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

• 0:08
  Speaker 1: Hey, Daniel, I have a question about dark matter. Oh, man,
• 0:11
  Speaker 1: don't we all? I mean, I know that we don't
• 0:13
  Speaker 1: know what it is, right, but what is it like?
• 0:16
  Speaker 1: I mean, is it quishy? We don't know. What does
• 0:19
  Speaker 1: it taste like? Well, you know, our tongues can't taste it,
• 0:22
  Speaker 1: so again we don't really know. But is it fuzzy
• 0:25
  Speaker 1: maybe we don't know, or scratchy? Probably not. But again
• 0:29
  Speaker 1: we just don't know. You know, for such a hot topic,
• 0:33
  Speaker 1: you would think you guys would know more about it. Well,
• 0:36
  Speaker 1: that's one thing we do know, whether dark matter is
• 0:38
  Speaker 1: hot or not. Hi am more Hammon, cartoonists and the
• 0:57
  Speaker 1: creator of PhD comments. I'm Daniel. I'm a particle physicist,
• 1:01
  Speaker 1: and I have no opinion about the attractiveness of dark matter. Well,
• 1:05
  Speaker 1: it's definitely attractive, right, gravitationally speaking on a cosmological level.
• 1:12
  Speaker 1: That's right. It is the great attractor from that point
• 1:14
  Speaker 1: of view. But welcome to our podcast, Daniel and Jorge
• 1:17
  Speaker 1: Explain the Universe, a production of I Heart Radio in
• 1:19
  Speaker 1: which we talk about all the amazing and crazy things
• 1:22
  Speaker 1: in our universe, the things that scientists have understood and
• 1:25
  Speaker 1: the things that scientists are now working to understand. We
• 1:28
  Speaker 1: break down all the crazy for you and explain it
• 1:31
  Speaker 1: in a way that hopefully makes you smile. It's right,
• 1:33
  Speaker 1: all the things that are hot in this universe and
• 1:35
  Speaker 1: all the things that are not hot or cold or
• 1:38
  Speaker 1: super cold, because the universe has a broad range. Right,
• 1:41
  Speaker 1: things can be as hot as a million degrees or
• 1:44
  Speaker 1: as cold as zero degree. That's right. Everything has a temperature,
• 1:47
  Speaker 1: even black holes, we all have a rating. That's right.
• 1:51
  Speaker 1: Most of the universe out there is at a very
• 1:53
  Speaker 1: cold two point seven three degrees kelvin. But there are
• 1:56
  Speaker 1: a few hot spots a place like Earth where hot
• 2:00
  Speaker 1: little bits of temperature clue to make life an interesting podcast.
• 2:03
  Speaker 1: And so we like to talk about in this podcast
• 2:06
  Speaker 1: about dark matter a lot. And I feel like we
• 2:08
  Speaker 1: talked about it a lot because it's such a huge mystery.
• 2:11
  Speaker 1: I mean, it's of the universe and we don't know
• 2:14
  Speaker 1: what it's made out of. I think it's one of
• 2:16
  Speaker 1: the biggest open questions in science. You know, the person
• 2:19
  Speaker 1: or the group that figures out, like what is dark matter? Anyway,
• 2:23
  Speaker 1: that will be a historic moment, that will be a
• 2:26
  Speaker 1: an understanding and achievement, a breakthrough that will go down
• 2:29
  Speaker 1: in history for sure. Do you think a Nobel Prize
• 2:32
  Speaker 1: would be enough for that discovery or they need to
• 2:34
  Speaker 1: like stack him up or something, or maybe make up
• 2:36
  Speaker 1: like a special Nobel Prize, the dark Nobel Prize. You know,
• 2:39
  Speaker 1: they should have already given a Nobel Prize to Vera
• 2:42
  Speaker 1: Reuben for the discovery that dark matter was out there.
• 2:44
  Speaker 1: Even if we don't know what it is, we know
• 2:46
  Speaker 1: it's there, we know it's matter, and Nobel Prize committee
• 2:50
  Speaker 1: overlooked very Reuben. Some say because she's a woman, that's terrible.
• 2:54
  Speaker 1: That's the dark history of the Nobel Prize. It's the
• 2:56
  Speaker 1: dark history of dark matter. But we know something. There's
• 3:00
  Speaker 1: a little bit about dark matter. Let it's there, and
• 3:02
  Speaker 1: that it's affecting things gravitationally and keeping galaxies together. But
• 3:06
  Speaker 1: the question is how much more do we know about it?
• 3:09
  Speaker 1: What else do we know about this mysterious thing, if
• 3:12
  Speaker 1: it even is a thing. That's right, We would love
• 3:14
  Speaker 1: to know what dark matter is made out of. And
• 3:17
  Speaker 1: particle physicists like me scratch their heads all day wondering
• 3:20
  Speaker 1: what kind of particle is it made out of? For
• 3:22
  Speaker 1: many particles or is it a particle at all? But
• 3:25
  Speaker 1: along the way while we're looking for its particle nature,
• 3:28
  Speaker 1: we have other ways to try to get clues as
• 3:30
  Speaker 1: to what it might be. By looking at how it
• 3:32
  Speaker 1: moves and how it clumps, and how it squishes and
• 3:34
  Speaker 1: how it buzzes, we can try to get a handle
• 3:37
  Speaker 1: on what it is or isn't. Yeah, and so to
• 3:39
  Speaker 1: be on the program, we'll be asking the question is
• 3:46
  Speaker 1: dark matter hot or not? Well, for those of you
• 3:49
  Speaker 1: who are a little bit older, you might remember a
• 3:51
  Speaker 1: popular website a few decades ago called hot or Not,
• 3:55
  Speaker 1: which was probably inappropriate these days, totally inappropriate exact Yeah,
• 4:00
  Speaker 1: rated people based on their hotness. And I'm guessing it
• 4:03
  Speaker 1: was not the temperature. No, it was not the temperature,
• 4:05
  Speaker 1: although maybe we should revive it in a physics version,
• 4:07
  Speaker 1: like is the top cork hot or not? Our neutrinos
• 4:11
  Speaker 1: hot or not? That might be or cold maybe based
• 4:14
  Speaker 1: on how much fun thing you can get for it.
• 4:18
  Speaker 1: That's right, And it's a weird combination of ideas, you know,
• 4:21
  Speaker 1: dark matter, mysterious blobs of stuff out there in the universe,
• 4:24
  Speaker 1: and temperature, But it turns out to be very important.
• 4:27
  Speaker 1: It's one of the most powerful handles we have on
• 4:29
  Speaker 1: the nature of dark matter and one of the most
• 4:32
  Speaker 1: valuable clues we have that tells us what it is
• 4:34
  Speaker 1: and what it can't be. Yeah, So, as usual, we
• 4:37
  Speaker 1: were wondering how many people out there had thought about
• 4:39
  Speaker 1: this question of whether dark matter is hot or cold,
• 4:43
  Speaker 1: and so as usual Daniel went out there into the
• 4:46
  Speaker 1: wilds of the Internet to ask people this question. That's right.
• 4:49
  Speaker 1: So thank you to everybody who was willing to participate
• 4:52
  Speaker 1: in our random person on the internet questions. And if
• 4:54
  Speaker 1: you'd like to answer random questions from me in preparation
• 4:58
  Speaker 1: for a future podcast, please is right to us two
• 5:01
  Speaker 1: questions at Daniel and Jorge dot com. To think about
• 5:03
  Speaker 1: it for a second, what would you answer if someone
• 5:05
  Speaker 1: asked you, is dark matter hot or cold? Here's what
• 5:09
  Speaker 1: people have to say. Yes, it seems most natural to
• 5:11
  Speaker 1: me that dark matter would interact with itself, so it's
• 5:16
  Speaker 1: doing so. It's reasonable think that it could have a
• 5:19
  Speaker 1: temperature um it's relative to other dark matter, so I
• 5:23
  Speaker 1: guess it will be hot. What I think it's that
• 5:29
  Speaker 1: are parts that of the dark matter that can be
• 5:33
  Speaker 1: hot and parts that are gonna be colder. I think
• 5:37
  Speaker 1: dark matter is cold, or at least cooler than normal matter.
• 5:41
  Speaker 1: On average average temperature of the universe is a few
• 5:45
  Speaker 1: codings about zero. And since we have no idea, what
• 5:48
  Speaker 1: what is what other constituent particles of dark mastra? I
• 5:52
  Speaker 1: think the answer is we have no idea. I don't
• 5:55
  Speaker 1: know a lot about dark matter, but I don't usually
• 6:00
  Speaker 1: ink of matter having a specific temperature. I'd say we
• 6:02
  Speaker 1: don't know, because we don't even know what it is.
• 6:05
  Speaker 1: I would say that it's probably not hot. Well, hot
• 6:08
  Speaker 1: and cold are relative terms, So if what you mean
• 6:11
  Speaker 1: is does dark matter have a temperature, then I would
• 6:14
  Speaker 1: say probably not, because everything with the temperature gives off
• 6:18
  Speaker 1: infrared radiation. I had to consult my eleven year old,
• 6:21
  Speaker 1: who is the cosmologist in our family. So we think
• 6:25
  Speaker 1: that dark matter is cold. The only reason we know
• 6:28
  Speaker 1: it exists is because it reacts with gravity, and I
• 6:32
  Speaker 1: don't think it will react with anything on the electromagnetic spectrum,
• 6:36
  Speaker 1: so it wouldn't be hot or cold. Both knowing scientists,
• 6:43
  Speaker 1: they fancim intrinsic property of dark matter and named it
• 6:47
  Speaker 1: hot and cold, even though it doesn't mean anything like
• 6:49
  Speaker 1: hot or cold. All right, I like how people evaded
• 6:52
  Speaker 1: the question very expertise. You're impressed by that are you disappointed?
• 6:57
  Speaker 1: I'm impressed. They're like, Oh, they're thinking, like physics avoid
• 7:01
  Speaker 1: answering the question. It's like what is hot and cold?
• 7:05
  Speaker 1: Let's divert into that discussion. Well, we do this a
• 7:07
  Speaker 1: lot in physics. We apply weird sounding characteristics to things,
• 7:10
  Speaker 1: you know, like when we're talking about particles, we're talking
• 7:13
  Speaker 1: about their spin that's not really spin, and we're talking
• 7:15
  Speaker 1: about their mass, but they don't have any stuff to them.
• 7:18
  Speaker 1: And so I understand why people are a little wary
• 7:21
  Speaker 1: of interpreting like the temperature of dark matter, Like what
• 7:24
  Speaker 1: does that actually mean? What are we really talking about? Yeah, Like,
• 7:28
  Speaker 1: we don't even know if it's a thing, So how
• 7:29
  Speaker 1: can I not thing have temperature? That's right? It feels
• 7:33
  Speaker 1: like a detail, Like are you worried about what color
• 7:35
  Speaker 1: it is? You don't even know if it exists. Why
• 7:37
  Speaker 1: do you care if it's purple or brown? Right? Yeah? Yeah?
• 7:39
  Speaker 1: What color is dark matter? Then you it's dark? Alright,
• 7:43
  Speaker 1: So let's break it down for folks. And first of all,
• 7:45
  Speaker 1: I guess the question is, how can dark matter even
• 7:48
  Speaker 1: have a temperature if we don't know what it is? Right? Well,
• 7:50
  Speaker 1: let's remember what temperature really means. For us, temperature is
• 7:54
  Speaker 1: a macroscopic quantity. Right, you touch something, it feels hot
• 7:58
  Speaker 1: or it feels cold, and that's really actually about the
• 8:00
  Speaker 1: heat difference, Like if something has more energy in it
• 8:03
  Speaker 1: than you do, then the heat flows from it into
• 8:06
  Speaker 1: your finger, like when you touch a hot burner, and
• 8:08
  Speaker 1: that's what you're feeling. So you don't actually measure temperature
• 8:11
  Speaker 1: with your finger. You measure like a relative heat. But
• 8:13
  Speaker 1: when we think about temperature, like microscopically, we try to
• 8:16
  Speaker 1: understand how that experience of feeling things being hot or
• 8:20
  Speaker 1: cold translates to like the motion of the particles inside it.
• 8:25
  Speaker 1: And so most loosely, we think about temperatures relating to
• 8:28
  Speaker 1: how fast those particles inside something are moving. Like a gas.
• 8:33
  Speaker 1: If it's a hot gas, then the particles in it
• 8:36
  Speaker 1: are moving really fast, that's right, and that's in fact
• 8:39
  Speaker 1: what's happening. But also for liquids and for solids, and
• 8:42
  Speaker 1: in fact that's why liquids and solids are more solid
• 8:45
  Speaker 1: than gases, right, because their particles are not moving as much,
• 8:48
  Speaker 1: they're more easily trapped by all the bonds, and solid
• 8:51
  Speaker 1: has various temperatures because the atoms and it can wiggle
• 8:54
  Speaker 1: more or less they can shake and vibrate in that
• 8:57
  Speaker 1: kind of stuff. So it's all about the energy stored
• 8:59
  Speaker 1: in those parts, like the motion of the particles inside,
• 9:02
  Speaker 1: like the speed almost. Yeah, if you're talking about a gas,
• 9:04
  Speaker 1: then it's mostly about the speed. And I think this
• 9:07
  Speaker 1: is really interesting stuff to like take something that's macroscopic
• 9:10
  Speaker 1: and kind of qualitative, you know, this feeling of temperature,
• 9:13
  Speaker 1: and try to understand it on the microscopic scale, and
• 9:16
  Speaker 1: it sometimes works and it doesn't always work. And we
• 9:19
  Speaker 1: had a whole podcast where we talked about like the
• 9:21
  Speaker 1: hottest things in the universe, and some of these things
• 9:24
  Speaker 1: are counterintuitive. Like some of the hottest stuff in the
• 9:27
  Speaker 1: universe is the interstellar plasma, which is like some crazy
• 9:31
  Speaker 1: high temperature like three thousand degrees kelvin. But if we
• 9:34
  Speaker 1: dropped you in it, you would freeze to death immediately.
• 9:38
  Speaker 1: And that feels counterintuitive because there isn't much of it
• 9:41
  Speaker 1: out there that's right of this plasma. It's very hot,
• 9:44
  Speaker 1: but it's very dilute, so it doesn't contain a lot
• 9:46
  Speaker 1: of heat, and so you're much denser blob of heat.
• 9:49
  Speaker 1: If we dropped doing it, most of your heat would
• 9:51
  Speaker 1: leak out but the particles of that plasma individually are
• 9:54
  Speaker 1: moving super duper fast, and so you can still call
• 9:57
  Speaker 1: it hot, right, So it's related to the speed and
• 10:00
  Speaker 1: the get or the vibration or like the kinetic energy
• 10:02
  Speaker 1: of the molecules and particles in something. But how does
• 10:07
  Speaker 1: that apply to dark matter, because we don't really know
• 10:09
  Speaker 1: if dark matter is made out of particles or not.
• 10:11
  Speaker 1: We don't really know. Well, we know that something is
• 10:13
  Speaker 1: out there creating gravity. We know there's a kind of matter,
• 10:17
  Speaker 1: and that's really about it. We know sort of where
• 10:20
  Speaker 1: it is in the universe, but you're right, we don't
• 10:22
  Speaker 1: know that it's a particle that could turn out to
• 10:24
  Speaker 1: be something else. And you know, all the matter that
• 10:26
  Speaker 1: we've ever seen in the universe so far has been
• 10:29
  Speaker 1: made out of particles. So it seems tempting to say, well,
• 10:32
  Speaker 1: then the dark matter must also be made out of particles.
• 10:35
  Speaker 1: But you know, remember that dark matter is most of
• 10:37
  Speaker 1: the stuff in the universe. We've only seen a little slice.
• 10:40
  Speaker 1: We've seen five percent of the universe, so it's dangerous
• 10:43
  Speaker 1: to extrapolate to like a full and say the rest
• 10:47
  Speaker 1: of it must also be made out of particles. But
• 10:50
  Speaker 1: we don't really have better ideas, and so we typically
• 10:53
  Speaker 1: just assume dark matters made out of particles. So that's
• 10:55
  Speaker 1: kind of like the working hypothesis. Yeah, it's like, let's
• 10:58
  Speaker 1: try this, let's see if it works. If it breaks,
• 11:01
  Speaker 1: then we'll go back and examine all the assumptions we made.
• 11:03
  Speaker 1: But when you're exploring the unknown, you've got to make
• 11:06
  Speaker 1: some assumptions just to like have something to do, because
• 11:09
  Speaker 1: you can't just sit at home and go like, I
• 11:10
  Speaker 1: don't know what dark matter is. You know, it sort
• 11:13
  Speaker 1: of ends there. So we say, maybe dark matters of
• 11:16
  Speaker 1: particle and then we can ask if dark matter is
• 11:18
  Speaker 1: made of particles, are those particles moving fast or are
• 11:22
  Speaker 1: they moving slow? Right? Are they hot or not? Are
• 11:24
  Speaker 1: they hot or not? That's exactly what that really means.
• 11:26
  Speaker 1: It means is dark matter made out of super fast,
• 11:29
  Speaker 1: zippie particles moving relativistic speeds or is it made of
• 11:32
  Speaker 1: like heavier, slower moving particles that just sort of like
• 11:35
  Speaker 1: float around its slower speeds. I guess it's kind of
• 11:38
  Speaker 1: weird to think of something being hot but not being
• 11:42
  Speaker 1: able to touch it, you know what I mean? Like,
• 11:44
  Speaker 1: that's weird, right, I was just thinking, do neutrinos have
• 11:47
  Speaker 1: a temperature, like in a neutrino who we can't interact
• 11:49
  Speaker 1: with through electromagnetism, Can that have a temperature? Yes, absolutely,
• 11:54
  Speaker 1: neutrinos are very hot, and the reason is that neutrinos
• 11:57
  Speaker 1: have almost no mass, and so these through the universe
• 12:00
  Speaker 1: at very very high speeds, and so they contain a
• 12:03
  Speaker 1: lot of energy. You would say they have a high temperature,
• 12:06
  Speaker 1: but you're right that you can't feel them, and the
• 12:08
  Speaker 1: reason is that you have no interaction in common with them,
• 12:10
  Speaker 1: or almost none, because all they feel is the weak force.
• 12:13
  Speaker 1: So they have all this energy, but they have no
• 12:15
  Speaker 1: way to transmit it to you. So it's like you
• 12:18
  Speaker 1: pass right through each other. And so they can have
• 12:21
  Speaker 1: that high temperature, they can have that high energy, but
• 12:23
  Speaker 1: if there's no common interaction, no way to communicate, there's
• 12:27
  Speaker 1: no way for that energy to flow to you, and
• 12:29
  Speaker 1: so you won't feel them being hot. What about like
• 12:32
  Speaker 1: if what if dark matter is not a particle? Can
• 12:34
  Speaker 1: it still have a temperature and something that's not a
• 12:37
  Speaker 1: particle still be hot? WHOA? You just blew my mind.
• 12:40
  Speaker 1: Could something that's not made of particles have a temperature?
• 12:44
  Speaker 1: We've never seen anything that's not made of particles, So
• 12:48
  Speaker 1: that's quite a reach. But I guess macroscopically you could
• 12:52
  Speaker 1: like see if it emits light, and everything in the
• 12:55
  Speaker 1: universe that does emit light has a temperature, it's black
• 12:58
  Speaker 1: body radiation. But I don't know. That would be an
• 13:01
  Speaker 1: amazing thing to explore if we discover the dark matter
• 13:05
  Speaker 1: wasn't made of particles, because we do know something about
• 13:07
  Speaker 1: its temperature, which is what we're going to talk about today.
• 13:10
  Speaker 1: I see, so that it's made out of particles is
• 13:12
  Speaker 1: not just a working hypothesis. It's it's like your only hypothesis.
• 13:16
  Speaker 1: It's all we got at this point. It's like the
• 13:17
  Speaker 1: one idea we've been using for a hundred years, or
• 13:20
  Speaker 1: you know, empty box for crazy new ideas somebody should
• 13:24
  Speaker 1: come up with. Really, like, could it be something that's
• 13:27
  Speaker 1: not a particle? It certainly could be. I mean, we
• 13:29
  Speaker 1: have no concrete evidence that it is a particle other
• 13:32
  Speaker 1: than all matter so far discovered is made of particles,
• 13:36
  Speaker 1: but it certainly could be. We're open to surprises. I mean,
• 13:38
  Speaker 1: dark matter itself is a surprise. Its existence was a surprise,
• 13:43
  Speaker 1: and there have been some ideas about un particles matter
• 13:47
  Speaker 1: made out of things that are not quite particles that
• 13:49
  Speaker 1: you know, don't have a definitive size, but it's a
• 13:52
  Speaker 1: bit fuzzy, and nobody's really worked out the math for
• 13:55
  Speaker 1: how it could be dark matter, So they're just sort
• 13:56
  Speaker 1: of like the beginnings of ideas, I guess. I mean,
• 13:59
  Speaker 1: you know, like energy, is energy also particle based, because
• 14:03
  Speaker 1: you know, energy can have gravity or exert gravity or
• 14:06
  Speaker 1: effect gravity. Energy density certainly has gravity, and some energy
• 14:10
  Speaker 1: is particle based, like photons, right, Photons are basically just energy.
• 14:15
  Speaker 1: They have no mass to them, and photons contribute to
• 14:18
  Speaker 1: the energy density of the universe, and therefore it's curvature.
• 14:21
  Speaker 1: So certainly, all right, Well, I guess there's no maybe
• 14:26
  Speaker 1: room and your equations so far to account for something
• 14:30
  Speaker 1: that's not a particle, Is that kind of what you're saying.
• 14:32
  Speaker 1: That's right, yeah, But I would love to blow up
• 14:34
  Speaker 1: those equations. I would love if we found something about
• 14:36
  Speaker 1: dark matter that proved that it wasn't the particle and
• 14:39
  Speaker 1: then we have to go back to the drawing board
• 14:40
  Speaker 1: and think from scratch. That would be a tremendous breakthrough,
• 14:44
  Speaker 1: an intellectual crack in the very foundations of physics, which
• 14:47
  Speaker 1: is the kind of thing we're all hoping will happen,
• 14:49
  Speaker 1: you know, because those are the moment you get, like
• 14:51
  Speaker 1: the real insights. It pulled back the curtain and discover
• 14:54
  Speaker 1: something surprising and fascinating about the universe. So yeah, this
• 14:57
  Speaker 1: is all we got so far, and I would love
• 14:59
  Speaker 1: to see it break into pieces. You'd love to prove
• 15:01
  Speaker 1: that they're not so hot, all these theories precisely. All right, Well,
• 15:06
  Speaker 1: let's get into how we could tell whether or not
• 15:09
  Speaker 1: dark matter has a temperature besides like I guess, feeling
• 15:13
  Speaker 1: its forehead. Daniel, that's right, These days were very sensitive
• 15:18
  Speaker 1: to high temperatures. But let's get into how we could
• 15:21
  Speaker 1: tell and what it tells us about dark matter. But first,
• 15:24
  Speaker 1: let's take a quick break, all right, Daniel, we're talking
• 15:37
  Speaker 1: about whether dark matter is hot or cold, and so
• 15:40
  Speaker 1: we talked about how we have to kind of assume
• 15:42
  Speaker 1: that it's a particle because that's the only idea that
• 15:45
  Speaker 1: we have. And so if it's a particle, then you
• 15:48
  Speaker 1: can talk about whether those dark matter particles are moving
• 15:51
  Speaker 1: a lot or vibrating a lot, which case wouldn't make
• 15:55
  Speaker 1: them technically hot even though we can't feel it. That's right,
• 15:57
  Speaker 1: and we're really interested in whether dark matter is fast
• 16:01
  Speaker 1: moving or slow moving, because it tells us also whether
• 16:04
  Speaker 1: the particle is heavy, in which case it is more
• 16:06
  Speaker 1: likely slow moving and cold, or very very low mass,
• 16:09
  Speaker 1: in which case is probably faster moving and hot. So
• 16:12
  Speaker 1: we're we're using this as a way to sort of
• 16:14
  Speaker 1: get a clue as to the nature of dark matter.
• 16:16
  Speaker 1: It's but it could dark matter be both, like I mean,
• 16:19
  Speaker 1: it could it be like regular matter that some of
• 16:21
  Speaker 1: it is hot and some of it is cold. Totally absolutely,
• 16:24
  Speaker 1: dark matter could be lots of different particles, some of
• 16:26
  Speaker 1: which are very heavy and some of which are very light.
• 16:29
  Speaker 1: But we know that dark matter sticks around for a
• 16:31
  Speaker 1: very very long time. It's like cosmologically stable. It's been
• 16:34
  Speaker 1: here since the beginning. It's affected the structure of the universe.
• 16:37
  Speaker 1: We've seen it put its imprint on the whole history
• 16:40
  Speaker 1: of the universe. And so that suggests that it's probably stable,
• 16:43
  Speaker 1: that it's not changing a lot from one kind of
• 16:46
  Speaker 1: mass to another. But you know, we really just don't know.
• 16:48
  Speaker 1: All right, Well, let's get into now how we could
• 16:51
  Speaker 1: tell whether dark matter has the temperature or not, Like,
• 16:53
  Speaker 1: how do you how would you even measure the temperature?
• 16:55
  Speaker 1: Of dark matter. If a particle of dark matter is
• 16:57
  Speaker 1: moving a lot or vibrating a lot, or could we
• 17:00
  Speaker 1: even tell the different We can actually tell the difference,
• 17:03
  Speaker 1: and I think this is really clever. It's one of
• 17:04
  Speaker 1: the most elegant pieces of science that I've seen recently.
• 17:08
  Speaker 1: We can tell whether dark matter is moving fast or
• 17:11
  Speaker 1: slow because of the way it makes an imprint on
• 17:14
  Speaker 1: the growth of the universe. You know, the universe started
• 17:17
  Speaker 1: from like the Big Bang, and back then things were
• 17:20
  Speaker 1: hot and dense and mostly uniform, and then you've got
• 17:24
  Speaker 1: little quantum fluctuations, little pockets of density here and less
• 17:28
  Speaker 1: density there, and those pockets are critical because that's what
• 17:31
  Speaker 1: seeds the whole structure of the universe. Like the reason
• 17:34
  Speaker 1: we have a galaxy here and not over there is
• 17:36
  Speaker 1: because some initial fluctuation made things a little dense, and
• 17:39
  Speaker 1: then gravity clumped them together and clumped them together even further.
• 17:43
  Speaker 1: So you've got these little fluctuations in the early universe,
• 17:46
  Speaker 1: which see the structure of the universe, right, because gravity
• 17:49
  Speaker 1: takes over from these little wrinkles. But dark matter plays
• 17:53
  Speaker 1: a really big role in that because dark matter basically
• 17:56
  Speaker 1: is gravity, right, It's the biggest source of gravity in
• 17:59
  Speaker 1: the universe, and so where dark matter is and how
• 18:02
  Speaker 1: it's distributed determines the shape and the structure of the
• 18:06
  Speaker 1: whole universe. And so we can tell from like pictures
• 18:08
  Speaker 1: of the Big Bang until the temperature of dark matter
• 18:11
  Speaker 1: at the beginning of time or right now. Well, we
• 18:13
  Speaker 1: can tell the temperature of dark matter over the history
• 18:16
  Speaker 1: of the universe. Everything is cooling down, but we can
• 18:19
  Speaker 1: tell whether dark matter was made hot or made cold.
• 18:22
  Speaker 1: Everything is getting colder over time, but we can tell
• 18:25
  Speaker 1: whether dark matter started out hotter or colder. And we
• 18:28
  Speaker 1: can do that by seeing whether or not it's moved
• 18:31
  Speaker 1: around a lot, whether or not it's been wiggling around
• 18:34
  Speaker 1: and that's affecting the structure universe, or whether it's been
• 18:36
  Speaker 1: mostly staying in the places it was made. I see
• 18:39
  Speaker 1: because I guess you assume that it's kind of like
• 18:41
  Speaker 1: a gas, right Like you don't assume it's a solid.
• 18:44
  Speaker 1: You assume that it's you know, kind of moving around freely.
• 18:48
  Speaker 1: It's not tied together to itself except with gravity. That's right,
• 18:51
  Speaker 1: only held together with gravity. And so we think of
• 18:53
  Speaker 1: it like a diffuse gas, like a pressureless gas that
• 18:56
  Speaker 1: doesn't even bounce against itself, and so basically it just
• 18:59
  Speaker 1: has gravity tational effects. And so we can sort of
• 19:02
  Speaker 1: walk through the history of the universe with a cold
• 19:04
  Speaker 1: version of dark matter, a version where dark matter is
• 19:06
  Speaker 1: mostly staying where it was, and then we can walk
• 19:09
  Speaker 1: through a version of the universe where dark matter is hot,
• 19:11
  Speaker 1: where it's zipping around really fast, and we see that
• 19:13
  Speaker 1: those two things predict different shapes of the universe that
• 19:17
  Speaker 1: we see today and also different histories of the universe,
• 19:20
  Speaker 1: and then we can compare those histories to what we
• 19:22
  Speaker 1: actually see because like, if the dark matter at the
• 19:24
  Speaker 1: beginning of time was super cold, then I guess it
• 19:31
  Speaker 1: particles themselves don't have enough speed to like go off
• 19:34
  Speaker 1: and spread out. They would sort of stay clumped together.
• 19:37
  Speaker 1: That's exactly right. So if dark matter is very cold,
• 19:40
  Speaker 1: then the structure of the universe forms sort of bottom up.
• 19:43
  Speaker 1: Everything is where it was and it's not zipping around
• 19:45
  Speaker 1: very much, and so you get these little clumps of
• 19:48
  Speaker 1: density from those initial wrinkles, and that's what seeds like
• 19:51
  Speaker 1: the formation of stars, and then stars get together and
• 19:54
  Speaker 1: they form galaxies, and galaxies pull themselves together to form
• 19:58
  Speaker 1: galaxy clusters. You get this structure formation that's sort of
• 20:01
  Speaker 1: bottom up. Everything starts clumping where it was and then
• 20:05
  Speaker 1: pulls together, so you get, for example, galaxies forming before
• 20:10
  Speaker 1: galaxy clusters. You get stars forming, then galaxies, then galaxy
• 20:14
  Speaker 1: clusters in that order. And we can look back through
• 20:17
  Speaker 1: the history of time because remember as we look out
• 20:20
  Speaker 1: through space, we're looking backwards in times, so we can
• 20:22
  Speaker 1: see where their galaxies a billion years after the universe started,
• 20:26
  Speaker 1: where their stars. Which order did things get made? We
• 20:29
  Speaker 1: can tell by looking deep into the history of the
• 20:31
  Speaker 1: universe just by looking far out into space. Right, And
• 20:34
  Speaker 1: I guess you're using relative terms right, like cold and
• 20:37
  Speaker 1: hot here. You're not thinking about a specific temperature because
• 20:41
  Speaker 1: that could maybe also depend on how heavy these particles are.
• 20:44
  Speaker 1: That's right. We're mostly talking about whether or not they're relativistic,
• 20:47
  Speaker 1: like are they moving it close to the speed of
• 20:49
  Speaker 1: light or are they not relativistic? You know, they're moving
• 20:52
  Speaker 1: a much less than when you say hot, you mean
• 20:55
  Speaker 1: like super duper hot light speed hot. Yeah, exactly. And
• 21:00
  Speaker 1: when we think about what hot dark matter would look like, well,
• 21:03
  Speaker 1: you have the early universe, and you know dark matter
• 21:06
  Speaker 1: is made just with everything else, and you get these
• 21:07
  Speaker 1: initial little clumps of density from quantum fluctuations. But if
• 21:11
  Speaker 1: dark matters most of the stuff and it's moving really
• 21:14
  Speaker 1: really fast, then those initial little blobs of density don't
• 21:17
  Speaker 1: really matter because dark matter sort of washes them all out,
• 21:20
  Speaker 1: Like the dark matter is flying everywhere super duper fast,
• 21:23
  Speaker 1: and so those initial little clumps get evened out, they
• 21:26
  Speaker 1: get smoothed out, so you don't get stars forming first.
• 21:29
  Speaker 1: Instead you get these like these really big supermassive blobs
• 21:34
  Speaker 1: of stuff because only the really big over densities, only
• 21:38
  Speaker 1: the really big clumps from the beginning stick around and
• 21:41
  Speaker 1: survived the dark matter spreading everything out to form some structure.
• 21:44
  Speaker 1: What do you mean? So if the dark matter is hot,
• 21:47
  Speaker 1: it means that the it's particles are moving a lot.
• 21:50
  Speaker 1: And so are you saying that dark matter is more
• 21:52
  Speaker 1: diffuse or like the blobs are moving around fast. Both,
• 21:56
  Speaker 1: they're moving around faster and so they spread out and
• 21:58
  Speaker 1: so it gets more even and so it's harder for
• 22:01
  Speaker 1: gravity to get a handle and start forming stars, for example,
• 22:05
  Speaker 1: because things get smooth. For gravity to form a star,
• 22:08
  Speaker 1: you need like a little blob that's denser than the
• 22:10
  Speaker 1: stuff around it that it can gather stuff together using gravity.
• 22:14
  Speaker 1: But if dark matter, which is most of the stuff,
• 22:16
  Speaker 1: is moving fast, then it's spread everything out, its smoothed
• 22:19
  Speaker 1: everything over. There's nothing for gravity get a handle on,
• 22:22
  Speaker 1: except for the really really big stuff because that's the
• 22:25
  Speaker 1: stuff that dark matter can't smooth out. And so instead
• 22:28
  Speaker 1: of getting stars and then galaxies and the galaxy clusters
• 22:32
  Speaker 1: and then superclusters, you start out with supercluster sized blobs
• 22:36
  Speaker 1: of stuff and then it breaks up into galaxy cluster
• 22:40
  Speaker 1: sized blobs of stuff, and those break up into galaxy
• 22:43
  Speaker 1: size blobs of stuff, and then you get stars forming.
• 22:46
  Speaker 1: So it's sort of like top down instead of bottom up.
• 22:49
  Speaker 1: Interesting just based off of the temperature of dark matter. Yeah,
• 22:53
  Speaker 1: so the temperature of dark matter totally determines the entire
• 22:57
  Speaker 1: history of the universe. Like the universe would be very
• 23:00
  Speaker 1: different if we had no dark matter because it wouldn't
• 23:02
  Speaker 1: have been around to clump the normal matter together into
• 23:05
  Speaker 1: stars and galaxies. And also the universe would be different
• 23:08
  Speaker 1: if we had hot or cold dark matter, just it's
• 23:11
  Speaker 1: such a dominant force. It's most of the gravity. So
• 23:14
  Speaker 1: it affects how the universe came together. And we can
• 23:17
  Speaker 1: actually tell the history of the universe whether things foreign,
• 23:21
  Speaker 1: buttom up or top down. Yeah, because we can look
• 23:24
  Speaker 1: back in time and we can say, well, we're there
• 23:26
  Speaker 1: galaxies in the first billion or two years after the
• 23:29
  Speaker 1: Big Bang, or did it take a while for galaxies
• 23:32
  Speaker 1: to form? And so we can look back in time
• 23:36
  Speaker 1: and we can ask whether these things were made, in
• 23:38
  Speaker 1: what order were they made. And also it affects the
• 23:41
  Speaker 1: way things look today, because things would be smoother today
• 23:45
  Speaker 1: if dark matter was hot, and things would be sort
• 23:47
  Speaker 1: of clumpier today if dark matter was cold, like for example,
• 23:51
  Speaker 1: our galaxy is the Milky Way, and if dark matter
• 23:54
  Speaker 1: was cold, then we expect that the Milky Way has
• 23:57
  Speaker 1: a bunch of like little galaxies orbiting it, the way
• 24:00
  Speaker 1: the Earth has the Moon. We expect that the Milky
• 24:02
  Speaker 1: Way has its own little like many galaxies that orbit
• 24:06
  Speaker 1: our galaxy. If dark matter was super cold, if dark
• 24:09
  Speaker 1: matter was cold, then there should have been these blobs
• 24:12
  Speaker 1: of stuff formed outside of our galaxy, these dwarf galaxies,
• 24:15
  Speaker 1: which would now be orbiting the Milky Way, and that
• 24:19
  Speaker 1: we should see that today, So that would be a
• 24:21
  Speaker 1: sign that dark matter is cold. Affects not just the
• 24:23
  Speaker 1: history of the universe, but also affects the shape of
• 24:26
  Speaker 1: the way things look today. Yeah, I guess it's. I mean,
• 24:29
  Speaker 1: it's such a huge part of the universe that you
• 24:32
  Speaker 1: know whether it's hot or not. It should be no
• 24:34
  Speaker 1: surprise that the term is the fate of the universe
• 24:36
  Speaker 1: because it's such a huge chunk of it. Yeah, exactly.
• 24:38
  Speaker 1: It's not a little detail. It's not like a tiny
• 24:41
  Speaker 1: bit of salt that you add to your recipe. Right,
• 24:43
  Speaker 1: it's most of the stuff in the universe, and so
• 24:46
  Speaker 1: of course it's going to have big consequences for how
• 24:48
  Speaker 1: the universe looks and how it comes together. All right,
• 24:52
  Speaker 1: it could be hot or cold, and we could probably
• 24:55
  Speaker 1: tell by looking at the structure and the history. I
• 24:58
  Speaker 1: guess the history is also important of the universe. The
• 25:00
  Speaker 1: history kind of tells us a clue about whether it's
• 25:02
  Speaker 1: hot or not. That's right. Did the structure form top
• 25:05
  Speaker 1: down big stuff first and then small stuff or to
• 25:08
  Speaker 1: the form bottom up like small stuff first, which then
• 25:10
  Speaker 1: came together to make the bigger stuff. And it also
• 25:14
  Speaker 1: affects the way things look in our universe today. Right,
• 25:17
  Speaker 1: all right, let's now answer the question whether dark matter
• 25:20
  Speaker 1: is hot or not and what that tells us about it.
• 25:23
  Speaker 1: The first, let's take another quick break. All right, Daniel,
• 25:39
  Speaker 1: is dark matter hot or not? Is it a swipe
• 25:42
  Speaker 1: laughter or a swipe right for you? Well, I love
• 25:45
  Speaker 1: dark matter. I'm very excited about dark matter. I'm very
• 25:48
  Speaker 1: attracted to dark matter. But I have to say that
• 25:51
  Speaker 1: the universe tells us that dark matter is quite cool.
• 25:55
  Speaker 1: It's not hot, It's definitely not hot. I mean, it's
• 25:58
  Speaker 1: still be beautiful. It's just you know, a little chilly.
• 26:00
  Speaker 1: That's right. It's got its own standards of beauty, and
• 26:03
  Speaker 1: it's pretty cool, you know, dark matter. And we know
• 26:07
  Speaker 1: that because we look at the history of the universe
• 26:09
  Speaker 1: and we see that stars formed first, and that then
• 26:12
  Speaker 1: galaxies formed, and that then galaxy structure is formed. Because
• 26:16
  Speaker 1: we look back in the very early universe and we
• 26:18
  Speaker 1: see galaxies forming before there were clusters, and we see
• 26:21
  Speaker 1: stars forming before there was galaxies. Can we tell that?
• 26:24
  Speaker 1: Can we? How can we tell? I felt like we
• 26:26
  Speaker 1: can only see really far out and tell the distance
• 26:29
  Speaker 1: and the age of things by looking at like supernovas,
• 26:33
  Speaker 1: And so how can we tell how things formed if
• 26:36
  Speaker 1: our only way of knowing is through stars? That's right? Well,
• 26:39
  Speaker 1: the supernovas tell us sort of like the distance ladder
• 26:42
  Speaker 1: and so we can tell how far away something is
• 26:46
  Speaker 1: and therefore when it happened. And you're right that we
• 26:49
  Speaker 1: need stars to happen to give us that distance ladder.
• 26:52
  Speaker 1: But we can go back and look at the early universe,
• 26:54
  Speaker 1: right that tells us like, okay, this is really really
• 26:57
  Speaker 1: far away. And for example, you would expect that there
• 27:00
  Speaker 1: would be galaxy clusters formed in the very early universe
• 27:04
  Speaker 1: if dark matter was hot. And so we look out
• 27:07
  Speaker 1: past the most distant supernovas into the deep early universe,
• 27:10
  Speaker 1: you know, and we can tell that these things happened,
• 27:13
  Speaker 1: you know, thirteen billion years ago, for example. We don't
• 27:16
  Speaker 1: see galaxy clusters forming out there in the very edges
• 27:19
  Speaker 1: of the things that we can observe. That's the very
• 27:21
  Speaker 1: earliest universe. And you're right. We we we can't get
• 27:24
  Speaker 1: as precise an estimate for those distances because we don't
• 27:27
  Speaker 1: have the supernovas, but we can extrapple it a little bit.
• 27:29
  Speaker 1: And also we know it's super duper old. So like
• 27:32
  Speaker 1: the the oldest stars that we can see tell us
• 27:36
  Speaker 1: that things were not as formed as they are closer
• 27:40
  Speaker 1: to us or closer to the present, that's right. They
• 27:42
  Speaker 1: tell us that the structure formed bottom up that things
• 27:45
  Speaker 1: came together in small clumps first, and then those small
• 27:48
  Speaker 1: clumps organized themselves into bigger stuff. So you get stars,
• 27:53
  Speaker 1: and then galaxies, and then galaxy clusters, and then superclusters
• 27:57
  Speaker 1: of galaxies, which is the latest structure to form. And
• 28:01
  Speaker 1: that's why they're the biggest gravitationally bound objects in the
• 28:04
  Speaker 1: universe because they have most recently come together. It takes
• 28:06
  Speaker 1: a while for gravity to do this, and galaxy superclusters
• 28:11
  Speaker 1: are the last thing to have formed. It's all that
• 28:13
  Speaker 1: we've had time to form so far in the universe.
• 28:15
  Speaker 1: All right, Well, I guess so then that tells us
• 28:17
  Speaker 1: that dork matter is cold, and I guess it. Do
• 28:22
  Speaker 1: we have a sense of how cold it is, like,
• 28:24
  Speaker 1: you know, not going at the speed of light. I
• 28:26
  Speaker 1: know that's how you define cold. But is it like chili?
• 28:30
  Speaker 1: Or is it like warm? Or is are we talking
• 28:32
  Speaker 1: like the temperature of the sun? What are we talking about?
• 28:34
  Speaker 1: It's definitely not the temperature of the sun. I mean,
• 28:37
  Speaker 1: if it's out there and it's a particle, it's going
• 28:39
  Speaker 1: to be very very cold. You know, it's going to
• 28:41
  Speaker 1: be a few degrees kelvin. Really, we think dark matter
• 28:45
  Speaker 1: is only a few degrees kelvin probably, yeah, And you know,
• 28:48
  Speaker 1: it's not interacting in the same way that like hydrogen
• 28:50
  Speaker 1: does in the core of the Sun to produce a
• 28:53
  Speaker 1: huge amount of energy. But there's still a lot we
• 28:55
  Speaker 1: don't know about dark matter that could have self interactions
• 28:57
  Speaker 1: that contain energy that we are not aware of. And
• 29:00
  Speaker 1: so everything we say here should be taken with a
• 29:02
  Speaker 1: big grain of salt because it's all pretty speculative. But
• 29:05
  Speaker 1: you know, also, the cold dark matter picture is pretty good.
• 29:09
  Speaker 1: It works pretty well, but it's not perfect, Like it
• 29:11
  Speaker 1: doesn't perfectly explain everything that we see, right, Like you're saying,
• 29:15
  Speaker 1: cold dark matter predicts that we would have baby galaxies
• 29:18
  Speaker 1: floating around us. That's right. We expect to see a
• 29:20
  Speaker 1: bunch of these dwarf galaxies orbiting the Milky Way, and
• 29:23
  Speaker 1: we see some, but we don't see nearly as many
• 29:26
  Speaker 1: as we expect, and we don't know yet. Is that
• 29:28
  Speaker 1: because dark matter isn't as cold as we thought, or
• 29:31
  Speaker 1: is it because those dwarf galaxies are harder to see
• 29:34
  Speaker 1: than we thought they would be. And recently people have
• 29:36
  Speaker 1: developed extra good techniques to find dwarf galaxies and they
• 29:39
  Speaker 1: found a few more, and that sort of closes the
• 29:41
  Speaker 1: gap a little bit, but there's still some tension there.
• 29:44
  Speaker 1: It's still something that we don't quite understand. And you know,
• 29:47
  Speaker 1: we like those details. We like getting those things right
• 29:50
  Speaker 1: because those are the things that tell us that our
• 29:52
  Speaker 1: theory is really working. And so there's still some question
• 29:55
  Speaker 1: marks about it. But it's definitely not hot. It's some
• 29:58
  Speaker 1: version of cold. I guess we can't make any version
• 30:02
  Speaker 1: in our simulations work out to be just like the
• 30:05
  Speaker 1: universe we have now, like you tweaking further, you don't
• 30:08
  Speaker 1: get the right proportion of dwarf or baby galaxies, not yet.
• 30:11
  Speaker 1: But you know, these simulations are very very hard to
• 30:14
  Speaker 1: do because you're simulating an enormous number of particles. And
• 30:18
  Speaker 1: when they do these simulations, they usually just like leave
• 30:21
  Speaker 1: out all the normal matter because the normal matter is
• 30:23
  Speaker 1: a small fraction and it's much harder to model because
• 30:26
  Speaker 1: normal matter has complicated interactions, right, you know, stars and
• 30:30
  Speaker 1: gas and all that stuff. It has pressure and complicated
• 30:34
  Speaker 1: flows because of the electromagnetic interactions and the strong interactions
• 30:37
  Speaker 1: and all that stuff. So until recently, these simulations have
• 30:40
  Speaker 1: mostly just removed all the bionic matter. But you know,
• 30:43
  Speaker 1: baryons are important. I'm a baryon, you're a baryon. Stars
• 30:47
  Speaker 1: or buryons. The whole visible part of the galaxy has
• 30:49
  Speaker 1: made a bury on. So what does it mean, Like,
• 30:52
  Speaker 1: that's the particles that we're made out of, regular matter
• 30:55
  Speaker 1: like quarks and electrons. And so when they do these
• 30:58
  Speaker 1: simulations to describe the structure the universe, they don't have
• 31:01
  Speaker 1: the computational power to describe all the barry on its
• 31:04
  Speaker 1: all the things that make me and you corks and
• 31:07
  Speaker 1: all that stuff, So they mostly just remove it as
• 31:09
  Speaker 1: a as a simplification because that's the most complicated stuff
• 31:12
  Speaker 1: to describe, and so our simulations are really approximate right now.
• 31:16
  Speaker 1: So people are working on ways to include normal matter
• 31:19
  Speaker 1: in these simulations and try to get more precise estimates,
• 31:22
  Speaker 1: more precise predictions for how many dwarf galaxies we should see. Yeah,
• 31:26
  Speaker 1: I guess people are complicated. They're hard to predict, for sure,
• 31:31
  Speaker 1: they are. They are hard to describe. So we know
• 31:33
  Speaker 1: we think dark matter is made out of particles, and
• 31:36
  Speaker 1: if it is, we think it's cold, because that's what
• 31:38
  Speaker 1: the universe is telling is So what does that tell
• 31:40
  Speaker 1: us about dark matter? Like, does it give us a
• 31:43
  Speaker 1: clue about what it is or what kind of particle
• 31:45
  Speaker 1: it is, or you know, is the fact that it's cold.
• 31:49
  Speaker 1: Does that tell you something about how it interacts with
• 31:52
  Speaker 1: other forces? Yeah, it tells us a lot. And what
• 31:54
  Speaker 1: it can do is remove candidate particles from the list,
• 31:58
  Speaker 1: and most specifically, it is the neutrino as a candidate
• 32:02
  Speaker 1: for dark matter. For a long time, people thought, oh,
• 32:05
  Speaker 1: there's a lot of invisible matter out there, a matter
• 32:07
  Speaker 1: that almost never or never interacts with us except for gravitationally.
• 32:11
  Speaker 1: Maybe it's just neutrinos. And it's a very tempting candidate
• 32:15
  Speaker 1: because we already know about neutrinos. We know neutrinos are
• 32:18
  Speaker 1: these whispy particles that can pass through a light year
• 32:20
  Speaker 1: of lead without interacting. The air around us is filled
• 32:24
  Speaker 1: with neutrinos, but we can't feel them or taste them.
• 32:27
  Speaker 1: They have a lot of energy, but they don't deposit
• 32:29
  Speaker 1: it on us. And so it's tempting to assign these
• 32:32
  Speaker 1: two mysteries together, right, the weirdness of neutrinos and the
• 32:36
  Speaker 1: mystery of the missing matter. Maybe one plus one just
• 32:38
  Speaker 1: equals too, And so for a long time people suspected
• 32:42
  Speaker 1: maybe the missing matter was just like a ridiculous number
• 32:45
  Speaker 1: of neutrinos. And remember neutrinos are very very light, that
• 32:49
  Speaker 1: hardly any mass per particle. It's not zero, but it's
• 32:52
  Speaker 1: a small number. So if you're gonna explain most of
• 32:55
  Speaker 1: the stuff in the universe with neutrinos, it would have
• 32:58
  Speaker 1: to be an ungodly number of neutrinos. Could it be
• 33:01
  Speaker 1: like a heavy neutrino, Like I know, neutrinos they can
• 33:04
  Speaker 1: have different masses, right, The neutrinos that we're aware of,
• 33:07
  Speaker 1: the three, the electron, mu and town netrinos all have
• 33:09
  Speaker 1: very very very small masses. And so what we can
• 33:12
  Speaker 1: do is we can rule out those We can say
• 33:14
  Speaker 1: it's not one of the neutrinos that we know one
• 33:16
  Speaker 1: of the neutrino lights. Yeah, exactly, because those neutrinos have
• 33:19
  Speaker 1: such small mass that they're always moving basically at the
• 33:22
  Speaker 1: speed of light, are very close to the speed of light.
• 33:24
  Speaker 1: For example, when neutrinos come from a supernova, they arrive,
• 33:28
  Speaker 1: you know, very close to the same time as the
• 33:31
  Speaker 1: photons arrive because they're traveling basically at the speed of light. Actually,
• 33:34
  Speaker 1: the neutrinos get here first because the photons get slowed
• 33:38
  Speaker 1: down by interacting with the star. But it's basically a race.
• 33:41
  Speaker 1: The neutrinos fly almost the speed of light. You're saying
• 33:44
  Speaker 1: they're faster than light, Thaniel, They're not faster than light.
• 33:47
  Speaker 1: They leave sooner. The photons spend more time packing, but
• 33:50
  Speaker 1: they do travel a little faster. But you're exactly right
• 33:53
  Speaker 1: that there's the possibility that there could be some weird
• 33:56
  Speaker 1: heavy neutrinos, so not the neutrinos that we're familiar with,
• 33:59
  Speaker 1: but if they're is another kind of neutrino, fourth neutrino,
• 34:02
  Speaker 1: or many other kinds of neutrinos that are very heavy,
• 34:05
  Speaker 1: then those are still valid candidates for the dark matter.
• 34:09
  Speaker 1: And those go by the terms like sterile neutrinos because
• 34:12
  Speaker 1: called sterile because maybe they interact with our kind of
• 34:14
  Speaker 1: matter even less. Wow, it's like a neutral neutrino. Yeah,
• 34:19
  Speaker 1: that's right. It's like an even more standoffish and snobbish
• 34:21
  Speaker 1: particle than the neutrino. And that's a hard standard to meet.
• 34:25
  Speaker 1: I was just thinking, like shy or you know, loth
• 34:27
  Speaker 1: to interact with other particles. They know the introvert neutrinos,
• 34:31
  Speaker 1: but you just assume that you know, it's just not
• 34:34
  Speaker 1: my apologies sterile neutrinos, I take it back, right, So
• 34:37
  Speaker 1: that tells that they can't be neutrinos because neutrinos you
• 34:41
  Speaker 1: usually go really fast, but they could be. Basically, that
• 34:44
  Speaker 1: doesn't leave you much, does It just tells you that
• 34:46
  Speaker 1: it's another kind of part of it. Yeah, and that
• 34:48
  Speaker 1: we don't know about that. That's an important clue because
• 34:50
  Speaker 1: that means that there's no particle on our current list
• 34:52
  Speaker 1: that fits the requirements. There's no particle out there that
• 34:56
  Speaker 1: doesn't have electromagnetic or strong interactions and is heavy. Right
• 35:01
  Speaker 1: there just isn't one. The only particle in our current
• 35:04
  Speaker 1: list that had any chance of being the dark matter
• 35:07
  Speaker 1: or new trinos, and this piece of evidence rules that out.
• 35:10
  Speaker 1: It says it can't be one of the new trinos
• 35:12
  Speaker 1: we know. So it has to be a new particle.
• 35:15
  Speaker 1: And that's exciting. A new heavy particle, a new heavy
• 35:17
  Speaker 1: particle exactly. It means that there's something new to discover.
• 35:20
  Speaker 1: It's not just oh, there are more of this particle
• 35:22
  Speaker 1: than we thought. It means there's a new particle. And
• 35:25
  Speaker 1: a new particle is interesting because you wonder, like why
• 35:27
  Speaker 1: does it exist? How many new particles are there, Where
• 35:29
  Speaker 1: did it come from? Why is it different from these
• 35:31
  Speaker 1: other particles? You know, it gives you a whole new
• 35:34
  Speaker 1: set of questions to ask, a whole new way to
• 35:36
  Speaker 1: look at the universe. And you guys are looking for
• 35:38
  Speaker 1: these in the particle colliders, right, you're smashing particles hoping
• 35:42
  Speaker 1: that a new kind of particle will pop out. And
• 35:44
  Speaker 1: you might say, hey, that's dark matter. That's right. And
• 35:46
  Speaker 1: we have specific ideas for what this new particle could be.
• 35:50
  Speaker 1: We have ideas like the whimp particle weakly interacting massive particle.
• 35:54
  Speaker 1: It's just a generic name meaning some big, heavy particle
• 35:59
  Speaker 1: that doesn't interact much. And it has to not interact
• 36:01
  Speaker 1: very much in order to be the dark matter. And
• 36:03
  Speaker 1: it has to be massive in order to be cold
• 36:06
  Speaker 1: because of the structure of the universe. And another idea
• 36:09
  Speaker 1: is the axion. The axon could be the dark matter,
• 36:13
  Speaker 1: and we have specific experiments to look for whimps and
• 36:16
  Speaker 1: for axons. We just did a podcast episode about axons.
• 36:20
  Speaker 1: They're not the same thing. They are not the same thing.
• 36:22
  Speaker 1: There are two very different kinds of particles. The axion
• 36:25
  Speaker 1: is like a heavier version of the photon, and the
• 36:27
  Speaker 1: whimp is like it's like a heavier version of the neutrino,
• 36:30
  Speaker 1: but maybe interacts even less. And we have experiments underground
• 36:34
  Speaker 1: to look for WIMPs, these big tanks of liquid argon,
• 36:38
  Speaker 1: for example, or liquid zenon that look for one whimp
• 36:42
  Speaker 1: coming through and knocking into a bunch of particles and
• 36:45
  Speaker 1: then giving us a signal. We're using space telescopes to
• 36:48
  Speaker 1: look to see if occasionally whimps bounce into each other
• 36:52
  Speaker 1: and give off a little flash of light that we
• 36:54
  Speaker 1: could see, which would be really really rare because dark
• 36:56
  Speaker 1: matter is dark. But you know, we look at places
• 36:59
  Speaker 1: where there is a lot of dark matter and try
• 37:01
  Speaker 1: to see the occasional blip, and then we try to
• 37:04
  Speaker 1: make dark matter in the collider to see if we
• 37:06
  Speaker 1: can create it and play with it there. So far,
• 37:09
  Speaker 1: none of these experiments have turned up any evidence for
• 37:12
  Speaker 1: dark matter that anybody believes, and so we're still in
• 37:15
  Speaker 1: the hunt. But you know, even though we don't know
• 37:18
  Speaker 1: what dark matter is, we're able to say some things
• 37:20
  Speaker 1: about what it isn't right, is it weird that you
• 37:22
  Speaker 1: haven't found dark matter in these colliders. I mean, like
• 37:26
  Speaker 1: in the universe there's five times more dark matter than
• 37:29
  Speaker 1: regular matter, which might make you think that it's like
• 37:32
  Speaker 1: it's more likely to happen, but in our colliders you
• 37:35
  Speaker 1: can't seem to make even a little bit of it.
• 37:37
  Speaker 1: That's right. It is a little weird. Now. On one hand,
• 37:40
  Speaker 1: it may be the dark matters everywhere, but we can't
• 37:43
  Speaker 1: make it because we're playing with our kind of matter,
• 37:45
  Speaker 1: like our kind of matter might not interact with dark matter,
• 37:48
  Speaker 1: which means that we can't use our matter to look
• 37:52
  Speaker 1: for dark matter, and we can't use our matter to
• 37:54
  Speaker 1: make dark matter like for that to work for any
• 37:56
  Speaker 1: of the experiments I just described to work to this
• 38:00
  Speaker 1: govern the particle nature of dark matter means there has
• 38:02
  Speaker 1: to be some way for our particles to talk to
• 38:05
  Speaker 1: the dark matter particles, to share some sort of new
• 38:08
  Speaker 1: dark photon, or some new force has to exist that
• 38:12
  Speaker 1: works on both particles. And it could be that it
• 38:15
  Speaker 1: just doesn't. It could be the dark matters out there.
• 38:17
  Speaker 1: It's a particle and it just feels nothing except for gravity,
• 38:21
  Speaker 1: in which case it's basically hopeless for us to discover
• 38:24
  Speaker 1: its particle nature because gravity is so weak that we
• 38:27
  Speaker 1: can only detect dark matter when you have enormous, like
• 38:31
  Speaker 1: galaxy sized blobs of it. Which makes it pretty hard
• 38:34
  Speaker 1: to do particle experiments. But I thought when you smash particles,
• 38:37
  Speaker 1: it turns into like pure energy, and then anything can
• 38:39
  Speaker 1: come out of it. You're saying that maybe it's possible
• 38:41
  Speaker 1: that not even dark matter can come out of that.
• 38:43
  Speaker 1: That's right. When you smash particles together, it's not exactly
• 38:46
  Speaker 1: pure energy. It turns into one of the bosons of
• 38:49
  Speaker 1: the forces that can interact with those particles. So, for example,
• 38:52
  Speaker 1: when you smash a quark and an antiquark together, you
• 38:55
  Speaker 1: can get a glue on, or you can get a photon,
• 38:58
  Speaker 1: or you can get a w boson. But if those forces,
• 39:01
  Speaker 1: the weak and the strong force and electromagnetism don't interact
• 39:04
  Speaker 1: with dark matter, then those bosons which represent that energy
• 39:08
  Speaker 1: can't then turn into dark matter. And so that is
• 39:11
  Speaker 1: one limitation I know that I like to say in
• 39:13
  Speaker 1: this podcast that we can use colliders to explore the
• 39:15
  Speaker 1: universe because anything that can be made will be made.
• 39:18
  Speaker 1: But there is an important caveat there that whatever can
• 39:22
  Speaker 1: be made has to somehow interact with the particles that
• 39:25
  Speaker 1: were smashing. If there's no way to interact, then you
• 39:28
  Speaker 1: just can't make it. You need a dark matter collider, Daniel. Obviously,
• 39:32
  Speaker 1: to discover dark matter, you have to build a dark
• 39:35
  Speaker 1: matter collider. All right, Well, um, it sounds like we
• 39:38
  Speaker 1: don't know what dark matter is still, but we know
• 39:41
  Speaker 1: that it's pretty cool. It's a pretty cool thing in
• 39:44
  Speaker 1: the universe. It's cool, that's right. Dark matter is pretty chill.
• 39:48
  Speaker 1: You know. It wants to come over and watch Netflix
• 39:50
  Speaker 1: with you, even if you don't think it's hot. Yeah,
• 39:54
  Speaker 1: all right, Well again, just makes me think about all
• 39:57
  Speaker 1: the crazy things we don't know, you know, and all
• 39:59
  Speaker 1: the sort of fun and clever ways we can tell
• 40:02
  Speaker 1: about things we don't know even though we don't know
• 40:05
  Speaker 1: anything about it. Yeah, And this is what science does,
• 40:08
  Speaker 1: is we probe things from every direction. We're trying to
• 40:11
  Speaker 1: uncover a real truth about the universe, and that has
• 40:13
  Speaker 1: lots of facets. And so if we get stumped in
• 40:16
  Speaker 1: one direction, like we can't seem to find it in
• 40:18
  Speaker 1: our detectors, then we go another route and say, well,
• 40:20
  Speaker 1: can we say anything about it from this perspective or
• 40:22
  Speaker 1: from that perspective? And we're trying to be clever in
• 40:25
  Speaker 1: the field of particle physics, and science in general is
• 40:28
  Speaker 1: filled with clever people having new ideas about ways to
• 40:31
  Speaker 1: answer these questions, and so to me, this is one
• 40:33
  Speaker 1: of the most elegant ways to put a really important,
• 40:36
  Speaker 1: really insightful constraint on what dark matter is and isn't.
• 40:40
  Speaker 1: All Right, Well, I think we answered that question pretty good,
• 40:43
  Speaker 1: and I think we can all learn a little bit
• 40:44
  Speaker 1: from dark matter to just be cool. Don't get too excited.
• 40:49
  Speaker 1: Thanks for joining us, See you next time. Thanks for listening,
• 41:00
  Speaker 1: and remember that Daniel and Jorge Explain the Universe is
• 41:02
  Speaker 1: a production of I Heart Radio. For More podcast. For
• 41:06
  Speaker 1: my heart Radio, visit the I heart Radio app, Apple Podcasts,
• 41:10
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