Speaker 1: Hey, Daniel, I have a question about dark matter. Oh, man,
Speaker 1: don't we all? I mean, I know that we don't
Speaker 1: know what it is, right, but what is it like?
Speaker 1: I mean, is it quishy? We don't know. What does
Speaker 1: it taste like? Well, you know, our tongues can't taste it,
Speaker 1: so again we don't really know. But is it fuzzy
Speaker 1: maybe we don't know, or scratchy? Probably not. But again
Speaker 1: we just don't know. You know, for such a hot topic,
Speaker 1: you would think you guys would know more about it. Well,
Speaker 1: that's one thing we do know, whether dark matter is
Speaker 1: hot or not. Hi am more Hammon, cartoonists and the
Speaker 1: creator of PhD comments. I'm Daniel. I'm a particle physicist,
Speaker 1: and I have no opinion about the attractiveness of dark matter. Well,
Speaker 1: it's definitely attractive, right, gravitationally speaking on a cosmological level.
Speaker 1: That's right. It is the great attractor from that point
Speaker 1: of view. But welcome to our podcast, Daniel and Jorge
Speaker 1: Explain the Universe, a production of I Heart Radio in
Speaker 1: which we talk about all the amazing and crazy things
Speaker 1: in our universe, the things that scientists have understood and
Speaker 1: the things that scientists are now working to understand. We
Speaker 1: break down all the crazy for you and explain it
Speaker 1: in a way that hopefully makes you smile. It's right,
Speaker 1: all the things that are hot in this universe and
Speaker 1: all the things that are not hot or cold or
Speaker 1: super cold, because the universe has a broad range. Right,
Speaker 1: things can be as hot as a million degrees or
Speaker 1: as cold as zero degree. That's right. Everything has a temperature,
Speaker 1: even black holes, we all have a rating. That's right.
Speaker 1: Most of the universe out there is at a very
Speaker 1: cold two point seven three degrees kelvin. But there are
Speaker 1: a few hot spots a place like Earth where hot
Speaker 1: little bits of temperature clue to make life an interesting podcast.
Speaker 1: And so we like to talk about in this podcast
Speaker 1: about dark matter a lot. And I feel like we
Speaker 1: talked about it a lot because it's such a huge mystery.
Speaker 1: I mean, it's of the universe and we don't know
Speaker 1: what it's made out of. I think it's one of
Speaker 1: the biggest open questions in science. You know, the person
Speaker 1: or the group that figures out, like what is dark matter? Anyway,
Speaker 1: that will be a historic moment, that will be a
Speaker 1: an understanding and achievement, a breakthrough that will go down
Speaker 1: in history for sure. Do you think a Nobel Prize
Speaker 1: would be enough for that discovery or they need to
Speaker 1: like stack him up or something, or maybe make up
Speaker 1: like a special Nobel Prize, the dark Nobel Prize. You know,
Speaker 1: they should have already given a Nobel Prize to Vera
Speaker 1: Reuben for the discovery that dark matter was out there.
Speaker 1: Even if we don't know what it is, we know
Speaker 1: it's there, we know it's matter, and Nobel Prize committee
Speaker 1: overlooked very Reuben. Some say because she's a woman, that's terrible.
Speaker 1: That's the dark history of the Nobel Prize. It's the
Speaker 1: dark history of dark matter. But we know something. There's
Speaker 1: a little bit about dark matter. Let it's there, and
Speaker 1: that it's affecting things gravitationally and keeping galaxies together. But
Speaker 1: the question is how much more do we know about it?
Speaker 1: What else do we know about this mysterious thing, if
Speaker 1: it even is a thing. That's right, We would love
Speaker 1: to know what dark matter is made out of. And
Speaker 1: particle physicists like me scratch their heads all day wondering
Speaker 1: what kind of particle is it made out of? For
Speaker 1: many particles or is it a particle at all? But
Speaker 1: along the way while we're looking for its particle nature,
Speaker 1: we have other ways to try to get clues as
Speaker 1: to what it might be. By looking at how it
Speaker 1: moves and how it clumps, and how it squishes and
Speaker 1: how it buzzes, we can try to get a handle
Speaker 1: on what it is or isn't. Yeah, and so to
Speaker 1: be on the program, we'll be asking the question is
Speaker 1: dark matter hot or not? Well, for those of you
Speaker 1: who are a little bit older, you might remember a
Speaker 1: popular website a few decades ago called hot or Not,
Speaker 1: which was probably inappropriate these days, totally inappropriate exact Yeah,
Speaker 1: rated people based on their hotness. And I'm guessing it
Speaker 1: was not the temperature. No, it was not the temperature,
Speaker 1: although maybe we should revive it in a physics version,
Speaker 1: like is the top cork hot or not? Our neutrinos
Speaker 1: hot or not? That might be or cold maybe based
Speaker 1: on how much fun thing you can get for it.
Speaker 1: That's right, And it's a weird combination of ideas, you know,
Speaker 1: dark matter, mysterious blobs of stuff out there in the universe,
Speaker 1: and temperature, But it turns out to be very important.
Speaker 1: It's one of the most powerful handles we have on
Speaker 1: the nature of dark matter and one of the most
Speaker 1: valuable clues we have that tells us what it is
Speaker 1: and what it can't be. Yeah, So, as usual, we
Speaker 1: were wondering how many people out there had thought about
Speaker 1: this question of whether dark matter is hot or cold,
Speaker 1: and so as usual Daniel went out there into the
Speaker 1: wilds of the Internet to ask people this question. That's right.
Speaker 1: So thank you to everybody who was willing to participate
Speaker 1: in our random person on the internet questions. And if
Speaker 1: you'd like to answer random questions from me in preparation
Speaker 1: for a future podcast, please is right to us two
Speaker 1: questions at Daniel and Jorge dot com. To think about
Speaker 1: it for a second, what would you answer if someone
Speaker 1: asked you, is dark matter hot or cold? Here's what
Speaker 1: people have to say. Yes, it seems most natural to
Speaker 1: me that dark matter would interact with itself, so it's
Speaker 1: doing so. It's reasonable think that it could have a
Speaker 1: temperature um it's relative to other dark matter, so I
Speaker 1: guess it will be hot. What I think it's that
Speaker 1: are parts that of the dark matter that can be
Speaker 1: hot and parts that are gonna be colder. I think
Speaker 1: dark matter is cold, or at least cooler than normal matter.
Speaker 1: On average average temperature of the universe is a few
Speaker 1: codings about zero. And since we have no idea, what
Speaker 1: what is what other constituent particles of dark mastra? I
Speaker 1: think the answer is we have no idea. I don't
Speaker 1: know a lot about dark matter, but I don't usually
Speaker 1: ink of matter having a specific temperature. I'd say we
Speaker 1: don't know, because we don't even know what it is.
Speaker 1: I would say that it's probably not hot. Well, hot
Speaker 1: and cold are relative terms, So if what you mean
Speaker 1: is does dark matter have a temperature, then I would
Speaker 1: say probably not, because everything with the temperature gives off
Speaker 1: infrared radiation. I had to consult my eleven year old,
Speaker 1: who is the cosmologist in our family. So we think
Speaker 1: that dark matter is cold. The only reason we know
Speaker 1: it exists is because it reacts with gravity, and I
Speaker 1: don't think it will react with anything on the electromagnetic spectrum,
Speaker 1: so it wouldn't be hot or cold. Both knowing scientists,
Speaker 1: they fancim intrinsic property of dark matter and named it
Speaker 1: hot and cold, even though it doesn't mean anything like
Speaker 1: hot or cold. All right, I like how people evaded
Speaker 1: the question very expertise. You're impressed by that are you disappointed?
Speaker 1: I'm impressed. They're like, Oh, they're thinking, like physics avoid
Speaker 1: answering the question. It's like what is hot and cold?
Speaker 1: Let's divert into that discussion. Well, we do this a
Speaker 1: lot in physics. We apply weird sounding characteristics to things,
Speaker 1: you know, like when we're talking about particles, we're talking
Speaker 1: about their spin that's not really spin, and we're talking
Speaker 1: about their mass, but they don't have any stuff to them.
Speaker 1: And so I understand why people are a little wary
Speaker 1: of interpreting like the temperature of dark matter, Like what
Speaker 1: does that actually mean? What are we really talking about? Yeah, Like,
Speaker 1: we don't even know if it's a thing, So how
Speaker 1: can I not thing have temperature? That's right? It feels
Speaker 1: like a detail, Like are you worried about what color
Speaker 1: it is? You don't even know if it exists. Why
Speaker 1: do you care if it's purple or brown? Right? Yeah? Yeah?
Speaker 1: What color is dark matter? Then you it's dark? Alright,
Speaker 1: So let's break it down for folks. And first of all,
Speaker 1: I guess the question is, how can dark matter even
Speaker 1: have a temperature if we don't know what it is? Right? Well,
Speaker 1: let's remember what temperature really means. For us, temperature is
Speaker 1: a macroscopic quantity. Right, you touch something, it feels hot
Speaker 1: or it feels cold, and that's really actually about the
Speaker 1: heat difference, Like if something has more energy in it
Speaker 1: than you do, then the heat flows from it into
Speaker 1: your finger, like when you touch a hot burner, and
Speaker 1: that's what you're feeling. So you don't actually measure temperature
Speaker 1: with your finger. You measure like a relative heat. But
Speaker 1: when we think about temperature, like microscopically, we try to
Speaker 1: understand how that experience of feeling things being hot or
Speaker 1: cold translates to like the motion of the particles inside it.
Speaker 1: And so most loosely, we think about temperatures relating to
Speaker 1: how fast those particles inside something are moving. Like a gas.
Speaker 1: If it's a hot gas, then the particles in it
Speaker 1: are moving really fast, that's right, and that's in fact
Speaker 1: what's happening. But also for liquids and for solids, and
Speaker 1: in fact that's why liquids and solids are more solid
Speaker 1: than gases, right, because their particles are not moving as much,
Speaker 1: they're more easily trapped by all the bonds, and solid
Speaker 1: has various temperatures because the atoms and it can wiggle
Speaker 1: more or less they can shake and vibrate in that
Speaker 1: kind of stuff. So it's all about the energy stored
Speaker 1: in those parts, like the motion of the particles inside,
Speaker 1: like the speed almost. Yeah, if you're talking about a gas,
Speaker 1: then it's mostly about the speed. And I think this
Speaker 1: is really interesting stuff to like take something that's macroscopic
Speaker 1: and kind of qualitative, you know, this feeling of temperature,
Speaker 1: and try to understand it on the microscopic scale, and
Speaker 1: it sometimes works and it doesn't always work. And we
Speaker 1: had a whole podcast where we talked about like the
Speaker 1: hottest things in the universe, and some of these things
Speaker 1: are counterintuitive. Like some of the hottest stuff in the
Speaker 1: universe is the interstellar plasma, which is like some crazy
Speaker 1: high temperature like three thousand degrees kelvin. But if we
Speaker 1: dropped you in it, you would freeze to death immediately.
Speaker 1: And that feels counterintuitive because there isn't much of it
Speaker 1: out there that's right of this plasma. It's very hot,
Speaker 1: but it's very dilute, so it doesn't contain a lot
Speaker 1: of heat, and so you're much denser blob of heat.
Speaker 1: If we dropped doing it, most of your heat would
Speaker 1: leak out but the particles of that plasma individually are
Speaker 1: moving super duper fast, and so you can still call
Speaker 1: it hot, right, So it's related to the speed and
Speaker 1: the get or the vibration or like the kinetic energy
Speaker 1: of the molecules and particles in something. But how does
Speaker 1: that apply to dark matter, because we don't really know
Speaker 1: if dark matter is made out of particles or not.
Speaker 1: We don't really know. Well, we know that something is
Speaker 1: out there creating gravity. We know there's a kind of matter,
Speaker 1: and that's really about it. We know sort of where
Speaker 1: it is in the universe, but you're right, we don't
Speaker 1: know that it's a particle that could turn out to
Speaker 1: be something else. And you know, all the matter that
Speaker 1: we've ever seen in the universe so far has been
Speaker 1: made out of particles. So it seems tempting to say, well,
Speaker 1: then the dark matter must also be made out of particles.
Speaker 1: But you know, remember that dark matter is most of
Speaker 1: the stuff in the universe. We've only seen a little slice.
Speaker 1: We've seen five percent of the universe, so it's dangerous
Speaker 1: to extrapolate to like a full and say the rest
Speaker 1: of it must also be made out of particles. But
Speaker 1: we don't really have better ideas, and so we typically
Speaker 1: just assume dark matters made out of particles. So that's
Speaker 1: kind of like the working hypothesis. Yeah, it's like, let's
Speaker 1: try this, let's see if it works. If it breaks,
Speaker 1: then we'll go back and examine all the assumptions we made.
Speaker 1: But when you're exploring the unknown, you've got to make
Speaker 1: some assumptions just to like have something to do, because
Speaker 1: you can't just sit at home and go like, I
Speaker 1: don't know what dark matter is. You know, it sort
Speaker 1: of ends there. So we say, maybe dark matters of
Speaker 1: particle and then we can ask if dark matter is
Speaker 1: made of particles, are those particles moving fast or are
Speaker 1: they moving slow? Right? Are they hot or not? Are
Speaker 1: they hot or not? That's exactly what that really means.
Speaker 1: It means is dark matter made out of super fast,
Speaker 1: zippie particles moving relativistic speeds or is it made of
Speaker 1: like heavier, slower moving particles that just sort of like
Speaker 1: float around its slower speeds. I guess it's kind of
Speaker 1: weird to think of something being hot but not being
Speaker 1: able to touch it, you know what I mean? Like,
Speaker 1: that's weird, right, I was just thinking, do neutrinos have
Speaker 1: a temperature, like in a neutrino who we can't interact
Speaker 1: with through electromagnetism, Can that have a temperature? Yes, absolutely,
Speaker 1: neutrinos are very hot, and the reason is that neutrinos
Speaker 1: have almost no mass, and so these through the universe
Speaker 1: at very very high speeds, and so they contain a
Speaker 1: lot of energy. You would say they have a high temperature,
Speaker 1: but you're right that you can't feel them, and the
Speaker 1: reason is that you have no interaction in common with them,
Speaker 1: or almost none, because all they feel is the weak force.
Speaker 1: So they have all this energy, but they have no
Speaker 1: way to transmit it to you. So it's like you
Speaker 1: pass right through each other. And so they can have
Speaker 1: that high temperature, they can have that high energy, but
Speaker 1: if there's no common interaction, no way to communicate, there's
Speaker 1: no way for that energy to flow to you, and
Speaker 1: so you won't feel them being hot. What about like
Speaker 1: if what if dark matter is not a particle? Can
Speaker 1: it still have a temperature and something that's not a
Speaker 1: particle still be hot? WHOA? You just blew my mind.
Speaker 1: Could something that's not made of particles have a temperature?
Speaker 1: We've never seen anything that's not made of particles, So
Speaker 1: that's quite a reach. But I guess macroscopically you could
Speaker 1: like see if it emits light, and everything in the
Speaker 1: universe that does emit light has a temperature, it's black
Speaker 1: body radiation. But I don't know. That would be an
Speaker 1: amazing thing to explore if we discover the dark matter
Speaker 1: wasn't made of particles, because we do know something about
Speaker 1: its temperature, which is what we're going to talk about today.
Speaker 1: I see, so that it's made out of particles is
Speaker 1: not just a working hypothesis. It's it's like your only hypothesis.
Speaker 1: It's all we got at this point. It's like the
Speaker 1: one idea we've been using for a hundred years, or
Speaker 1: you know, empty box for crazy new ideas somebody should
Speaker 1: come up with. Really, like, could it be something that's
Speaker 1: not a particle? It certainly could be. I mean, we
Speaker 1: have no concrete evidence that it is a particle other
Speaker 1: than all matter so far discovered is made of particles,
Speaker 1: but it certainly could be. We're open to surprises. I mean,
Speaker 1: dark matter itself is a surprise. Its existence was a surprise,
Speaker 1: and there have been some ideas about un particles matter
Speaker 1: made out of things that are not quite particles that
Speaker 1: you know, don't have a definitive size, but it's a
Speaker 1: bit fuzzy, and nobody's really worked out the math for
Speaker 1: how it could be dark matter, So they're just sort
Speaker 1: of like the beginnings of ideas, I guess. I mean,
Speaker 1: you know, like energy, is energy also particle based, because
Speaker 1: you know, energy can have gravity or exert gravity or
Speaker 1: effect gravity. Energy density certainly has gravity, and some energy
Speaker 1: is particle based, like photons, right, Photons are basically just energy.
Speaker 1: They have no mass to them, and photons contribute to
Speaker 1: the energy density of the universe, and therefore it's curvature.
Speaker 1: So certainly, all right, Well, I guess there's no maybe
Speaker 1: room and your equations so far to account for something
Speaker 1: that's not a particle, Is that kind of what you're saying.
Speaker 1: That's right, yeah, But I would love to blow up
Speaker 1: those equations. I would love if we found something about
Speaker 1: dark matter that proved that it wasn't the particle and
Speaker 1: then we have to go back to the drawing board
Speaker 1: and think from scratch. That would be a tremendous breakthrough,
Speaker 1: an intellectual crack in the very foundations of physics, which
Speaker 1: is the kind of thing we're all hoping will happen,
Speaker 1: you know, because those are the moment you get, like
Speaker 1: the real insights. It pulled back the curtain and discover
Speaker 1: something surprising and fascinating about the universe. So yeah, this
Speaker 1: is all we got so far, and I would love
Speaker 1: to see it break into pieces. You'd love to prove
Speaker 1: that they're not so hot, all these theories precisely. All right, Well,
Speaker 1: let's get into how we could tell whether or not
Speaker 1: dark matter has a temperature besides like I guess, feeling
Speaker 1: its forehead. Daniel, that's right, These days were very sensitive
Speaker 1: to high temperatures. But let's get into how we could
Speaker 1: tell and what it tells us about dark matter. But first,
Speaker 1: let's take a quick break, all right, Daniel, we're talking
Speaker 1: about whether dark matter is hot or cold, and so
Speaker 1: we talked about how we have to kind of assume
Speaker 1: that it's a particle because that's the only idea that
Speaker 1: we have. And so if it's a particle, then you
Speaker 1: can talk about whether those dark matter particles are moving
Speaker 1: a lot or vibrating a lot, which case wouldn't make
Speaker 1: them technically hot even though we can't feel it. That's right,
Speaker 1: and we're really interested in whether dark matter is fast
Speaker 1: moving or slow moving, because it tells us also whether
Speaker 1: the particle is heavy, in which case it is more
Speaker 1: likely slow moving and cold, or very very low mass,
Speaker 1: in which case is probably faster moving and hot. So
Speaker 1: we're we're using this as a way to sort of
Speaker 1: get a clue as to the nature of dark matter.
Speaker 1: It's but it could dark matter be both, like I mean,
Speaker 1: it could it be like regular matter that some of
Speaker 1: it is hot and some of it is cold. Totally absolutely,
Speaker 1: dark matter could be lots of different particles, some of
Speaker 1: which are very heavy and some of which are very light.
Speaker 1: But we know that dark matter sticks around for a
Speaker 1: very very long time. It's like cosmologically stable. It's been
Speaker 1: here since the beginning. It's affected the structure of the universe.
Speaker 1: We've seen it put its imprint on the whole history
Speaker 1: of the universe. And so that suggests that it's probably stable,
Speaker 1: that it's not changing a lot from one kind of
Speaker 1: mass to another. But you know, we really just don't know.
Speaker 1: All right, Well, let's get into now how we could
Speaker 1: tell whether dark matter has the temperature or not, Like,
Speaker 1: how do you how would you even measure the temperature?
Speaker 1: Of dark matter. If a particle of dark matter is
Speaker 1: moving a lot or vibrating a lot, or could we
Speaker 1: even tell the different We can actually tell the difference,
Speaker 1: and I think this is really clever. It's one of
Speaker 1: the most elegant pieces of science that I've seen recently.
Speaker 1: We can tell whether dark matter is moving fast or
Speaker 1: slow because of the way it makes an imprint on
Speaker 1: the growth of the universe. You know, the universe started
Speaker 1: from like the Big Bang, and back then things were
Speaker 1: hot and dense and mostly uniform, and then you've got
Speaker 1: little quantum fluctuations, little pockets of density here and less
Speaker 1: density there, and those pockets are critical because that's what
Speaker 1: seeds the whole structure of the universe. Like the reason
Speaker 1: we have a galaxy here and not over there is
Speaker 1: because some initial fluctuation made things a little dense, and
Speaker 1: then gravity clumped them together and clumped them together even further.
Speaker 1: So you've got these little fluctuations in the early universe,
Speaker 1: which see the structure of the universe, right, because gravity
Speaker 1: takes over from these little wrinkles. But dark matter plays
Speaker 1: a really big role in that because dark matter basically
Speaker 1: is gravity, right, It's the biggest source of gravity in
Speaker 1: the universe, and so where dark matter is and how
Speaker 1: it's distributed determines the shape and the structure of the
Speaker 1: whole universe. And so we can tell from like pictures
Speaker 1: of the Big Bang until the temperature of dark matter
Speaker 1: at the beginning of time or right now. Well, we
Speaker 1: can tell the temperature of dark matter over the history
Speaker 1: of the universe. Everything is cooling down, but we can
Speaker 1: tell whether dark matter was made hot or made cold.
Speaker 1: Everything is getting colder over time, but we can tell
Speaker 1: whether dark matter started out hotter or colder. And we
Speaker 1: can do that by seeing whether or not it's moved
Speaker 1: around a lot, whether or not it's been wiggling around
Speaker 1: and that's affecting the structure universe, or whether it's been
Speaker 1: mostly staying in the places it was made. I see
Speaker 1: because I guess you assume that it's kind of like
Speaker 1: a gas, right Like you don't assume it's a solid.
Speaker 1: You assume that it's you know, kind of moving around freely.
Speaker 1: It's not tied together to itself except with gravity. That's right,
Speaker 1: only held together with gravity. And so we think of
Speaker 1: it like a diffuse gas, like a pressureless gas that
Speaker 1: doesn't even bounce against itself, and so basically it just
Speaker 1: has gravity tational effects. And so we can sort of
Speaker 1: walk through the history of the universe with a cold
Speaker 1: version of dark matter, a version where dark matter is
Speaker 1: mostly staying where it was, and then we can walk
Speaker 1: through a version of the universe where dark matter is hot,
Speaker 1: where it's zipping around really fast, and we see that
Speaker 1: those two things predict different shapes of the universe that
Speaker 1: we see today and also different histories of the universe,
Speaker 1: and then we can compare those histories to what we
Speaker 1: actually see because like, if the dark matter at the
Speaker 1: beginning of time was super cold, then I guess it
Speaker 1: particles themselves don't have enough speed to like go off
Speaker 1: and spread out. They would sort of stay clumped together.
Speaker 1: That's exactly right. So if dark matter is very cold,
Speaker 1: then the structure of the universe forms sort of bottom up.
Speaker 1: Everything is where it was and it's not zipping around
Speaker 1: very much, and so you get these little clumps of
Speaker 1: density from those initial wrinkles, and that's what seeds like
Speaker 1: the formation of stars, and then stars get together and
Speaker 1: they form galaxies, and galaxies pull themselves together to form
Speaker 1: galaxy clusters. You get this structure formation that's sort of
Speaker 1: bottom up. Everything starts clumping where it was and then
Speaker 1: pulls together, so you get, for example, galaxies forming before
Speaker 1: galaxy clusters. You get stars forming, then galaxies, then galaxy
Speaker 1: clusters in that order. And we can look back through
Speaker 1: the history of time because remember as we look out
Speaker 1: through space, we're looking backwards in times, so we can
Speaker 1: see where their galaxies a billion years after the universe started,
Speaker 1: where their stars. Which order did things get made? We
Speaker 1: can tell by looking deep into the history of the
Speaker 1: universe just by looking far out into space. Right, And
Speaker 1: I guess you're using relative terms right, like cold and
Speaker 1: hot here. You're not thinking about a specific temperature because
Speaker 1: that could maybe also depend on how heavy these particles are.
Speaker 1: That's right. We're mostly talking about whether or not they're relativistic,
Speaker 1: like are they moving it close to the speed of
Speaker 1: light or are they not relativistic? You know, they're moving
Speaker 1: a much less than when you say hot, you mean
Speaker 1: like super duper hot light speed hot. Yeah, exactly. And
Speaker 1: when we think about what hot dark matter would look like, well,
Speaker 1: you have the early universe, and you know dark matter
Speaker 1: is made just with everything else, and you get these
Speaker 1: initial little clumps of density from quantum fluctuations. But if
Speaker 1: dark matters most of the stuff and it's moving really
Speaker 1: really fast, then those initial little blobs of density don't
Speaker 1: really matter because dark matter sort of washes them all out,
Speaker 1: Like the dark matter is flying everywhere super duper fast,
Speaker 1: and so those initial little clumps get evened out, they
Speaker 1: get smoothed out, so you don't get stars forming first.
Speaker 1: Instead you get these like these really big supermassive blobs
Speaker 1: of stuff because only the really big over densities, only
Speaker 1: the really big clumps from the beginning stick around and
Speaker 1: survived the dark matter spreading everything out to form some structure.
Speaker 1: What do you mean? So if the dark matter is hot,
Speaker 1: it means that the it's particles are moving a lot.
Speaker 1: And so are you saying that dark matter is more
Speaker 1: diffuse or like the blobs are moving around fast. Both,
Speaker 1: they're moving around faster and so they spread out and
Speaker 1: so it gets more even and so it's harder for
Speaker 1: gravity to get a handle and start forming stars, for example,
Speaker 1: because things get smooth. For gravity to form a star,
Speaker 1: you need like a little blob that's denser than the
Speaker 1: stuff around it that it can gather stuff together using gravity.
Speaker 1: But if dark matter, which is most of the stuff,
Speaker 1: is moving fast, then it's spread everything out, its smoothed
Speaker 1: everything over. There's nothing for gravity get a handle on,
Speaker 1: except for the really really big stuff because that's the
Speaker 1: stuff that dark matter can't smooth out. And so instead
Speaker 1: of getting stars and then galaxies and the galaxy clusters
Speaker 1: and then superclusters, you start out with supercluster sized blobs
Speaker 1: of stuff and then it breaks up into galaxy cluster
Speaker 1: sized blobs of stuff, and those break up into galaxy
Speaker 1: size blobs of stuff, and then you get stars forming.
Speaker 1: So it's sort of like top down instead of bottom up.
Speaker 1: Interesting just based off of the temperature of dark matter. Yeah,
Speaker 1: so the temperature of dark matter totally determines the entire
Speaker 1: history of the universe. Like the universe would be very
Speaker 1: different if we had no dark matter because it wouldn't
Speaker 1: have been around to clump the normal matter together into
Speaker 1: stars and galaxies. And also the universe would be different
Speaker 1: if we had hot or cold dark matter, just it's
Speaker 1: such a dominant force. It's most of the gravity. So
Speaker 1: it affects how the universe came together. And we can
Speaker 1: actually tell the history of the universe whether things foreign,
Speaker 1: buttom up or top down. Yeah, because we can look
Speaker 1: back in time and we can say, well, we're there
Speaker 1: galaxies in the first billion or two years after the
Speaker 1: Big Bang, or did it take a while for galaxies
Speaker 1: to form? And so we can look back in time
Speaker 1: and we can ask whether these things were made, in
Speaker 1: what order were they made. And also it affects the
Speaker 1: way things look today, because things would be smoother today
Speaker 1: if dark matter was hot, and things would be sort
Speaker 1: of clumpier today if dark matter was cold, like for example,
Speaker 1: our galaxy is the Milky Way, and if dark matter
Speaker 1: was cold, then we expect that the Milky Way has
Speaker 1: a bunch of like little galaxies orbiting it, the way
Speaker 1: the Earth has the Moon. We expect that the Milky
Speaker 1: Way has its own little like many galaxies that orbit
Speaker 1: our galaxy. If dark matter was super cold, if dark
Speaker 1: matter was cold, then there should have been these blobs
Speaker 1: of stuff formed outside of our galaxy, these dwarf galaxies,
Speaker 1: which would now be orbiting the Milky Way, and that
Speaker 1: we should see that today, So that would be a
Speaker 1: sign that dark matter is cold. Affects not just the
Speaker 1: history of the universe, but also affects the shape of
Speaker 1: the way things look today. Yeah, I guess it's. I mean,
Speaker 1: it's such a huge part of the universe that you
Speaker 1: know whether it's hot or not. It should be no
Speaker 1: surprise that the term is the fate of the universe
Speaker 1: because it's such a huge chunk of it. Yeah, exactly.
Speaker 1: It's not a little detail. It's not like a tiny
Speaker 1: bit of salt that you add to your recipe. Right,
Speaker 1: it's most of the stuff in the universe, and so
Speaker 1: of course it's going to have big consequences for how
Speaker 1: the universe looks and how it comes together. All right,
Speaker 1: it could be hot or cold, and we could probably
Speaker 1: tell by looking at the structure and the history. I
Speaker 1: guess the history is also important of the universe. The
Speaker 1: history kind of tells us a clue about whether it's
Speaker 1: hot or not. That's right. Did the structure form top
Speaker 1: down big stuff first and then small stuff or to
Speaker 1: the form bottom up like small stuff first, which then
Speaker 1: came together to make the bigger stuff. And it also
Speaker 1: affects the way things look in our universe today. Right,
Speaker 1: all right, let's now answer the question whether dark matter
Speaker 1: is hot or not and what that tells us about it.
Speaker 1: The first, let's take another quick break. All right, Daniel,
Speaker 1: is dark matter hot or not? Is it a swipe
Speaker 1: laughter or a swipe right for you? Well, I love
Speaker 1: dark matter. I'm very excited about dark matter. I'm very
Speaker 1: attracted to dark matter. But I have to say that
Speaker 1: the universe tells us that dark matter is quite cool.
Speaker 1: It's not hot, It's definitely not hot. I mean, it's
Speaker 1: still be beautiful. It's just you know, a little chilly.
Speaker 1: That's right. It's got its own standards of beauty, and
Speaker 1: it's pretty cool, you know, dark matter. And we know
Speaker 1: that because we look at the history of the universe
Speaker 1: and we see that stars formed first, and that then
Speaker 1: galaxies formed, and that then galaxy structure is formed. Because
Speaker 1: we look back in the very early universe and we
Speaker 1: see galaxies forming before there were clusters, and we see
Speaker 1: stars forming before there was galaxies. Can we tell that?
Speaker 1: Can we? How can we tell? I felt like we
Speaker 1: can only see really far out and tell the distance
Speaker 1: and the age of things by looking at like supernovas,
Speaker 1: And so how can we tell how things formed if
Speaker 1: our only way of knowing is through stars? That's right? Well,
Speaker 1: the supernovas tell us sort of like the distance ladder
Speaker 1: and so we can tell how far away something is
Speaker 1: and therefore when it happened. And you're right that we
Speaker 1: need stars to happen to give us that distance ladder.
Speaker 1: But we can go back and look at the early universe,
Speaker 1: right that tells us like, okay, this is really really
Speaker 1: far away. And for example, you would expect that there
Speaker 1: would be galaxy clusters formed in the very early universe
Speaker 1: if dark matter was hot. And so we look out
Speaker 1: past the most distant supernovas into the deep early universe,
Speaker 1: you know, and we can tell that these things happened,
Speaker 1: you know, thirteen billion years ago, for example. We don't
Speaker 1: see galaxy clusters forming out there in the very edges
Speaker 1: of the things that we can observe. That's the very
Speaker 1: earliest universe. And you're right. We we we can't get
Speaker 1: as precise an estimate for those distances because we don't
Speaker 1: have the supernovas, but we can extrapple it a little bit.
Speaker 1: And also we know it's super duper old. So like
Speaker 1: the the oldest stars that we can see tell us
Speaker 1: that things were not as formed as they are closer
Speaker 1: to us or closer to the present, that's right. They
Speaker 1: tell us that the structure formed bottom up that things
Speaker 1: came together in small clumps first, and then those small
Speaker 1: clumps organized themselves into bigger stuff. So you get stars,
Speaker 1: and then galaxies, and then galaxy clusters, and then superclusters
Speaker 1: of galaxies, which is the latest structure to form. And
Speaker 1: that's why they're the biggest gravitationally bound objects in the
Speaker 1: universe because they have most recently come together. It takes
Speaker 1: a while for gravity to do this, and galaxy superclusters
Speaker 1: are the last thing to have formed. It's all that
Speaker 1: we've had time to form so far in the universe.
Speaker 1: All right, Well, I guess so then that tells us
Speaker 1: that dork matter is cold, and I guess it. Do
Speaker 1: we have a sense of how cold it is, like,
Speaker 1: you know, not going at the speed of light. I
Speaker 1: know that's how you define cold. But is it like chili?
Speaker 1: Or is it like warm? Or is are we talking
Speaker 1: like the temperature of the sun? What are we talking about?
Speaker 1: It's definitely not the temperature of the sun. I mean,
Speaker 1: if it's out there and it's a particle, it's going
Speaker 1: to be very very cold. You know, it's going to
Speaker 1: be a few degrees kelvin. Really, we think dark matter
Speaker 1: is only a few degrees kelvin probably, yeah, And you know,
Speaker 1: it's not interacting in the same way that like hydrogen
Speaker 1: does in the core of the Sun to produce a
Speaker 1: huge amount of energy. But there's still a lot we
Speaker 1: don't know about dark matter that could have self interactions
Speaker 1: that contain energy that we are not aware of. And
Speaker 1: so everything we say here should be taken with a
Speaker 1: big grain of salt because it's all pretty speculative. But
Speaker 1: you know, also, the cold dark matter picture is pretty good.
Speaker 1: It works pretty well, but it's not perfect, Like it
Speaker 1: doesn't perfectly explain everything that we see, right, Like you're saying,
Speaker 1: cold dark matter predicts that we would have baby galaxies
Speaker 1: floating around us. That's right. We expect to see a
Speaker 1: bunch of these dwarf galaxies orbiting the Milky Way, and
Speaker 1: we see some, but we don't see nearly as many
Speaker 1: as we expect, and we don't know yet. Is that
Speaker 1: because dark matter isn't as cold as we thought, or
Speaker 1: is it because those dwarf galaxies are harder to see
Speaker 1: than we thought they would be. And recently people have
Speaker 1: developed extra good techniques to find dwarf galaxies and they
Speaker 1: found a few more, and that sort of closes the
Speaker 1: gap a little bit, but there's still some tension there.
Speaker 1: It's still something that we don't quite understand. And you know,
Speaker 1: we like those details. We like getting those things right
Speaker 1: because those are the things that tell us that our
Speaker 1: theory is really working. And so there's still some question
Speaker 1: marks about it. But it's definitely not hot. It's some
Speaker 1: version of cold. I guess we can't make any version
Speaker 1: in our simulations work out to be just like the
Speaker 1: universe we have now, like you tweaking further, you don't
Speaker 1: get the right proportion of dwarf or baby galaxies, not yet.
Speaker 1: But you know, these simulations are very very hard to
Speaker 1: do because you're simulating an enormous number of particles. And
Speaker 1: when they do these simulations, they usually just like leave
Speaker 1: out all the normal matter because the normal matter is
Speaker 1: a small fraction and it's much harder to model because
Speaker 1: normal matter has complicated interactions, right, you know, stars and
Speaker 1: gas and all that stuff. It has pressure and complicated
Speaker 1: flows because of the electromagnetic interactions and the strong interactions
Speaker 1: and all that stuff. So until recently, these simulations have
Speaker 1: mostly just removed all the bionic matter. But you know,
Speaker 1: baryons are important. I'm a baryon, you're a baryon. Stars
Speaker 1: or buryons. The whole visible part of the galaxy has
Speaker 1: made a bury on. So what does it mean, Like,
Speaker 1: that's the particles that we're made out of, regular matter
Speaker 1: like quarks and electrons. And so when they do these
Speaker 1: simulations to describe the structure the universe, they don't have
Speaker 1: the computational power to describe all the barry on its
Speaker 1: all the things that make me and you corks and
Speaker 1: all that stuff, So they mostly just remove it as
Speaker 1: a as a simplification because that's the most complicated stuff
Speaker 1: to describe, and so our simulations are really approximate right now.
Speaker 1: So people are working on ways to include normal matter
Speaker 1: in these simulations and try to get more precise estimates,
Speaker 1: more precise predictions for how many dwarf galaxies we should see. Yeah,
Speaker 1: I guess people are complicated. They're hard to predict, for sure,
Speaker 1: they are. They are hard to describe. So we know
Speaker 1: we think dark matter is made out of particles, and
Speaker 1: if it is, we think it's cold, because that's what
Speaker 1: the universe is telling is So what does that tell
Speaker 1: us about dark matter? Like, does it give us a
Speaker 1: clue about what it is or what kind of particle
Speaker 1: it is, or you know, is the fact that it's cold.
Speaker 1: Does that tell you something about how it interacts with
Speaker 1: other forces? Yeah, it tells us a lot. And what
Speaker 1: it can do is remove candidate particles from the list,
Speaker 1: and most specifically, it is the neutrino as a candidate
Speaker 1: for dark matter. For a long time, people thought, oh,
Speaker 1: there's a lot of invisible matter out there, a matter
Speaker 1: that almost never or never interacts with us except for gravitationally.
Speaker 1: Maybe it's just neutrinos. And it's a very tempting candidate
Speaker 1: because we already know about neutrinos. We know neutrinos are
Speaker 1: these whispy particles that can pass through a light year
Speaker 1: of lead without interacting. The air around us is filled
Speaker 1: with neutrinos, but we can't feel them or taste them.
Speaker 1: They have a lot of energy, but they don't deposit
Speaker 1: it on us. And so it's tempting to assign these
Speaker 1: two mysteries together, right, the weirdness of neutrinos and the
Speaker 1: mystery of the missing matter. Maybe one plus one just
Speaker 1: equals too, And so for a long time people suspected
Speaker 1: maybe the missing matter was just like a ridiculous number
Speaker 1: of neutrinos. And remember neutrinos are very very light, that
Speaker 1: hardly any mass per particle. It's not zero, but it's
Speaker 1: a small number. So if you're gonna explain most of
Speaker 1: the stuff in the universe with neutrinos, it would have
Speaker 1: to be an ungodly number of neutrinos. Could it be
Speaker 1: like a heavy neutrino, Like I know, neutrinos they can
Speaker 1: have different masses, right, The neutrinos that we're aware of,
Speaker 1: the three, the electron, mu and town netrinos all have
Speaker 1: very very very small masses. And so what we can
Speaker 1: do is we can rule out those We can say
Speaker 1: it's not one of the neutrinos that we know one
Speaker 1: of the neutrino lights. Yeah, exactly, because those neutrinos have
Speaker 1: such small mass that they're always moving basically at the
Speaker 1: speed of light, are very close to the speed of light.
Speaker 1: For example, when neutrinos come from a supernova, they arrive,
Speaker 1: you know, very close to the same time as the
Speaker 1: photons arrive because they're traveling basically at the speed of light. Actually,
Speaker 1: the neutrinos get here first because the photons get slowed
Speaker 1: down by interacting with the star. But it's basically a race.
Speaker 1: The neutrinos fly almost the speed of light. You're saying
Speaker 1: they're faster than light, Thaniel, They're not faster than light.
Speaker 1: They leave sooner. The photons spend more time packing, but
Speaker 1: they do travel a little faster. But you're exactly right
Speaker 1: that there's the possibility that there could be some weird
Speaker 1: heavy neutrinos, so not the neutrinos that we're familiar with,
Speaker 1: but if they're is another kind of neutrino, fourth neutrino,
Speaker 1: or many other kinds of neutrinos that are very heavy,
Speaker 1: then those are still valid candidates for the dark matter.
Speaker 1: And those go by the terms like sterile neutrinos because
Speaker 1: called sterile because maybe they interact with our kind of
Speaker 1: matter even less. Wow, it's like a neutral neutrino. Yeah,
Speaker 1: that's right. It's like an even more standoffish and snobbish
Speaker 1: particle than the neutrino. And that's a hard standard to meet.
Speaker 1: I was just thinking, like shy or you know, loth
Speaker 1: to interact with other particles. They know the introvert neutrinos,
Speaker 1: but you just assume that you know, it's just not
Speaker 1: my apologies sterile neutrinos, I take it back, right, So
Speaker 1: that tells that they can't be neutrinos because neutrinos you
Speaker 1: usually go really fast, but they could be. Basically, that
Speaker 1: doesn't leave you much, does It just tells you that
Speaker 1: it's another kind of part of it. Yeah, and that
Speaker 1: we don't know about that. That's an important clue because
Speaker 1: that means that there's no particle on our current list
Speaker 1: that fits the requirements. There's no particle out there that
Speaker 1: doesn't have electromagnetic or strong interactions and is heavy. Right
Speaker 1: there just isn't one. The only particle in our current
Speaker 1: list that had any chance of being the dark matter
Speaker 1: or new trinos, and this piece of evidence rules that out.
Speaker 1: It says it can't be one of the new trinos
Speaker 1: we know. So it has to be a new particle.
Speaker 1: And that's exciting. A new heavy particle, a new heavy
Speaker 1: particle exactly. It means that there's something new to discover.
Speaker 1: It's not just oh, there are more of this particle
Speaker 1: than we thought. It means there's a new particle. And
Speaker 1: a new particle is interesting because you wonder, like why
Speaker 1: does it exist? How many new particles are there, Where
Speaker 1: did it come from? Why is it different from these
Speaker 1: other particles? You know, it gives you a whole new
Speaker 1: set of questions to ask, a whole new way to
Speaker 1: look at the universe. And you guys are looking for
Speaker 1: these in the particle colliders, right, you're smashing particles hoping
Speaker 1: that a new kind of particle will pop out. And
Speaker 1: you might say, hey, that's dark matter. That's right. And
Speaker 1: we have specific ideas for what this new particle could be.
Speaker 1: We have ideas like the whimp particle weakly interacting massive particle.
Speaker 1: It's just a generic name meaning some big, heavy particle
Speaker 1: that doesn't interact much. And it has to not interact
Speaker 1: very much in order to be the dark matter. And
Speaker 1: it has to be massive in order to be cold
Speaker 1: because of the structure of the universe. And another idea
Speaker 1: is the axion. The axon could be the dark matter,
Speaker 1: and we have specific experiments to look for whimps and
Speaker 1: for axons. We just did a podcast episode about axons.
Speaker 1: They're not the same thing. They are not the same thing.
Speaker 1: There are two very different kinds of particles. The axion
Speaker 1: is like a heavier version of the photon, and the
Speaker 1: whimp is like it's like a heavier version of the neutrino,
Speaker 1: but maybe interacts even less. And we have experiments underground
Speaker 1: to look for WIMPs, these big tanks of liquid argon,
Speaker 1: for example, or liquid zenon that look for one whimp
Speaker 1: coming through and knocking into a bunch of particles and
Speaker 1: then giving us a signal. We're using space telescopes to
Speaker 1: look to see if occasionally whimps bounce into each other
Speaker 1: and give off a little flash of light that we
Speaker 1: could see, which would be really really rare because dark
Speaker 1: matter is dark. But you know, we look at places
Speaker 1: where there is a lot of dark matter and try
Speaker 1: to see the occasional blip, and then we try to
Speaker 1: make dark matter in the collider to see if we
Speaker 1: can create it and play with it there. So far,
Speaker 1: none of these experiments have turned up any evidence for
Speaker 1: dark matter that anybody believes, and so we're still in
Speaker 1: the hunt. But you know, even though we don't know
Speaker 1: what dark matter is, we're able to say some things
Speaker 1: about what it isn't right, is it weird that you
Speaker 1: haven't found dark matter in these colliders. I mean, like
Speaker 1: in the universe there's five times more dark matter than
Speaker 1: regular matter, which might make you think that it's like
Speaker 1: it's more likely to happen, but in our colliders you
Speaker 1: can't seem to make even a little bit of it.
Speaker 1: That's right. It is a little weird. Now. On one hand,
Speaker 1: it may be the dark matters everywhere, but we can't
Speaker 1: make it because we're playing with our kind of matter,
Speaker 1: like our kind of matter might not interact with dark matter,
Speaker 1: which means that we can't use our matter to look
Speaker 1: for dark matter, and we can't use our matter to
Speaker 1: make dark matter like for that to work for any
Speaker 1: of the experiments I just described to work to this
Speaker 1: govern the particle nature of dark matter means there has
Speaker 1: to be some way for our particles to talk to
Speaker 1: the dark matter particles, to share some sort of new
Speaker 1: dark photon, or some new force has to exist that
Speaker 1: works on both particles. And it could be that it
Speaker 1: just doesn't. It could be the dark matters out there.
Speaker 1: It's a particle and it just feels nothing except for gravity,
Speaker 1: in which case it's basically hopeless for us to discover
Speaker 1: its particle nature because gravity is so weak that we
Speaker 1: can only detect dark matter when you have enormous, like
Speaker 1: galaxy sized blobs of it. Which makes it pretty hard
Speaker 1: to do particle experiments. But I thought when you smash particles,
Speaker 1: it turns into like pure energy, and then anything can
Speaker 1: come out of it. You're saying that maybe it's possible
Speaker 1: that not even dark matter can come out of that.
Speaker 1: That's right. When you smash particles together, it's not exactly
Speaker 1: pure energy. It turns into one of the bosons of
Speaker 1: the forces that can interact with those particles. So, for example,
Speaker 1: when you smash a quark and an antiquark together, you
Speaker 1: can get a glue on, or you can get a photon,
Speaker 1: or you can get a w boson. But if those forces,
Speaker 1: the weak and the strong force and electromagnetism don't interact
Speaker 1: with dark matter, then those bosons which represent that energy
Speaker 1: can't then turn into dark matter. And so that is
Speaker 1: one limitation I know that I like to say in
Speaker 1: this podcast that we can use colliders to explore the
Speaker 1: universe because anything that can be made will be made.
Speaker 1: But there is an important caveat there that whatever can
Speaker 1: be made has to somehow interact with the particles that
Speaker 1: were smashing. If there's no way to interact, then you
Speaker 1: just can't make it. You need a dark matter collider, Daniel. Obviously,
Speaker 1: to discover dark matter, you have to build a dark
Speaker 1: matter collider. All right, Well, um, it sounds like we
Speaker 1: don't know what dark matter is still, but we know
Speaker 1: that it's pretty cool. It's a pretty cool thing in
Speaker 1: the universe. It's cool, that's right. Dark matter is pretty chill.
Speaker 1: You know. It wants to come over and watch Netflix
Speaker 1: with you, even if you don't think it's hot. Yeah,
Speaker 1: all right, Well again, just makes me think about all
Speaker 1: the crazy things we don't know, you know, and all
Speaker 1: the sort of fun and clever ways we can tell
Speaker 1: about things we don't know even though we don't know
Speaker 1: anything about it. Yeah, And this is what science does,
Speaker 1: is we probe things from every direction. We're trying to
Speaker 1: uncover a real truth about the universe, and that has
Speaker 1: lots of facets. And so if we get stumped in
Speaker 1: one direction, like we can't seem to find it in
Speaker 1: our detectors, then we go another route and say, well,
Speaker 1: can we say anything about it from this perspective or
Speaker 1: from that perspective? And we're trying to be clever in
Speaker 1: the field of particle physics, and science in general is
Speaker 1: filled with clever people having new ideas about ways to
Speaker 1: answer these questions, and so to me, this is one
Speaker 1: of the most elegant ways to put a really important,
Speaker 1: really insightful constraint on what dark matter is and isn't.
Speaker 1: All Right, Well, I think we answered that question pretty good,
Speaker 1: and I think we can all learn a little bit
Speaker 1: from dark matter to just be cool. Don't get too excited.
Speaker 1: Thanks for joining us, See you next time. Thanks for listening,
Speaker 1: and remember that Daniel and Jorge Explain the Universe is
Speaker 1: a production of I Heart Radio. For More podcast. For
Speaker 1: my heart Radio, visit the I heart Radio app, Apple Podcasts,
Speaker 1: or wherever you listen to your favorite shows. H