let's try this. Let's see if it works. If it breaks, then we'll go back and examine all the assumptions we made. But when you're exploring the unknown, you've got to make some assumptions just to like have something to do. Because you can't just sit at home and go like, I don't know what dark matter is, you know, it sort of ends there. So we say maybe dark matter is a particle. And then we can ask if dark matter is made of particles, are those particles moving fast or are they moving slow? Right. Are they hot or not? Are they hot or not? That's exactly what that really means. It means is dark matter made out of super fast zippy particles moving at relativistic speeds or is it made of like heavier, slower moving particles that just sort of like float around

at slower speeds? I guess it's kind of weird to think of something being hot, but not being able to touch it. You know what I mean? Like, that's weird, right? I was just thinking, do neutrinos have a temperature? Like, can a neutrino who we can't interact with through electromagnetism? Can that have a temperature? Yes, absolutely.

Neutrinos are very hot. And the reason is that neutrinos have almost no mass. And so they zip through the universe at very, very high speeds. And so they contain a lot of energy. You would say they have a high temperature, but you're right that you can't feel them. And the reason is that you have no interaction in common with them. or almost none because all they feel is the weak force.

So they have all this energy, but they have no way to transmit it to you. So it's like you pass right through each other. And so they can have that high temperature. They can have that high energy. But if there's no common interaction, no way to communicate, there's no way for that energy to flow to you. And so you won't feel them being hot. What about like if dark matter is not a particle, can it still have a temperature?

Can something that's not a particle still be hot? Whoa, you just blew my mind. could something that's not made of particles have a temperature? We've never seen anything that's not made of particles. So that's quite a reach. But I guess macroscopically, you could like see if it emits light and everything in the universe that does emit light has a temperature. It's black body radiation.

But I don't know. That would be an amazing thing to explore if we discovered that dark matter wasn't made of particles because we do know something about its temperature, which is what we're going to talk about today. Oh, I see. So that it's made out of particles is not just a working hypothesis. It's like your only hypothesis. It's all we got at this point. It's like the one idea we've been using for 100 years or, you know, empty box for crazy new ideas somebody should come up with. Oh, really? Like, could it be something that's not a particle? It certainly could be. I mean, we have no concrete evidence that it is a particle other than all matter so far discovered is made of particles. Right. But it certainly could be. We're open to surprises. I mean, dark matter itself is a certain.

surprise. Its existence was a surprise. And there are been some ideas about unparticles, matter made out of things that are not quite particles that, you know, don't have a definitive size. But it's a bit fuzzy and nobody's really worked out the math for how it could be dark matter. So they're just sort of like the beginnings of ideas. I guess I mean, you know, like energy is energy also particle-based? Because you know, energy can have gravity or exert gravity or affect gravity. Energy density certainly has gravity and some energy is particle base like photons right photons are basically just energy they have no mass to them and photons contribute to the energy density of the universe and therefore it's curvature so certainly all right well i guess there's no maybe room in your equations so far to account for

something that's not a particle is that kind of what you're saying that's right yeah but i would love to blow up those equations i would love if we found something about dark matter that proved that it wasn't the particle and then we had to go back to the drawing board and think from scratch. That would be a tremendous breakthrough, an intellectual crack in the very foundations of physics, which is the kind of thing we're all hoping will happen, you know, because those are the moments you get like the real insights. It pull back the curtain and discover something surprising and fascinating about the universe. So, yeah, this is all we got so far and I would love to see it break into pieces. You'd love to prove that they're not so hot. Yeah. All these theories. Precisely. All right, well, let's get

into how we could tell whether or not dark matter has a temperature besides i guess feeling its forehead daniel that's right these days we're very sensitive to high temperatures but let's get into how we could tell and what it tells us about dark matter but first let's take a quick break the u.s open is here and on my podcast good game with sarah spain i'm breaking down the players from rising stars to legends chasing history, the predictions, well, we see a first-time winner, and the pressure. Billy Jean King says pressure is a privilege, you know. Plus, the stories and events off the court,

and of course the honey deuses, the signature cocktail of the U.S. Open. The U.S. Open has gotten to be a very fancy, wonderfully experiential sporting event. I mean, listen, the whole aim is to be accessible and inclusive for all tennis fans, whether you play tennis or not. Tennis is full of compelling stories of late. Have you heard about Icon Venus Williams' recent wildcard bids or the young Canadian, Victoria Mboko, making a name for herself? How about Naomi Osaka getting back to form?

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Think you could do it? It turns out that nearly 50% of men think that they could land the plane with the help of air traffic control. And they're saying like, okay, pull this, pull that, turn this. It's just, I can do my eyes close. I'm Mani. I'm Noah. This is Devon.

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DNA. Using new scientific tools, they're finding clues in evidence so tiny you might just miss it. He never thought he was going to get caught. And I just looked at my computer screen. I was just like, ah, gotcha. On America's Crime Lab, we'll learn about victims and survivors, and you'll meet the team behind the scenes at Othrum, the Houston Lab that takes on the most hopeless cases to finally solve the unsolvable. Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, Daniel, we're talking about whether dark matter is hot or cold. And so we're talking about how we have to kind of assume that it's a particle, because that's the only idea that we have.

And so if it's a particle, then you can talk about whether those dark matter particles are moving a lot or vibrating a lot, in which case would make them technically hot, even though we can't feel it. That's right. And we're really interested in whether dark matter is fast moving. or slow moving because it tells us also whether the particle is heavy, in which case it's more likely slow moving and cold, or very, very low mass, in which case it's probably faster moving and hot. So we're using this as a way to sort of get a clue

as to the nature of dark matter itself. But could dark matter be both? Like, I mean, could it be like regular matter that some of it is hot and some of it is cold? Totally. Absolutely. Dark matter could be lots of different particles, some of which are very heavy and some of which are very light. But we know that dark matter sticks around for a very, very, long time. It's like cosmologically stable. It's been here since the beginning. It's affected

the structure of the universe. We've seen it, put its imprint on the whole history of the universe. And so that suggests that it's probably stable, that it's not changing a lot from one kind of mass to another. But, you know, we really just don't know. All right. Well, let's get into now how we could tell whether dark matter has the temperature or not. Like, how do you, how would you even measure the temperature of dark matter? If a particle of dark matter was moving a lot or briberating a lot or not, could we even tell the difference? can actually tell the difference, and I think this is really clever. It's one of the most elegant pieces of science that I've seen recently. We can tell whether dark matter is moving

fast or slow because of the way it makes an imprint on the growth of the universe. You know, the universe started from like the Big Bang, and back then things were hot and dense and mostly uniform. And then you got little quantum fluctuations, little pockets of density here and less density there. And those pockets are critical because that's what seeds the whole structure of the universe. Like the reason we have a galaxy here and not over there is because some initial fluctuation made things a little dense and then gravity clumped them together and clump them together even further. So you got these little fluctuations in the early universe which see the structure of the universe, right? Because gravity takes over from these little

wrinkles. But dark matter plays a really big role in that because dark matter basically is gravity, right? It's the biggest source of gravity in the universe. And so where dark matter is, is and how it's distributed determines the shape and the structure of the whole universe. And so we can tell from like pictures of the Big Bang, can tell the temperature of dark matter at the beginning of time or right now? Well, we can tell the temperature of dark matter sort of over the history of the universe. Everything is cooling down, but we can tell whether dark matter was made hot or made cold. Everything is getting colder over time, but we can tell whether dark matter started out

hotter or colder. And we can do that by seeing whether or not it's moved around a lot. whether or not it's been wiggling around and that's affecting the structure of universe or whether it's been mostly staying in the places it was made. Oh, I see. Because I guess you assume that it's kind of like a gas, right? Like you don't assume it's a solid. You assume that it's, you know, kind of moving around freely. It's not tied together to itself, except with gravity. That's right. Only held together with gravity. And so we think of it like a diffuse gas, like a pressureless gas that doesn't even bounce against itself. And so basically it just has gravitational effects. And so we can sort of walk through the history of the universe with a cold version of dark matter,

a version where dark matter is mostly staying where it was. And then we can walk through a version of the universe where dark matter is hot, where it's zipping around really fast. And we see that those two things predict different shapes of the universe that we see today and also different histories of the universe. And then we can compare those histories to what we actually see. Because like if the dark matter at the beginning of time was super cold, then I guess it, particles themselves don't have enough speed to, like, go off and spread out. They would sort of stay clumped together. That's exactly right.

So if dark matter is very cold, then the structure of the universe forms sort of bottom up. Everything is where it was, and it's not zipping around very much. And so you get these little clumps of density from those initial wrinkles, and that's what seeds like the formation of stars. And then stars get together and they form galaxies, and galaxies pull themselves together to form galaxy clusters. So you get this structure formation that's sort of bottom up. Everything starts clumping where it was and then pulls together.

So you get, for example, galaxies forming before galaxy clusters. You get stars forming, then galaxies, then galaxy clusters in that order. And we can look back through the history of time. Because remember, as we look out through space, we're looking backwards in time. So we can see where there are galaxies a billion years after the universe started, where there's stars, which order did things get made? we can tell by looking deep into the history of the universe just by looking far out into space. Right.

And I guess you're using relative terms, right? Like cold and hot here. You're not thinking about a specific temperature because that could maybe also depend on how heavy these particles are. That's right. We're mostly talking about whether or not they're relativistic. Like are they moving it close to the speed of light or are they not relativistic? You know, they're moving on much less than the speed of light. So when you say hot, you mean like super duper hot.

Light speed hot. Yeah, exactly. And when we think about what hot dark matter would look like, well, you have the early universe and dark matter is made just with everything else. And you get these initial little clumps of density from quantum fluctuations. But if dark matter is most of the stuff and it's moving really, really fast, then those initial little blobs of density don't really matter because dark matter sort of washes them all out. Like the dark matter's flying everywhere, super duper fast.

And so those initial little clumps get evened out. They get smoothed out. So you don't get stars forming first. Instead, you get these, like, these really big, super massive blobs of stuff because only the really big over densities, only the really big clumps from the beginning stick around and survive the dark matter spreading everything out to form some structure. What do you mean? So if the dark matter is hot, it means that the, its particles are moving a lot.

And so are you saying the dark matter is more diffuse or like the blobs are moving around fast? Both. They're moving around fast. and so they spread out, and so it gets more even. And so it's harder for gravity to get a handle and start forming stars, for example, because things get smooth. For gravity to form a star, you need like a little blob that's denser than the stuff around it, that it can gather stuff together using gravity.

But if dark matter, which is most of the stuff, it's moving fast, then it's spread everything out. It's smoothed everything over. There's nothing for gravity to get a handle on except for the really, really big stuff, because that's the stuff that dark matter can't smooth out. Right. And so instead of getting stars and then galaxies and the galaxy clusters and then superclusters, you start out with super cluster sized blobs of stuff and then it breaks up into galaxy cluster size blobs of stuff. And those break up into galaxy size blobs of stuff and then you get stars forming. So it's sort of like top down instead of bottom up. Interesting. Just based off of the temperature of dark matter.

Yeah. So the temperature of dark matter totally determines the entire history. of the universe like the universe would be very different if we had no dark matter because it wouldn't have been around to clump the normal matter together into stars and galaxies and also the universe would be different if we had hot or cold dark matter just it's such a dominant force it's most of the gravity so it affects how the universe came together wow and we can actually tell the history of the universe whether things form bottom up or top down yeah because we can look back in time And we can say, well, were there galaxies in the first billion or two years after the Big Bang? Or did it take a while for galaxies to form?

And so we can look back in time and we can ask whether these things were made in what order were they made. And also, it affects the way things look today because things would be smoother today if dark matter was hot. And things would be sort of clumpier today if dark matter was cold. Like, for example, our galaxy is the Milky Way. And if dark matter was cold, then we expect that the Milky Way has a bunch of like little galaxies orbiting it, like the way the Earth has the moon. We expect that the Milky Way has its own little like mini galaxies that orbit our galaxy. If dark matter was super cold. If dark matter was cold, then there should have been these blobs of stuff formed outside of our galaxy, these dwarf galaxies, which would now be orbiting the Milky Way.

And that we should see that today. So that would be a sign that dark matter is cold. It affects not just the history of the universe, but it also affects the shape of the way things look today. Yeah. I guess it's, I mean, it's such a huge part of the universe that, you know, whether it's hot or not, it should be no surprise that it determines the fate of the universe

because it's such a huge chunk of it. Yeah, exactly. It's not a little detail. It's not like a tiny bit of salt that you add to your recipe, right? It's most of the stuff in the universe. And so, of course, it's going to have big consequences for how the universe looks and how it comes together. All right.

It could be hot or cold, and we could probably tell by looking at the structure and the history. I guess the history is also important of the universe. The history kind of tells us a clue about whether it's hot or not. That's right. Did the structure form top down, big stuff first and then small stuff? Or did it form bottom up, like small stuff first, which then came together to make the bigger stuff? And it also affects the way things look in our universe today. Right.

All right. Let's now answer the question whether dark matter is hot or not. and what that tells us about it. But first, let's take another quick break. The U.S. Open is here, and on my podcast, Good Game with Sarah Spain, I'm breaking down the players from rising stars to legends chasing history, the predictions, well, we see a first-time winner, and the pressure. Billy Jean King says pressure is a privilege, you know.

Plus, the stories and events off the court, and of course the honey deuses, the signature cocktail of the U.S. Open. The U.S. Open has gotten to be a very fancy, wonderfully experiential sporting event. I mean, listen, the whole aim is to be accessible and inclusive for all tennis fans, whether you play tennis or not. Tennis is full of compelling stories of late. Have you heard about icon Venus Williams' recent wildcard bids or the young Canadian, Victoria Mboko, making a name for herself? How about Naomi Osaka getting back to form? To hear this and more, listen to Good Game with Sarah Spain.

I heart women's sports production in partnership with deep blue sports and entertainment on the IHeart radio app, Apple Podcasts, or wherever you get your podcasts. Presented by Capital One, founding partner of IHeart Women's Sports. Imagine that you're on an airplane and all of a sudden you hear this. Attention passengers. The pilot is having an emergency and we need someone, anyone, to land this plane. Think you could do it? It turns out that nearly 50% of men think that they could land the plane with. the help of air traffic control. And they're saying like, okay, pull this, do this, pull that, turn this.

It's just, I can do it my eyes close. I'm Manny. I'm Noah. This is Devin. And on our new show, no such thing. We get to the bottom of questions like these. Join us as we talk to the leading expert on overconfidence. Those who lack expertise lack the expertise they need

to recognize that they lack expertise. And then as we try the whole thing out for real. Wait, what? Oh, that's the run right. I'm looking at this thing. Listen to no such thing on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. A foot washed up a shoe with some bones in it.

They had no idea who it was. Most everything was burned up pretty good from the fire that not a whole lot was salvageable. These are the coldest of cold cases, but everything is about to change. Every case that is a cold case that has DNA right now in a backlog. will be identified in our lifetime.

A small lab in Texas is cracking the code on DNA. Using new scientific tools, they're finding clues in evidence so tiny you might just miss it. He never thought he was going to get caught, and I just looked at my computer screen. I was just like, ah, gotcha. On America's Crime Lab, we'll learn about victims and survivors, and you'll meet the team behind the scenes at Othrum,

the Houston Lab that takes on the most hopeless cases, to finally solve the unsolvable. Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, Daniel, is Dark Matter hot or not? Is it a swipe left or a swipe right for you? Well, I love Dark Matter.

I'm very excited by Dark Matter. I'm very attracted to dark matter, but I have to say that the universe tells us that dark matter is quite cold. It's not hot. It's definitely not hot. I mean, it'll be beautiful. It's just a little chilly. That's right. It's got its own standards of beauty.

And it's pretty cool, you know, dark matter. And we know that because we look at the history of the universe and we see that stars formed first and that then galaxies formed and that then galaxy structure is formed because we look back in the very early universe. and we see galaxies forming before there were clusters. And we see stars forming before there was galaxies. Can we tell that? Can we, how can we tell? I thought, like, we can only see really far out until the distance and the age of things

by looking at, like, supernovas. So how could we tell how things formed if our only way of knowing is through stars? That's right. Well, the supernovas tell us sort of like the distance ladder. And so we can tell how far away something is and therefore when it happened. And you're right that we need stars to happen to give us that distance ladder.

But we can go back and look at the early universe, right? That tells us like, okay, this is really, really far away. And for example, you would expect that there would be galaxy clusters formed in the very early universe if dark matter was hot. And so we look out past the most distant supernovas into the deep early universe, you know. And we can tell that these things happened, you know, 13 billion years ago, for example. And we don't see galaxy clusters form. out there in the very edges of the things that we can observe. That's the very earliest universe.

You're right. We can't get as precise an estimate for those distances because we don't have the supernovas, but we can extrapolate a little bit. And also we know it's super duper old. Oh, I see. So like the oldest stars that we can see tell us that things were not as formed as they are closer to us or closer to the present. That's right.

They tell us that the structure formed bottom up. That things came together in small clumps first. and then those small clumps organize themselves into bigger stuff. So you get stars and then galaxies and then galaxy clusters and then super clusters of galaxies, which is the latest structure to form. And that's why they're the biggest gravitationally bound objects in the universe because they have most recently come together. It takes a while for gravity to do this.

And galaxy super clusters are the last thing to have formed. It's all that we've had time to form so far in the universe. All right. Well, I guess so then that tells us. that dark matter is cold and I guess do we have a sense of how cold it is like you know not going at the speed of light I know that's how you define cold but is it like chilly or is it like warm or is are we talking like the temperature of the sun what are we talking about it's definitely not the temperature of the sun I mean if it's out there and it's a particle it's going to be very very

cold you know it's going to be a few degrees Kelvin really we think dark matter is only a few degrees Kelvin probably yeah and you know it's not interacting in the same way that like hydrogen does in the core of the sun to produce a huge amount of energy but there's still a lot we don't know about dark matter that could have self interactions that contain energy that we are not aware of and so everything we say here should be taken with a big grin of salt because it's all pretty speculative but you know also the cold dark matter picture is pretty good it works pretty well but it's not perfect like it doesn't perfectly explain everything that we see right like you were saying cold dark Matter predicts that we would have baby galaxies floating around us.

That's right. We expect to see a bunch of these dwarf galaxies orbiting the Milky Way, and we see some, but we don't see nearly as many as we expect. And we don't know yet, is that because Dark Matter isn't as cold as we thought, or is it because those dwarf galaxies are harder to see than we thought they would be? And recently, people have developed extra good techniques to find dwarf galaxies, and they found a few more, and that sort of closes the gap a little bit. But there's still some tension there.

It's still something that we don't quite understand. And, you know, we like those details. We like getting those things right because those are the things that tell us that our theory is really working. And so there's still some question marks about it, but it's definitely not hot. It's some version of cold. I guess we can't make any version in our simulations work out to be just like the universe we have now. Like if you tweak it further, you don't get the right proportion of dwarf or baby galaxies. Not yet.

But, you know, these simulations are very, very hard to do. because you're simulating an enormous number of particles. And when they do these simulations, they usually just like leave out all the normal matter, because the normal matter is a small fraction, and it's much harder to model because normal matter has complicated interactions, right? You know, stars and gas and all that stuff, it has pressure and complicated flows because of the electromagnetic interactions

and the strong interactions and all that stuff. So until recently, these simulations have mostly just removed all the baryonic matter. But, you know, baryons are, important. I'm a baryon, you're a baryon, stars are baryons. The whole visible part of the galaxy is made of baryon. What does that mean? That's the particles that we're made out of, regular matter, like quarks and electrons. And so when they do these simulations to describe the structure of the universe, they don't have the computational power to describe all the baryons, all the things that make me and you, corks and all that stuff. So they mostly just remove it as a simplification because that's the most complicated stuff to describe. And so

our simulations are really approximate right now. So people are working on ways to include normal matter in these simulations and try to get more precise estimates, more precise predictions for how many dwarf galaxies we should see. Yeah, I guess people are complicated. They're hard to predict, for sure. They are. They are hard to describe.

So we know, we think dark matter is made out of particles. And if it is, we think it's cold because that's what the universe is telling us. So what does that tell us about dark matter? Like, does it give us a clue about what it is? or what kind of particle it is? Or, you know, is the fact that it's cold? Does that tell you something about how it interacts with other forces? Yeah, it tells us a lot.

And what it can do is remove candidate particles from the list. And most specifically, it acts as the neutrino as a candidate for dark matter. For a long time, people thought, oh, there's a lot of invisible matter out there, a matter that almost never or never interacts with us except for gravitationally. Maybe it's just neutrinos. And it's a very tempting candidate because we already know about neutrinos. We know neutrinos are these wispy particles that can pass through a light year of lead

without interacting. The air around us is filled with neutrinos, but we can't feel them or taste them. They have a lot of energy, but they don't deposit it on us. And so it's tempting to assign these two mysteries together, right? The weirdness of neutrinos and the mystery of the missing matter. Maybe one plus one just equals two. And so for a long time, people suspected maybe, the missing matter was just like a ridiculous number of neutrinos.

And remember, neutrinos are very, very light that have hardly any mass per particle. It's not zero, but it's a small number. So if you're going to explain most of the stuff in the universe with neutrinos, it would have to be an ungodly number of neutrinos. Could it be like a heavy neutrino? Like I know neutrinos, they can have different masses, right? The neutrinos that we're aware of the three, the electron muon and town neutrinos all have very, very, very small masses.

Right. And so what we can do is we can rule out those. We can say it's not one of the neutrinos that we know. It's not one of the neutrino lights. Yeah, exactly, because those neutrinos have such small mass that they're always moving basically at the speed of light or very close to the speed of light. For example, when neutrinos come from a supernova, they arrive very close to the same time as the photons arrive because they're traveling basically at the speed of light.

Actually, the neutrinos get here first because the photons get slowed down by interacting with the star. but it's basically a race. The neutrinos fly at almost the speed of light. You're saying they're faster than light, Daniel? They are not faster than light. They leave sooner. The photons spend more time packing, but they do travel a little faster.

But you're exactly right that there's the possibility that there could be some weird, heavy neutrino. So not the neutrinos that we're familiar with. But if there is another kind of neutrino, a fourth neutrino or many other kinds of neutrinos that are very heavy, then those are still valid candidates for the dark matter. And those go by the terms like sterile neutrinos called sterile because maybe they interact with our kind of matter even less.

Wow. It's like a neutral neutrino. Yeah, that's right. It's like an even more standoffish and snobbish particle than the neutrino. And that's a hard standard to me. Oh, I was just thinking like shy or, you know, loathe to interact with other particles.

There you go, the introvert neutrino. That's right. But you just assume that, you know, it's a snob. My apologies, sterile neutrinos. I dig it back. All right. So that tells us that they can't, be neutrinos because neutrinos usually go really fast, but they could be basically that doesn't

leave you much. It just tells you that it's another kind of particle that we don't know about. And that's an important clue because that means that there's no particle on our current list that fits the requirements. There's no particle out there that doesn't have electromagnetic or strong interactions and is heavy. There just isn't one. The only particle in our current list that had any chance of being the dark matter were neutrinos. And this people, piece of evidence rules that out. It says it can't be one of the neutrinos we know. So it has to be a new particle. And that's exciting. A new heavy particle. A new heavy particle, exactly. It means that there's something new to discover. It's not just, oh, there are more of this particle than we thought. It means

there's a new particle. Right. And a new particle is interesting because you wonder like, why does it exist? How many new particles are there? Where did it come from? Why is it different from these other particles? You know, it gives you a whole new set of questions to ask a whole new way to look at the universe. And you guys are looking for these in the particle colliders, right? You're smashing particles, hoping that a new kind of particle will pop out and you might say, hey, that's dark matter. That's right. And we have specific ideas for what this new particle could be. We have ideas like the WIMP particle, weakly interacting massive particle. It's just a generic name, meaning some big, heavy particle that doesn't interact very much. And it has to not interact very much in order to be the dark matter,

and it has to be massive in order to be cold because of the structure of the universe. And another idea is the axion, the axon could be the dark matter. Right. And we have specific experiments to look for wimps and for axions. We just did a podcast episode about axions. Right. They're not the same thing. They are not the same thing.

They're two very different kinds of particles. The axon is like a heavier version of the photon. And the wimp is like, it's like a heavier version of the neutrino, but maybe interacts even less. And we have experiments underground to look for wimps, these big tanks of liquid argon for example, or liquid xenon that look for one wimp coming through and knocking into a bunch of particles and then giving us a signal. We're using space telescopes to look to see if occasionally wimps bounce into each other and give off a little flash of light that we could see, which would be really, really rare

because dark matter is dark. But, you know, we look at places where there's a lot of dark matter and try to see the occasional blip. And then we try to make dark matter in the collider to see if we can create it in place. with it there. So far, none of these experiments have turned up any evidence for dark matter that anybody believes. And so we're still in the hunt. But, you know, even though we don't know what dark matter is, we're able to say some things about what it isn't. Right. Is it weird that you haven't found dark matter in these colliders? I mean, like in the universe, there's five times more dark matter than regular matter, which might make you think that it's like it's more likely

to happen. But in our colliders, you can't seem to make even a little bit of it. That's right. is a little weird. Now, on one hand, it may be that dark matter's everywhere, but we can't make it because we're playing with our kind of matter. Like, our kind of matter might not interact with dark matter, which means that we can't use our matter to look for dark matter. And we can't use our matter to make dark matter. Like for that to work, for any of the experiments I just described to work, to discover the particle nature of dark matter means there has to be some way for our particles to talk to the dark matter particles, to share some sort of new dark photon

or some new force has to exist that works on both particles. And it could be that it just doesn't. It could be that dark matter is out there. It's a particle. And it just feels nothing except for gravity, in which case, basically hopeless for us to discover its particle nature. Because gravity is so weak that we can only detect dark matter when you have enormous, like, galaxy-sized blobs of it.

which makes it pretty hard to do particle experiments. But I thought when you smash particles, it turns into like pure energy and then anything can come out of it. Are you saying that maybe it's possible that not even dark matter can come out of that? That's right. When you smash particles together, it's not exactly pure energy. It turns into one of the bosons of the forces that can interact with those particles. So, for example, when you smash a quark and an anti-quark together, you can get a glue on or you can get a photon or you can get a W boson. But if those forces, the weak and the strong force and electromagnetism, don't interact with dark matter,

then those bosons, which represent that energy, can't then turn into dark matter. And so that is one limitation. I know that I like to say on this podcast that we can use colliders to explore the universe because anything that can be made will be made. But there is an important caveat there, that whatever can be made has to somehow interact with the particles that we're smashing. And if there's no way to interact, then you just can't make it. Wow. You need a dark matter collider, Daniel, obviously. To discover dark matter, you have to build a dark matter collider. All right. Well, it sounds like we don't know what dark matter is still, but we know that it's pretty cool.

It's a pretty cool thing in the universe. It's cold. That's right. Dark matter is pretty chill. You know, it wants to come over and watch Netflix with you, even if you don't think it's hot. Yeah. All right. Well, again, just makes you think about all the crazy things we don't know, you know. and all the sort of fun and clever ways we can tell about things we don't know, even though we don't know anything about it. Yeah. And this is what science does is we probe things from every direction. We're trying to uncover a real truth about the universe,

and that has lots of facets. And so if we get stumped in one direction, like we can't seem to find it in our detectors, then we go another route and say, well, can we say anything about it from this perspective or from that perspective? And we're trying to be clever in the field of particle physics and science in general is filled with, clever people, having new ideas about ways to answer these questions.

And so to me, this is one of the most elegant ways to put a really important, really insightful constraint on what dark matter is and isn't. All right. Well, I think we answered that question pretty good. And I think we can all learn a little bit from dark matter to just be cool. Don't get too excited. Thanks for joining us. See you next time. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of IHeartRadio. For more podcasts from IHeartRadio, visit the IHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.

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