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Speaker 1: Hey, Daniel, I have a question about dark matter. Oh, man,

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Speaker 1: don't we all? I mean, I know that we don't

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Speaker 1: know what it is, right, but what is it like?

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Speaker 1: I mean, is it quishy? We don't know. What does

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Speaker 1: it taste like? Well, you know, our tongues can't taste it,

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Speaker 1: so again we don't really know. But is it fuzzy

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Speaker 1: maybe we don't know, or scratchy? Probably not. But again

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Speaker 1: we just don't know. You know, for such a hot topic,

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Speaker 1: you would think you guys would know more about it. Well,

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Speaker 1: that's one thing we do know, whether dark matter is

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Speaker 1: hot or not. Hi am more Hammon, cartoonists and the

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Speaker 1: creator of PhD comments. I'm Daniel. I'm a particle physicist,

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Speaker 1: and I have no opinion about the attractiveness of dark matter. Well,

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Speaker 1: it's definitely attractive, right, gravitationally speaking on a cosmological level.

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Speaker 1: That's right. It is the great attractor from that point

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Speaker 1: of view. But welcome to our podcast, Daniel and Jorge

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Speaker 1: Explain the Universe, a production of I Heart Radio in

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Speaker 1: which we talk about all the amazing and crazy things

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Speaker 1: in our universe, the things that scientists have understood and

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Speaker 1: the things that scientists are now working to understand. We

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Speaker 1: break down all the crazy for you and explain it

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Speaker 1: in a way that hopefully makes you smile. It's right,

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Speaker 1: all the things that are hot in this universe and

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Speaker 1: all the things that are not hot or cold or

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Speaker 1: super cold, because the universe has a broad range. Right,

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Speaker 1: things can be as hot as a million degrees or

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Speaker 1: as cold as zero degree. That's right. Everything has a temperature,

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Speaker 1: even black holes, we all have a rating. That's right.

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Speaker 1: Most of the universe out there is at a very

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Speaker 1: cold two point seven three degrees kelvin. But there are

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Speaker 1: a few hot spots a place like Earth where hot

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Speaker 1: little bits of temperature clue to make life an interesting podcast.

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Speaker 1: And so we like to talk about in this podcast

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Speaker 1: about dark matter a lot. And I feel like we

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Speaker 1: talked about it a lot because it's such a huge mystery.

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Speaker 1: I mean, it's of the universe and we don't know

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Speaker 1: what it's made out of. I think it's one of

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Speaker 1: the biggest open questions in science. You know, the person

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Speaker 1: or the group that figures out, like what is dark matter? Anyway,

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Speaker 1: that will be a historic moment, that will be a

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Speaker 1: an understanding and achievement, a breakthrough that will go down

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Speaker 1: in history for sure. Do you think a Nobel Prize

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Speaker 1: would be enough for that discovery or they need to

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Speaker 1: like stack him up or something, or maybe make up

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Speaker 1: like a special Nobel Prize, the dark Nobel Prize. You know,

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Speaker 1: they should have already given a Nobel Prize to Vera

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Speaker 1: Reuben for the discovery that dark matter was out there.

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Speaker 1: Even if we don't know what it is, we know

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Speaker 1: it's there, we know it's matter, and Nobel Prize committee

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Speaker 1: overlooked very Reuben. Some say because she's a woman, that's terrible.

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Speaker 1: That's the dark history of the Nobel Prize. It's the

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Speaker 1: dark history of dark matter. But we know something. There's

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Speaker 1: a little bit about dark matter. Let it's there, and

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Speaker 1: that it's affecting things gravitationally and keeping galaxies together. But

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Speaker 1: the question is how much more do we know about it?

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Speaker 1: What else do we know about this mysterious thing, if

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Speaker 1: it even is a thing. That's right, We would love

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Speaker 1: to know what dark matter is made out of. And

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Speaker 1: particle physicists like me scratch their heads all day wondering

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Speaker 1: what kind of particle is it made out of? For

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Speaker 1: many particles or is it a particle at all? But

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Speaker 1: along the way while we're looking for its particle nature,

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Speaker 1: we have other ways to try to get clues as

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Speaker 1: to what it might be. By looking at how it

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Speaker 1: moves and how it clumps, and how it squishes and

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Speaker 1: how it buzzes, we can try to get a handle

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Speaker 1: on what it is or isn't. Yeah, and so to

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Speaker 1: be on the program, we'll be asking the question is

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Speaker 1: dark matter hot or not? Well, for those of you

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Speaker 1: who are a little bit older, you might remember a

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Speaker 1: popular website a few decades ago called hot or Not,

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Speaker 1: which was probably inappropriate these days, totally inappropriate exact Yeah,

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Speaker 1: rated people based on their hotness. And I'm guessing it

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Speaker 1: was not the temperature. No, it was not the temperature,

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Speaker 1: although maybe we should revive it in a physics version,

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Speaker 1: like is the top cork hot or not? Our neutrinos

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Speaker 1: hot or not? That might be or cold maybe based

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Speaker 1: on how much fun thing you can get for it.

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Speaker 1: That's right, And it's a weird combination of ideas, you know,

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Speaker 1: dark matter, mysterious blobs of stuff out there in the universe,

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Speaker 1: and temperature, But it turns out to be very important.

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Speaker 1: It's one of the most powerful handles we have on

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Speaker 1: the nature of dark matter and one of the most

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Speaker 1: valuable clues we have that tells us what it is

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Speaker 1: and what it can't be. Yeah, So, as usual, we

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Speaker 1: were wondering how many people out there had thought about

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Speaker 1: this question of whether dark matter is hot or cold,

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Speaker 1: and so as usual Daniel went out there into the

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Speaker 1: wilds of the Internet to ask people this question. That's right.

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Speaker 1: So thank you to everybody who was willing to participate

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Speaker 1: in our random person on the internet questions. And if

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Speaker 1: you'd like to answer random questions from me in preparation

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Speaker 1: for a future podcast, please is right to us two

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Speaker 1: questions at Daniel and Jorge dot com. To think about

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Speaker 1: it for a second, what would you answer if someone

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Speaker 1: asked you, is dark matter hot or cold? Here's what

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Speaker 1: people have to say. Yes, it seems most natural to

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Speaker 1: me that dark matter would interact with itself, so it's

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Speaker 1: doing so. It's reasonable think that it could have a

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Speaker 1: temperature um it's relative to other dark matter, so I

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Speaker 1: guess it will be hot. What I think it's that

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Speaker 1: are parts that of the dark matter that can be

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Speaker 1: hot and parts that are gonna be colder. I think

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Speaker 1: dark matter is cold, or at least cooler than normal matter.

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Speaker 1: On average average temperature of the universe is a few

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Speaker 1: codings about zero. And since we have no idea, what

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Speaker 1: what is what other constituent particles of dark mastra? I

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Speaker 1: think the answer is we have no idea. I don't

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Speaker 1: know a lot about dark matter, but I don't usually

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Speaker 1: ink of matter having a specific temperature. I'd say we

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Speaker 1: don't know, because we don't even know what it is.

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Speaker 1: I would say that it's probably not hot. Well, hot

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Speaker 1: and cold are relative terms, So if what you mean

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Speaker 1: is does dark matter have a temperature, then I would

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Speaker 1: say probably not, because everything with the temperature gives off

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Speaker 1: infrared radiation. I had to consult my eleven year old,

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Speaker 1: who is the cosmologist in our family. So we think

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Speaker 1: that dark matter is cold. The only reason we know

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Speaker 1: it exists is because it reacts with gravity, and I

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Speaker 1: don't think it will react with anything on the electromagnetic spectrum,

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Speaker 1: so it wouldn't be hot or cold. Both knowing scientists,

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Speaker 1: they fancim intrinsic property of dark matter and named it

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Speaker 1: hot and cold, even though it doesn't mean anything like

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Speaker 1: hot or cold. All right, I like how people evaded

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Speaker 1: the question very expertise. You're impressed by that are you disappointed?

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Speaker 1: I'm impressed. They're like, Oh, they're thinking, like physics avoid

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Speaker 1: answering the question. It's like what is hot and cold?

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Speaker 1: Let's divert into that discussion. Well, we do this a

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Speaker 1: lot in physics. We apply weird sounding characteristics to things,

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Speaker 1: you know, like when we're talking about particles, we're talking

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Speaker 1: about their spin that's not really spin, and we're talking

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Speaker 1: about their mass, but they don't have any stuff to them.

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Speaker 1: And so I understand why people are a little wary

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Speaker 1: of interpreting like the temperature of dark matter, Like what

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Speaker 1: does that actually mean? What are we really talking about? Yeah, Like,

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Speaker 1: we don't even know if it's a thing, So how

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Speaker 1: can I not thing have temperature? That's right? It feels

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Speaker 1: like a detail, Like are you worried about what color

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Speaker 1: it is? You don't even know if it exists. Why

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Speaker 1: do you care if it's purple or brown? Right? Yeah? Yeah?

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Speaker 1: What color is dark matter? Then you it's dark? Alright,

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Speaker 1: So let's break it down for folks. And first of all,

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Speaker 1: I guess the question is, how can dark matter even

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Speaker 1: have a temperature if we don't know what it is? Right? Well,

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Speaker 1: let's remember what temperature really means. For us, temperature is

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Speaker 1: a macroscopic quantity. Right, you touch something, it feels hot

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Speaker 1: or it feels cold, and that's really actually about the

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Speaker 1: heat difference, Like if something has more energy in it

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Speaker 1: than you do, then the heat flows from it into

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Speaker 1: your finger, like when you touch a hot burner, and

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Speaker 1: that's what you're feeling. So you don't actually measure temperature

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Speaker 1: with your finger. You measure like a relative heat. But

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Speaker 1: when we think about temperature, like microscopically, we try to

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Speaker 1: understand how that experience of feeling things being hot or

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Speaker 1: cold translates to like the motion of the particles inside it.

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Speaker 1: And so most loosely, we think about temperatures relating to

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Speaker 1: how fast those particles inside something are moving. Like a gas.

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Speaker 1: If it's a hot gas, then the particles in it

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Speaker 1: are moving really fast, that's right, and that's in fact

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Speaker 1: what's happening. But also for liquids and for solids, and

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Speaker 1: in fact that's why liquids and solids are more solid

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Speaker 1: than gases, right, because their particles are not moving as much,

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Speaker 1: they're more easily trapped by all the bonds, and solid

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Speaker 1: has various temperatures because the atoms and it can wiggle

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Speaker 1: more or less they can shake and vibrate in that

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Speaker 1: kind of stuff. So it's all about the energy stored

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Speaker 1: in those parts, like the motion of the particles inside,

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Speaker 1: like the speed almost. Yeah, if you're talking about a gas,

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Speaker 1: then it's mostly about the speed. And I think this

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Speaker 1: is really interesting stuff to like take something that's macroscopic

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Speaker 1: and kind of qualitative, you know, this feeling of temperature,

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Speaker 1: and try to understand it on the microscopic scale, and

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Speaker 1: it sometimes works and it doesn't always work. And we

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Speaker 1: had a whole podcast where we talked about like the

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Speaker 1: hottest things in the universe, and some of these things

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Speaker 1: are counterintuitive. Like some of the hottest stuff in the

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Speaker 1: universe is the interstellar plasma, which is like some crazy

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Speaker 1: high temperature like three thousand degrees kelvin. But if we

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Speaker 1: dropped you in it, you would freeze to death immediately.

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Speaker 1: And that feels counterintuitive because there isn't much of it

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Speaker 1: out there that's right of this plasma. It's very hot,

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Speaker 1: but it's very dilute, so it doesn't contain a lot

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Speaker 1: of heat, and so you're much denser blob of heat.

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Speaker 1: If we dropped doing it, most of your heat would

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Speaker 1: leak out but the particles of that plasma individually are

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Speaker 1: moving super duper fast, and so you can still call

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Speaker 1: it hot, right, So it's related to the speed and

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Speaker 1: the get or the vibration or like the kinetic energy

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Speaker 1: of the molecules and particles in something. But how does

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Speaker 1: that apply to dark matter, because we don't really know

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Speaker 1: if dark matter is made out of particles or not.

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Speaker 1: We don't really know. Well, we know that something is

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Speaker 1: out there creating gravity. We know there's a kind of matter,

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Speaker 1: and that's really about it. We know sort of where

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Speaker 1: it is in the universe, but you're right, we don't

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Speaker 1: know that it's a particle that could turn out to

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Speaker 1: be something else. And you know, all the matter that

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Speaker 1: we've ever seen in the universe so far has been

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Speaker 1: made out of particles. So it seems tempting to say, well,

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Speaker 1: then the dark matter must also be made out of particles.

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Speaker 1: But you know, remember that dark matter is most of

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Speaker 1: the stuff in the universe. We've only seen a little slice.

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Speaker 1: We've seen five percent of the universe, so it's dangerous

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Speaker 1: to extrapolate to like a full and say the rest

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Speaker 1: of it must also be made out of particles. But

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Speaker 1: we don't really have better ideas, and so we typically

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Speaker 1: just assume dark matters made out of particles. So that's

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Speaker 1: kind of like the working hypothesis. Yeah, it's like, let's

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Speaker 1: try this, let's see if it works. If it breaks,

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Speaker 1: then we'll go back and examine all the assumptions we made.

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Speaker 1: But when you're exploring the unknown, you've got to make

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Speaker 1: some assumptions just to like have something to do, because

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Speaker 1: you can't just sit at home and go like, I

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Speaker 1: don't know what dark matter is. You know, it sort

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Speaker 1: of ends there. So we say, maybe dark matters of

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Speaker 1: particle and then we can ask if dark matter is

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Speaker 1: made of particles, are those particles moving fast or are

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Speaker 1: they moving slow? Right? Are they hot or not? Are

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Speaker 1: they hot or not? That's exactly what that really means.

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Speaker 1: It means is dark matter made out of super fast,

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Speaker 1: zippie particles moving relativistic speeds or is it made of

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Speaker 1: like heavier, slower moving particles that just sort of like

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Speaker 1: float around its slower speeds. I guess it's kind of

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Speaker 1: weird to think of something being hot but not being

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Speaker 1: able to touch it, you know what I mean? Like,

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Speaker 1: that's weird, right, I was just thinking, do neutrinos have

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Speaker 1: a temperature, like in a neutrino who we can't interact

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Speaker 1: with through electromagnetism, Can that have a temperature? Yes, absolutely,

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Speaker 1: neutrinos are very hot, and the reason is that neutrinos

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Speaker 1: have almost no mass, and so these through the universe

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Speaker 1: at very very high speeds, and so they contain a

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Speaker 1: lot of energy. You would say they have a high temperature,

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Speaker 1: but you're right that you can't feel them, and the

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Speaker 1: reason is that you have no interaction in common with them,

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Speaker 1: or almost none, because all they feel is the weak force.

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Speaker 1: So they have all this energy, but they have no

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Speaker 1: way to transmit it to you. So it's like you

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Speaker 1: pass right through each other. And so they can have

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Speaker 1: that high temperature, they can have that high energy, but

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Speaker 1: if there's no common interaction, no way to communicate, there's

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Speaker 1: no way for that energy to flow to you, and

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Speaker 1: so you won't feel them being hot. What about like

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Speaker 1: if what if dark matter is not a particle? Can

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Speaker 1: it still have a temperature and something that's not a

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Speaker 1: particle still be hot? WHOA? You just blew my mind.

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Speaker 1: Could something that's not made of particles have a temperature?

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Speaker 1: We've never seen anything that's not made of particles, So

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Speaker 1: that's quite a reach. But I guess macroscopically you could

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Speaker 1: like see if it emits light, and everything in the

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Speaker 1: universe that does emit light has a temperature, it's black

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Speaker 1: body radiation. But I don't know. That would be an

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Speaker 1: amazing thing to explore if we discover the dark matter

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Speaker 1: wasn't made of particles, because we do know something about

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Speaker 1: its temperature, which is what we're going to talk about today.

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Speaker 1: I see, so that it's made out of particles is

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Speaker 1: not just a working hypothesis. It's it's like your only hypothesis.

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Speaker 1: It's all we got at this point. It's like the

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Speaker 1: one idea we've been using for a hundred years, or

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Speaker 1: you know, empty box for crazy new ideas somebody should

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Speaker 1: come up with. Really, like, could it be something that's

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Speaker 1: not a particle? It certainly could be. I mean, we

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Speaker 1: have no concrete evidence that it is a particle other

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Speaker 1: than all matter so far discovered is made of particles,

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Speaker 1: but it certainly could be. We're open to surprises. I mean,

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Speaker 1: dark matter itself is a surprise. Its existence was a surprise,

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Speaker 1: and there have been some ideas about un particles matter

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Speaker 1: made out of things that are not quite particles that

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Speaker 1: you know, don't have a definitive size, but it's a

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Speaker 1: bit fuzzy, and nobody's really worked out the math for

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Speaker 1: how it could be dark matter, So they're just sort

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Speaker 1: of like the beginnings of ideas, I guess. I mean,

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Speaker 1: you know, like energy, is energy also particle based, because

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Speaker 1: you know, energy can have gravity or exert gravity or

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Speaker 1: effect gravity. Energy density certainly has gravity, and some energy

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Speaker 1: is particle based, like photons, right, Photons are basically just energy.

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Speaker 1: They have no mass to them, and photons contribute to

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Speaker 1: the energy density of the universe, and therefore it's curvature.

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Speaker 1: So certainly, all right, Well, I guess there's no maybe

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Speaker 1: room and your equations so far to account for something

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Speaker 1: that's not a particle, Is that kind of what you're saying.

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Speaker 1: That's right, yeah, But I would love to blow up

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Speaker 1: those equations. I would love if we found something about

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Speaker 1: dark matter that proved that it wasn't the particle and

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Speaker 1: then we have to go back to the drawing board

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Speaker 1: and think from scratch. That would be a tremendous breakthrough,

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Speaker 1: an intellectual crack in the very foundations of physics, which

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Speaker 1: is the kind of thing we're all hoping will happen,

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Speaker 1: you know, because those are the moment you get, like

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Speaker 1: the real insights. It pulled back the curtain and discover

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Speaker 1: something surprising and fascinating about the universe. So yeah, this

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Speaker 1: is all we got so far, and I would love

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Speaker 1: to see it break into pieces. You'd love to prove

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Speaker 1: that they're not so hot, all these theories precisely. All right, Well,

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Speaker 1: let's get into how we could tell whether or not

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Speaker 1: dark matter has a temperature besides like I guess, feeling

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Speaker 1: its forehead. Daniel, that's right, These days were very sensitive

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Speaker 1: to high temperatures. But let's get into how we could

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Speaker 1: tell and what it tells us about dark matter. But first,

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Speaker 1: let's take a quick break, all right, Daniel, we're talking

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Speaker 1: about whether dark matter is hot or cold, and so

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Speaker 1: we talked about how we have to kind of assume

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Speaker 1: that it's a particle because that's the only idea that

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Speaker 1: we have. And so if it's a particle, then you

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Speaker 1: can talk about whether those dark matter particles are moving

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Speaker 1: a lot or vibrating a lot, which case wouldn't make

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Speaker 1: them technically hot even though we can't feel it. That's right,

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Speaker 1: and we're really interested in whether dark matter is fast

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Speaker 1: moving or slow moving, because it tells us also whether

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Speaker 1: the particle is heavy, in which case it is more

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Speaker 1: likely slow moving and cold, or very very low mass,

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Speaker 1: in which case is probably faster moving and hot. So

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Speaker 1: we're we're using this as a way to sort of

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Speaker 1: get a clue as to the nature of dark matter.

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Speaker 1: It's but it could dark matter be both, like I mean,

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Speaker 1: it could it be like regular matter that some of

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Speaker 1: it is hot and some of it is cold. Totally absolutely,

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Speaker 1: dark matter could be lots of different particles, some of

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Speaker 1: which are very heavy and some of which are very light.

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Speaker 1: But we know that dark matter sticks around for a

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Speaker 1: very very long time. It's like cosmologically stable. It's been

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Speaker 1: here since the beginning. It's affected the structure of the universe.

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Speaker 1: We've seen it put its imprint on the whole history

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Speaker 1: of the universe. And so that suggests that it's probably stable,

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Speaker 1: that it's not changing a lot from one kind of

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Speaker 1: mass to another. But you know, we really just don't know.

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Speaker 1: All right, Well, let's get into now how we could

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Speaker 1: tell whether dark matter has the temperature or not, Like,

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Speaker 1: how do you how would you even measure the temperature?

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Speaker 1: Of dark matter. If a particle of dark matter is

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Speaker 1: moving a lot or vibrating a lot, or could we

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Speaker 1: even tell the different We can actually tell the difference,

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Speaker 1: and I think this is really clever. It's one of

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Speaker 1: the most elegant pieces of science that I've seen recently.

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Speaker 1: We can tell whether dark matter is moving fast or

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Speaker 1: slow because of the way it makes an imprint on

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Speaker 1: the growth of the universe. You know, the universe started

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Speaker 1: from like the Big Bang, and back then things were

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Speaker 1: hot and dense and mostly uniform, and then you've got

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Speaker 1: little quantum fluctuations, little pockets of density here and less

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Speaker 1: density there, and those pockets are critical because that's what

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Speaker 1: seeds the whole structure of the universe. Like the reason

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Speaker 1: we have a galaxy here and not over there is

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Speaker 1: because some initial fluctuation made things a little dense, and

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Speaker 1: then gravity clumped them together and clumped them together even further.

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Speaker 1: So you've got these little fluctuations in the early universe,

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Speaker 1: which see the structure of the universe, right, because gravity

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Speaker 1: takes over from these little wrinkles. But dark matter plays

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Speaker 1: a really big role in that because dark matter basically

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Speaker 1: is gravity, right, It's the biggest source of gravity in

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Speaker 1: the universe, and so where dark matter is and how

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Speaker 1: it's distributed determines the shape and the structure of the

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Speaker 1: whole universe. And so we can tell from like pictures

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Speaker 1: of the Big Bang until the temperature of dark matter

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Speaker 1: at the beginning of time or right now. Well, we

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Speaker 1: can tell the temperature of dark matter over the history

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Speaker 1: of the universe. Everything is cooling down, but we can

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Speaker 1: tell whether dark matter was made hot or made cold.

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Speaker 1: Everything is getting colder over time, but we can tell

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Speaker 1: whether dark matter started out hotter or colder. And we

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Speaker 1: can do that by seeing whether or not it's moved

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Speaker 1: around a lot, whether or not it's been wiggling around

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Speaker 1: and that's affecting the structure universe, or whether it's been

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Speaker 1: mostly staying in the places it was made. I see

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Speaker 1: because I guess you assume that it's kind of like

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Speaker 1: a gas, right Like you don't assume it's a solid.

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Speaker 1: You assume that it's you know, kind of moving around freely.

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Speaker 1: It's not tied together to itself except with gravity. That's right,

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Speaker 1: only held together with gravity. And so we think of

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Speaker 1: it like a diffuse gas, like a pressureless gas that

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Speaker 1: doesn't even bounce against itself, and so basically it just

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Speaker 1: has gravity tational effects. And so we can sort of

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Speaker 1: walk through the history of the universe with a cold

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Speaker 1: version of dark matter, a version where dark matter is

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Speaker 1: mostly staying where it was, and then we can walk

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Speaker 1: through a version of the universe where dark matter is hot,

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Speaker 1: where it's zipping around really fast, and we see that

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Speaker 1: those two things predict different shapes of the universe that

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Speaker 1: we see today and also different histories of the universe,

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Speaker 1: and then we can compare those histories to what we

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Speaker 1: actually see because like, if the dark matter at the

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Speaker 1: beginning of time was super cold, then I guess it

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Speaker 1: particles themselves don't have enough speed to like go off

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Speaker 1: and spread out. They would sort of stay clumped together.

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Speaker 1: That's exactly right. So if dark matter is very cold,

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Speaker 1: then the structure of the universe forms sort of bottom up.

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Speaker 1: Everything is where it was and it's not zipping around

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Speaker 1: very much, and so you get these little clumps of

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Speaker 1: density from those initial wrinkles, and that's what seeds like

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Speaker 1: the formation of stars, and then stars get together and

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Speaker 1: they form galaxies, and galaxies pull themselves together to form

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Speaker 1: galaxy clusters. You get this structure formation that's sort of

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Speaker 1: bottom up. Everything starts clumping where it was and then

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Speaker 1: pulls together, so you get, for example, galaxies forming before

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Speaker 1: galaxy clusters. You get stars forming, then galaxies, then galaxy

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Speaker 1: clusters in that order. And we can look back through

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Speaker 1: the history of time because remember as we look out

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Speaker 1: through space, we're looking backwards in times, so we can

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Speaker 1: see where their galaxies a billion years after the universe started,

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Speaker 1: where their stars. Which order did things get made? We

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Speaker 1: can tell by looking deep into the history of the

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Speaker 1: universe just by looking far out into space. Right, And

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Speaker 1: I guess you're using relative terms right, like cold and

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Speaker 1: hot here. You're not thinking about a specific temperature because

404
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Speaker 1: that could maybe also depend on how heavy these particles are.

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Speaker 1: That's right. We're mostly talking about whether or not they're relativistic,

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Speaker 1: like are they moving it close to the speed of

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Speaker 1: light or are they not relativistic? You know, they're moving

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Speaker 1: a much less than when you say hot, you mean

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Speaker 1: like super duper hot light speed hot. Yeah, exactly. And

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Speaker 1: when we think about what hot dark matter would look like, well,

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Speaker 1: you have the early universe, and you know dark matter

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Speaker 1: is made just with everything else, and you get these

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Speaker 1: initial little clumps of density from quantum fluctuations. But if

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Speaker 1: dark matters most of the stuff and it's moving really

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Speaker 1: really fast, then those initial little blobs of density don't

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Speaker 1: really matter because dark matter sort of washes them all out,

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Speaker 1: Like the dark matter is flying everywhere super duper fast,

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Speaker 1: and so those initial little clumps get evened out, they

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Speaker 1: get smoothed out, so you don't get stars forming first.

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Speaker 1: Instead you get these like these really big supermassive blobs

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Speaker 1: of stuff because only the really big over densities, only

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Speaker 1: the really big clumps from the beginning stick around and

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Speaker 1: survived the dark matter spreading everything out to form some structure.

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Speaker 1: What do you mean? So if the dark matter is hot,

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Speaker 1: it means that the it's particles are moving a lot.

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Speaker 1: And so are you saying that dark matter is more

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Speaker 1: diffuse or like the blobs are moving around fast. Both,

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Speaker 1: they're moving around faster and so they spread out and

429
00:21:58,760 --> 00:22:01,400
Speaker 1: so it gets more even and so it's harder for

430
00:22:01,520 --> 00:22:05,000
Speaker 1: gravity to get a handle and start forming stars, for example,

431
00:22:05,040 --> 00:22:08,120
Speaker 1: because things get smooth. For gravity to form a star,

432
00:22:08,240 --> 00:22:10,879
Speaker 1: you need like a little blob that's denser than the

433
00:22:10,880 --> 00:22:14,320
Speaker 1: stuff around it that it can gather stuff together using gravity.

434
00:22:14,400 --> 00:22:16,560
Speaker 1: But if dark matter, which is most of the stuff,

435
00:22:16,880 --> 00:22:19,879
Speaker 1: is moving fast, then it's spread everything out, its smoothed

436
00:22:19,960 --> 00:22:22,320
Speaker 1: everything over. There's nothing for gravity get a handle on,

437
00:22:22,680 --> 00:22:25,520
Speaker 1: except for the really really big stuff because that's the

438
00:22:25,560 --> 00:22:28,679
Speaker 1: stuff that dark matter can't smooth out. And so instead

439
00:22:28,720 --> 00:22:32,240
Speaker 1: of getting stars and then galaxies and the galaxy clusters

440
00:22:32,280 --> 00:22:36,520
Speaker 1: and then superclusters, you start out with supercluster sized blobs

441
00:22:36,520 --> 00:22:40,240
Speaker 1: of stuff and then it breaks up into galaxy cluster

442
00:22:40,320 --> 00:22:43,480
Speaker 1: sized blobs of stuff, and those break up into galaxy

443
00:22:43,520 --> 00:22:46,600
Speaker 1: size blobs of stuff, and then you get stars forming.

444
00:22:46,680 --> 00:22:49,760
Speaker 1: So it's sort of like top down instead of bottom up.

445
00:22:49,960 --> 00:22:53,800
Speaker 1: Interesting just based off of the temperature of dark matter. Yeah,

446
00:22:53,840 --> 00:22:56,960
Speaker 1: so the temperature of dark matter totally determines the entire

447
00:22:57,160 --> 00:22:59,920
Speaker 1: history of the universe. Like the universe would be very

448
00:23:00,080 --> 00:23:02,639
Speaker 1: different if we had no dark matter because it wouldn't

449
00:23:02,640 --> 00:23:05,280
Speaker 1: have been around to clump the normal matter together into

450
00:23:05,359 --> 00:23:08,240
Speaker 1: stars and galaxies. And also the universe would be different

451
00:23:08,240 --> 00:23:10,960
Speaker 1: if we had hot or cold dark matter, just it's

452
00:23:11,040 --> 00:23:14,040
Speaker 1: such a dominant force. It's most of the gravity. So

453
00:23:14,119 --> 00:23:17,240
Speaker 1: it affects how the universe came together. And we can

454
00:23:17,280 --> 00:23:21,639
Speaker 1: actually tell the history of the universe whether things foreign,

455
00:23:21,720 --> 00:23:24,200
Speaker 1: buttom up or top down. Yeah, because we can look

456
00:23:24,200 --> 00:23:26,440
Speaker 1: back in time and we can say, well, we're there

457
00:23:26,520 --> 00:23:29,439
Speaker 1: galaxies in the first billion or two years after the

458
00:23:29,440 --> 00:23:32,800
Speaker 1: Big Bang, or did it take a while for galaxies

459
00:23:32,800 --> 00:23:35,520
Speaker 1: to form? And so we can look back in time

460
00:23:36,000 --> 00:23:38,199
Speaker 1: and we can ask whether these things were made, in

461
00:23:38,240 --> 00:23:41,000
Speaker 1: what order were they made. And also it affects the

462
00:23:41,000 --> 00:23:45,080
Speaker 1: way things look today, because things would be smoother today

463
00:23:45,200 --> 00:23:47,040
Speaker 1: if dark matter was hot, and things would be sort

464
00:23:47,040 --> 00:23:51,240
Speaker 1: of clumpier today if dark matter was cold, like for example,

465
00:23:51,640 --> 00:23:54,680
Speaker 1: our galaxy is the Milky Way, and if dark matter

466
00:23:54,800 --> 00:23:57,199
Speaker 1: was cold, then we expect that the Milky Way has

467
00:23:57,240 --> 00:24:00,320
Speaker 1: a bunch of like little galaxies orbiting it, the way

468
00:24:00,359 --> 00:24:02,960
Speaker 1: the Earth has the Moon. We expect that the Milky

469
00:24:02,960 --> 00:24:06,560
Speaker 1: Way has its own little like many galaxies that orbit

470
00:24:06,600 --> 00:24:09,240
Speaker 1: our galaxy. If dark matter was super cold, if dark

471
00:24:09,240 --> 00:24:12,400
Speaker 1: matter was cold, then there should have been these blobs

472
00:24:12,400 --> 00:24:15,719
Speaker 1: of stuff formed outside of our galaxy, these dwarf galaxies,

473
00:24:15,920 --> 00:24:18,840
Speaker 1: which would now be orbiting the Milky Way, and that

474
00:24:19,040 --> 00:24:21,199
Speaker 1: we should see that today, So that would be a

475
00:24:21,200 --> 00:24:23,719
Speaker 1: sign that dark matter is cold. Affects not just the

476
00:24:23,760 --> 00:24:26,280
Speaker 1: history of the universe, but also affects the shape of

477
00:24:26,320 --> 00:24:29,480
Speaker 1: the way things look today. Yeah, I guess it's. I mean,

478
00:24:29,480 --> 00:24:32,280
Speaker 1: it's such a huge part of the universe that you

479
00:24:32,320 --> 00:24:34,240
Speaker 1: know whether it's hot or not. It should be no

480
00:24:34,320 --> 00:24:36,560
Speaker 1: surprise that the term is the fate of the universe

481
00:24:36,600 --> 00:24:38,920
Speaker 1: because it's such a huge chunk of it. Yeah, exactly.

482
00:24:38,960 --> 00:24:41,160
Speaker 1: It's not a little detail. It's not like a tiny

483
00:24:41,160 --> 00:24:43,320
Speaker 1: bit of salt that you add to your recipe. Right,

484
00:24:43,359 --> 00:24:46,000
Speaker 1: it's most of the stuff in the universe, and so

485
00:24:46,080 --> 00:24:48,920
Speaker 1: of course it's going to have big consequences for how

486
00:24:48,960 --> 00:24:52,359
Speaker 1: the universe looks and how it comes together. All right,

487
00:24:52,520 --> 00:24:55,639
Speaker 1: it could be hot or cold, and we could probably

488
00:24:55,640 --> 00:24:58,439
Speaker 1: tell by looking at the structure and the history. I

489
00:24:58,440 --> 00:25:00,439
Speaker 1: guess the history is also important of the universe. The

490
00:25:00,480 --> 00:25:02,360
Speaker 1: history kind of tells us a clue about whether it's

491
00:25:02,359 --> 00:25:05,000
Speaker 1: hot or not. That's right. Did the structure form top

492
00:25:05,080 --> 00:25:07,920
Speaker 1: down big stuff first and then small stuff or to

493
00:25:08,000 --> 00:25:10,600
Speaker 1: the form bottom up like small stuff first, which then

494
00:25:10,960 --> 00:25:13,960
Speaker 1: came together to make the bigger stuff. And it also

495
00:25:14,000 --> 00:25:17,119
Speaker 1: affects the way things look in our universe today. Right,

496
00:25:17,359 --> 00:25:20,680
Speaker 1: all right, let's now answer the question whether dark matter

497
00:25:20,920 --> 00:25:23,399
Speaker 1: is hot or not and what that tells us about it.

498
00:25:23,480 --> 00:25:39,120
Speaker 1: The first, let's take another quick break. All right, Daniel,

499
00:25:39,600 --> 00:25:42,080
Speaker 1: is dark matter hot or not? Is it a swipe

500
00:25:42,119 --> 00:25:45,320
Speaker 1: laughter or a swipe right for you? Well, I love

501
00:25:45,400 --> 00:25:48,239
Speaker 1: dark matter. I'm very excited about dark matter. I'm very

502
00:25:48,280 --> 00:25:51,320
Speaker 1: attracted to dark matter. But I have to say that

503
00:25:51,359 --> 00:25:54,639
Speaker 1: the universe tells us that dark matter is quite cool.

504
00:25:55,480 --> 00:25:58,040
Speaker 1: It's not hot, It's definitely not hot. I mean, it's

505
00:25:58,040 --> 00:26:00,720
Speaker 1: still be beautiful. It's just you know, a little chilly.

506
00:26:00,760 --> 00:26:03,880
Speaker 1: That's right. It's got its own standards of beauty, and

507
00:26:03,960 --> 00:26:06,879
Speaker 1: it's pretty cool, you know, dark matter. And we know

508
00:26:07,000 --> 00:26:09,119
Speaker 1: that because we look at the history of the universe

509
00:26:09,160 --> 00:26:12,240
Speaker 1: and we see that stars formed first, and that then

510
00:26:12,320 --> 00:26:16,240
Speaker 1: galaxies formed, and that then galaxy structure is formed. Because

511
00:26:16,280 --> 00:26:18,119
Speaker 1: we look back in the very early universe and we

512
00:26:18,160 --> 00:26:21,560
Speaker 1: see galaxies forming before there were clusters, and we see

513
00:26:21,600 --> 00:26:24,520
Speaker 1: stars forming before there was galaxies. Can we tell that?

514
00:26:24,600 --> 00:26:26,560
Speaker 1: Can we? How can we tell? I felt like we

515
00:26:26,600 --> 00:26:29,440
Speaker 1: can only see really far out and tell the distance

516
00:26:29,760 --> 00:26:33,600
Speaker 1: and the age of things by looking at like supernovas,

517
00:26:33,760 --> 00:26:36,320
Speaker 1: And so how can we tell how things formed if

518
00:26:36,320 --> 00:26:39,480
Speaker 1: our only way of knowing is through stars? That's right? Well,

519
00:26:39,480 --> 00:26:42,439
Speaker 1: the supernovas tell us sort of like the distance ladder

520
00:26:42,800 --> 00:26:46,000
Speaker 1: and so we can tell how far away something is

521
00:26:46,080 --> 00:26:48,959
Speaker 1: and therefore when it happened. And you're right that we

522
00:26:49,000 --> 00:26:51,840
Speaker 1: need stars to happen to give us that distance ladder.

523
00:26:52,080 --> 00:26:54,320
Speaker 1: But we can go back and look at the early universe,

524
00:26:54,400 --> 00:26:57,000
Speaker 1: right that tells us like, okay, this is really really

525
00:26:57,000 --> 00:27:00,680
Speaker 1: far away. And for example, you would expect that there

526
00:27:00,720 --> 00:27:04,760
Speaker 1: would be galaxy clusters formed in the very early universe

527
00:27:04,800 --> 00:27:07,000
Speaker 1: if dark matter was hot. And so we look out

528
00:27:07,080 --> 00:27:10,600
Speaker 1: past the most distant supernovas into the deep early universe,

529
00:27:10,640 --> 00:27:12,920
Speaker 1: you know, and we can tell that these things happened,

530
00:27:13,200 --> 00:27:16,160
Speaker 1: you know, thirteen billion years ago, for example. We don't

531
00:27:16,200 --> 00:27:19,439
Speaker 1: see galaxy clusters forming out there in the very edges

532
00:27:19,520 --> 00:27:21,600
Speaker 1: of the things that we can observe. That's the very

533
00:27:21,640 --> 00:27:24,160
Speaker 1: earliest universe. And you're right. We we we can't get

534
00:27:24,200 --> 00:27:27,000
Speaker 1: as precise an estimate for those distances because we don't

535
00:27:27,000 --> 00:27:29,560
Speaker 1: have the supernovas, but we can extrapple it a little bit.

536
00:27:29,560 --> 00:27:32,840
Speaker 1: And also we know it's super duper old. So like

537
00:27:32,880 --> 00:27:36,480
Speaker 1: the the oldest stars that we can see tell us

538
00:27:36,520 --> 00:27:40,240
Speaker 1: that things were not as formed as they are closer

539
00:27:40,280 --> 00:27:42,479
Speaker 1: to us or closer to the present, that's right. They

540
00:27:42,520 --> 00:27:45,760
Speaker 1: tell us that the structure formed bottom up that things

541
00:27:45,840 --> 00:27:48,600
Speaker 1: came together in small clumps first, and then those small

542
00:27:48,600 --> 00:27:52,880
Speaker 1: clumps organized themselves into bigger stuff. So you get stars,

543
00:27:53,000 --> 00:27:57,280
Speaker 1: and then galaxies, and then galaxy clusters, and then superclusters

544
00:27:57,280 --> 00:28:00,919
Speaker 1: of galaxies, which is the latest structure to form. And

545
00:28:01,040 --> 00:28:04,199
Speaker 1: that's why they're the biggest gravitationally bound objects in the

546
00:28:04,280 --> 00:28:06,840
Speaker 1: universe because they have most recently come together. It takes

547
00:28:06,840 --> 00:28:11,080
Speaker 1: a while for gravity to do this, and galaxy superclusters

548
00:28:11,119 --> 00:28:13,280
Speaker 1: are the last thing to have formed. It's all that

549
00:28:13,320 --> 00:28:15,720
Speaker 1: we've had time to form so far in the universe.

550
00:28:15,720 --> 00:28:17,879
Speaker 1: All right, Well, I guess so then that tells us

551
00:28:17,920 --> 00:28:21,960
Speaker 1: that dork matter is cold, and I guess it. Do

552
00:28:22,040 --> 00:28:23,840
Speaker 1: we have a sense of how cold it is, like,

553
00:28:24,280 --> 00:28:26,439
Speaker 1: you know, not going at the speed of light. I

554
00:28:26,480 --> 00:28:29,840
Speaker 1: know that's how you define cold. But is it like chili?

555
00:28:30,119 --> 00:28:32,280
Speaker 1: Or is it like warm? Or is are we talking

556
00:28:32,280 --> 00:28:34,840
Speaker 1: like the temperature of the sun? What are we talking about?

557
00:28:34,920 --> 00:28:37,000
Speaker 1: It's definitely not the temperature of the sun. I mean,

558
00:28:37,160 --> 00:28:39,520
Speaker 1: if it's out there and it's a particle, it's going

559
00:28:39,560 --> 00:28:41,760
Speaker 1: to be very very cold. You know, it's going to

560
00:28:41,840 --> 00:28:45,200
Speaker 1: be a few degrees kelvin. Really, we think dark matter

561
00:28:45,320 --> 00:28:48,120
Speaker 1: is only a few degrees kelvin probably, yeah, And you know,

562
00:28:48,160 --> 00:28:50,800
Speaker 1: it's not interacting in the same way that like hydrogen

563
00:28:50,880 --> 00:28:53,040
Speaker 1: does in the core of the Sun to produce a

564
00:28:53,120 --> 00:28:55,200
Speaker 1: huge amount of energy. But there's still a lot we

565
00:28:55,200 --> 00:28:57,760
Speaker 1: don't know about dark matter that could have self interactions

566
00:28:57,760 --> 00:29:00,240
Speaker 1: that contain energy that we are not aware of. And

567
00:29:00,240 --> 00:29:02,120
Speaker 1: so everything we say here should be taken with a

568
00:29:02,200 --> 00:29:05,480
Speaker 1: big grain of salt because it's all pretty speculative. But

569
00:29:05,600 --> 00:29:08,880
Speaker 1: you know, also, the cold dark matter picture is pretty good.

570
00:29:09,000 --> 00:29:11,440
Speaker 1: It works pretty well, but it's not perfect, Like it

571
00:29:11,520 --> 00:29:15,200
Speaker 1: doesn't perfectly explain everything that we see, right, Like you're saying,

572
00:29:15,240 --> 00:29:18,160
Speaker 1: cold dark matter predicts that we would have baby galaxies

573
00:29:18,280 --> 00:29:20,640
Speaker 1: floating around us. That's right. We expect to see a

574
00:29:20,640 --> 00:29:23,520
Speaker 1: bunch of these dwarf galaxies orbiting the Milky Way, and

575
00:29:23,560 --> 00:29:26,160
Speaker 1: we see some, but we don't see nearly as many

576
00:29:26,240 --> 00:29:28,719
Speaker 1: as we expect, and we don't know yet. Is that

577
00:29:28,800 --> 00:29:31,320
Speaker 1: because dark matter isn't as cold as we thought, or

578
00:29:31,400 --> 00:29:33,960
Speaker 1: is it because those dwarf galaxies are harder to see

579
00:29:34,000 --> 00:29:36,520
Speaker 1: than we thought they would be. And recently people have

580
00:29:36,600 --> 00:29:39,560
Speaker 1: developed extra good techniques to find dwarf galaxies and they

581
00:29:39,560 --> 00:29:41,800
Speaker 1: found a few more, and that sort of closes the

582
00:29:41,840 --> 00:29:44,760
Speaker 1: gap a little bit, but there's still some tension there.

583
00:29:44,760 --> 00:29:47,520
Speaker 1: It's still something that we don't quite understand. And you know,

584
00:29:47,600 --> 00:29:50,400
Speaker 1: we like those details. We like getting those things right

585
00:29:50,600 --> 00:29:52,280
Speaker 1: because those are the things that tell us that our

586
00:29:52,320 --> 00:29:55,680
Speaker 1: theory is really working. And so there's still some question

587
00:29:55,720 --> 00:29:58,719
Speaker 1: marks about it. But it's definitely not hot. It's some

588
00:29:58,920 --> 00:30:02,240
Speaker 1: version of cold. I guess we can't make any version

589
00:30:02,440 --> 00:30:04,920
Speaker 1: in our simulations work out to be just like the

590
00:30:05,000 --> 00:30:08,040
Speaker 1: universe we have now, like you tweaking further, you don't

591
00:30:08,440 --> 00:30:11,840
Speaker 1: get the right proportion of dwarf or baby galaxies, not yet.

592
00:30:11,880 --> 00:30:14,360
Speaker 1: But you know, these simulations are very very hard to

593
00:30:14,400 --> 00:30:18,120
Speaker 1: do because you're simulating an enormous number of particles. And

594
00:30:18,160 --> 00:30:21,040
Speaker 1: when they do these simulations, they usually just like leave

595
00:30:21,080 --> 00:30:23,680
Speaker 1: out all the normal matter because the normal matter is

596
00:30:23,680 --> 00:30:26,720
Speaker 1: a small fraction and it's much harder to model because

597
00:30:26,760 --> 00:30:30,520
Speaker 1: normal matter has complicated interactions, right, you know, stars and

598
00:30:30,800 --> 00:30:34,160
Speaker 1: gas and all that stuff. It has pressure and complicated

599
00:30:34,160 --> 00:30:37,640
Speaker 1: flows because of the electromagnetic interactions and the strong interactions

600
00:30:37,640 --> 00:30:40,680
Speaker 1: and all that stuff. So until recently, these simulations have

601
00:30:40,760 --> 00:30:43,680
Speaker 1: mostly just removed all the bionic matter. But you know,

602
00:30:43,960 --> 00:30:47,200
Speaker 1: baryons are important. I'm a baryon, you're a baryon. Stars

603
00:30:47,280 --> 00:30:49,800
Speaker 1: or buryons. The whole visible part of the galaxy has

604
00:30:49,840 --> 00:30:51,560
Speaker 1: made a bury on. So what does it mean, Like,

605
00:30:52,040 --> 00:30:54,760
Speaker 1: that's the particles that we're made out of, regular matter

606
00:30:55,240 --> 00:30:58,040
Speaker 1: like quarks and electrons. And so when they do these

607
00:30:58,120 --> 00:31:01,480
Speaker 1: simulations to describe the structure the universe, they don't have

608
00:31:01,560 --> 00:31:04,760
Speaker 1: the computational power to describe all the barry on its

609
00:31:04,760 --> 00:31:07,280
Speaker 1: all the things that make me and you corks and

610
00:31:07,320 --> 00:31:09,719
Speaker 1: all that stuff, So they mostly just remove it as

611
00:31:09,760 --> 00:31:12,920
Speaker 1: a as a simplification because that's the most complicated stuff

612
00:31:12,920 --> 00:31:16,600
Speaker 1: to describe, and so our simulations are really approximate right now.

613
00:31:16,600 --> 00:31:19,800
Speaker 1: So people are working on ways to include normal matter

614
00:31:19,840 --> 00:31:22,880
Speaker 1: in these simulations and try to get more precise estimates,

615
00:31:22,920 --> 00:31:26,680
Speaker 1: more precise predictions for how many dwarf galaxies we should see. Yeah,

616
00:31:26,680 --> 00:31:29,920
Speaker 1: I guess people are complicated. They're hard to predict, for sure,

617
00:31:31,320 --> 00:31:33,280
Speaker 1: they are. They are hard to describe. So we know

618
00:31:33,560 --> 00:31:36,280
Speaker 1: we think dark matter is made out of particles, and

619
00:31:36,320 --> 00:31:38,880
Speaker 1: if it is, we think it's cold, because that's what

620
00:31:38,920 --> 00:31:40,880
Speaker 1: the universe is telling is So what does that tell

621
00:31:40,920 --> 00:31:43,000
Speaker 1: us about dark matter? Like, does it give us a

622
00:31:43,000 --> 00:31:45,440
Speaker 1: clue about what it is or what kind of particle

623
00:31:45,800 --> 00:31:48,960
Speaker 1: it is, or you know, is the fact that it's cold.

624
00:31:49,120 --> 00:31:52,520
Speaker 1: Does that tell you something about how it interacts with

625
00:31:52,520 --> 00:31:54,600
Speaker 1: other forces? Yeah, it tells us a lot. And what

626
00:31:54,640 --> 00:31:58,000
Speaker 1: it can do is remove candidate particles from the list,

627
00:31:58,320 --> 00:32:02,440
Speaker 1: and most specifically, it is the neutrino as a candidate

628
00:32:02,480 --> 00:32:05,320
Speaker 1: for dark matter. For a long time, people thought, oh,

629
00:32:05,360 --> 00:32:07,680
Speaker 1: there's a lot of invisible matter out there, a matter

630
00:32:07,720 --> 00:32:11,040
Speaker 1: that almost never or never interacts with us except for gravitationally.

631
00:32:11,480 --> 00:32:15,120
Speaker 1: Maybe it's just neutrinos. And it's a very tempting candidate

632
00:32:15,160 --> 00:32:18,120
Speaker 1: because we already know about neutrinos. We know neutrinos are

633
00:32:18,160 --> 00:32:20,880
Speaker 1: these whispy particles that can pass through a light year

634
00:32:20,920 --> 00:32:24,400
Speaker 1: of lead without interacting. The air around us is filled

635
00:32:24,400 --> 00:32:27,120
Speaker 1: with neutrinos, but we can't feel them or taste them.

636
00:32:27,280 --> 00:32:29,080
Speaker 1: They have a lot of energy, but they don't deposit

637
00:32:29,160 --> 00:32:32,320
Speaker 1: it on us. And so it's tempting to assign these

638
00:32:32,360 --> 00:32:36,200
Speaker 1: two mysteries together, right, the weirdness of neutrinos and the

639
00:32:36,240 --> 00:32:38,800
Speaker 1: mystery of the missing matter. Maybe one plus one just

640
00:32:38,960 --> 00:32:42,160
Speaker 1: equals too, And so for a long time people suspected

641
00:32:42,480 --> 00:32:45,920
Speaker 1: maybe the missing matter was just like a ridiculous number

642
00:32:45,920 --> 00:32:49,200
Speaker 1: of neutrinos. And remember neutrinos are very very light, that

643
00:32:49,360 --> 00:32:52,640
Speaker 1: hardly any mass per particle. It's not zero, but it's

644
00:32:52,640 --> 00:32:55,760
Speaker 1: a small number. So if you're gonna explain most of

645
00:32:55,800 --> 00:32:58,160
Speaker 1: the stuff in the universe with neutrinos, it would have

646
00:32:58,200 --> 00:33:01,640
Speaker 1: to be an ungodly number of neutrinos. Could it be

647
00:33:01,680 --> 00:33:04,440
Speaker 1: like a heavy neutrino, Like I know, neutrinos they can

648
00:33:04,440 --> 00:33:07,160
Speaker 1: have different masses, right, The neutrinos that we're aware of,

649
00:33:07,200 --> 00:33:09,720
Speaker 1: the three, the electron, mu and town netrinos all have

650
00:33:09,880 --> 00:33:12,440
Speaker 1: very very very small masses. And so what we can

651
00:33:12,440 --> 00:33:14,000
Speaker 1: do is we can rule out those We can say

652
00:33:14,000 --> 00:33:16,120
Speaker 1: it's not one of the neutrinos that we know one

653
00:33:16,160 --> 00:33:19,280
Speaker 1: of the neutrino lights. Yeah, exactly, because those neutrinos have

654
00:33:19,480 --> 00:33:22,640
Speaker 1: such small mass that they're always moving basically at the

655
00:33:22,640 --> 00:33:24,680
Speaker 1: speed of light, are very close to the speed of light.

656
00:33:24,840 --> 00:33:28,480
Speaker 1: For example, when neutrinos come from a supernova, they arrive,

657
00:33:28,920 --> 00:33:31,200
Speaker 1: you know, very close to the same time as the

658
00:33:31,200 --> 00:33:34,960
Speaker 1: photons arrive because they're traveling basically at the speed of light. Actually,

659
00:33:34,960 --> 00:33:38,320
Speaker 1: the neutrinos get here first because the photons get slowed

660
00:33:38,360 --> 00:33:41,840
Speaker 1: down by interacting with the star. But it's basically a race.

661
00:33:41,840 --> 00:33:44,280
Speaker 1: The neutrinos fly almost the speed of light. You're saying

662
00:33:44,280 --> 00:33:47,080
Speaker 1: they're faster than light, Thaniel, They're not faster than light.

663
00:33:47,320 --> 00:33:50,480
Speaker 1: They leave sooner. The photons spend more time packing, but

664
00:33:50,520 --> 00:33:53,600
Speaker 1: they do travel a little faster. But you're exactly right

665
00:33:53,640 --> 00:33:56,040
Speaker 1: that there's the possibility that there could be some weird

666
00:33:56,160 --> 00:33:59,320
Speaker 1: heavy neutrinos, so not the neutrinos that we're familiar with,

667
00:33:59,560 --> 00:34:02,360
Speaker 1: but if they're is another kind of neutrino, fourth neutrino,

668
00:34:02,760 --> 00:34:05,560
Speaker 1: or many other kinds of neutrinos that are very heavy,

669
00:34:05,960 --> 00:34:08,640
Speaker 1: then those are still valid candidates for the dark matter.

670
00:34:09,000 --> 00:34:11,720
Speaker 1: And those go by the terms like sterile neutrinos because

671
00:34:12,040 --> 00:34:14,759
Speaker 1: called sterile because maybe they interact with our kind of

672
00:34:14,800 --> 00:34:18,960
Speaker 1: matter even less. Wow, it's like a neutral neutrino. Yeah,

673
00:34:19,040 --> 00:34:21,880
Speaker 1: that's right. It's like an even more standoffish and snobbish

674
00:34:21,920 --> 00:34:24,680
Speaker 1: particle than the neutrino. And that's a hard standard to meet.

675
00:34:25,440 --> 00:34:27,879
Speaker 1: I was just thinking, like shy or you know, loth

676
00:34:27,960 --> 00:34:31,280
Speaker 1: to interact with other particles. They know the introvert neutrinos,

677
00:34:31,520 --> 00:34:33,359
Speaker 1: but you just assume that you know, it's just not

678
00:34:34,280 --> 00:34:37,279
Speaker 1: my apologies sterile neutrinos, I take it back, right, So

679
00:34:37,360 --> 00:34:40,920
Speaker 1: that tells that they can't be neutrinos because neutrinos you

680
00:34:41,000 --> 00:34:44,080
Speaker 1: usually go really fast, but they could be. Basically, that

681
00:34:44,120 --> 00:34:46,360
Speaker 1: doesn't leave you much, does It just tells you that

682
00:34:46,400 --> 00:34:48,040
Speaker 1: it's another kind of part of it. Yeah, and that

683
00:34:48,080 --> 00:34:50,120
Speaker 1: we don't know about that. That's an important clue because

684
00:34:50,120 --> 00:34:52,680
Speaker 1: that means that there's no particle on our current list

685
00:34:52,920 --> 00:34:55,799
Speaker 1: that fits the requirements. There's no particle out there that

686
00:34:56,239 --> 00:35:01,239
Speaker 1: doesn't have electromagnetic or strong interactions and is heavy. Right

687
00:35:01,239 --> 00:35:04,120
Speaker 1: there just isn't one. The only particle in our current

688
00:35:04,120 --> 00:35:06,800
Speaker 1: list that had any chance of being the dark matter

689
00:35:07,239 --> 00:35:10,080
Speaker 1: or new trinos, and this piece of evidence rules that out.

690
00:35:10,160 --> 00:35:12,320
Speaker 1: It says it can't be one of the new trinos

691
00:35:12,320 --> 00:35:14,880
Speaker 1: we know. So it has to be a new particle.

692
00:35:15,040 --> 00:35:17,360
Speaker 1: And that's exciting. A new heavy particle, a new heavy

693
00:35:17,400 --> 00:35:20,480
Speaker 1: particle exactly. It means that there's something new to discover.

694
00:35:20,520 --> 00:35:22,760
Speaker 1: It's not just oh, there are more of this particle

695
00:35:22,800 --> 00:35:25,480
Speaker 1: than we thought. It means there's a new particle. And

696
00:35:25,480 --> 00:35:27,439
Speaker 1: a new particle is interesting because you wonder, like why

697
00:35:27,480 --> 00:35:29,759
Speaker 1: does it exist? How many new particles are there, Where

698
00:35:29,760 --> 00:35:31,879
Speaker 1: did it come from? Why is it different from these

699
00:35:31,880 --> 00:35:34,040
Speaker 1: other particles? You know, it gives you a whole new

700
00:35:34,040 --> 00:35:36,040
Speaker 1: set of questions to ask, a whole new way to

701
00:35:36,080 --> 00:35:38,640
Speaker 1: look at the universe. And you guys are looking for

702
00:35:38,680 --> 00:35:42,160
Speaker 1: these in the particle colliders, right, you're smashing particles hoping

703
00:35:42,200 --> 00:35:44,000
Speaker 1: that a new kind of particle will pop out. And

704
00:35:44,040 --> 00:35:46,799
Speaker 1: you might say, hey, that's dark matter. That's right. And

705
00:35:46,840 --> 00:35:49,799
Speaker 1: we have specific ideas for what this new particle could be.

706
00:35:50,120 --> 00:35:54,760
Speaker 1: We have ideas like the whimp particle weakly interacting massive particle.

707
00:35:54,760 --> 00:35:58,640
Speaker 1: It's just a generic name meaning some big, heavy particle

708
00:35:59,000 --> 00:36:01,640
Speaker 1: that doesn't interact much. And it has to not interact

709
00:36:01,719 --> 00:36:03,520
Speaker 1: very much in order to be the dark matter. And

710
00:36:03,560 --> 00:36:06,400
Speaker 1: it has to be massive in order to be cold

711
00:36:06,440 --> 00:36:09,240
Speaker 1: because of the structure of the universe. And another idea

712
00:36:09,360 --> 00:36:12,680
Speaker 1: is the axion. The axon could be the dark matter,

713
00:36:13,120 --> 00:36:16,319
Speaker 1: and we have specific experiments to look for whimps and

714
00:36:16,440 --> 00:36:20,040
Speaker 1: for axons. We just did a podcast episode about axons.

715
00:36:20,400 --> 00:36:22,160
Speaker 1: They're not the same thing. They are not the same thing.

716
00:36:22,200 --> 00:36:25,080
Speaker 1: There are two very different kinds of particles. The axion

717
00:36:25,200 --> 00:36:27,680
Speaker 1: is like a heavier version of the photon, and the

718
00:36:27,719 --> 00:36:30,120
Speaker 1: whimp is like it's like a heavier version of the neutrino,

719
00:36:30,239 --> 00:36:34,399
Speaker 1: but maybe interacts even less. And we have experiments underground

720
00:36:34,520 --> 00:36:37,839
Speaker 1: to look for WIMPs, these big tanks of liquid argon,

721
00:36:38,000 --> 00:36:42,280
Speaker 1: for example, or liquid zenon that look for one whimp

722
00:36:42,360 --> 00:36:45,319
Speaker 1: coming through and knocking into a bunch of particles and

723
00:36:45,320 --> 00:36:48,520
Speaker 1: then giving us a signal. We're using space telescopes to

724
00:36:48,600 --> 00:36:52,120
Speaker 1: look to see if occasionally whimps bounce into each other

725
00:36:52,160 --> 00:36:54,000
Speaker 1: and give off a little flash of light that we

726
00:36:54,000 --> 00:36:56,359
Speaker 1: could see, which would be really really rare because dark

727
00:36:56,360 --> 00:36:59,360
Speaker 1: matter is dark. But you know, we look at places

728
00:36:59,360 --> 00:37:01,319
Speaker 1: where there is a lot of dark matter and try

729
00:37:01,360 --> 00:37:03,920
Speaker 1: to see the occasional blip, and then we try to

730
00:37:04,000 --> 00:37:06,880
Speaker 1: make dark matter in the collider to see if we

731
00:37:06,960 --> 00:37:09,520
Speaker 1: can create it and play with it there. So far,

732
00:37:09,920 --> 00:37:12,319
Speaker 1: none of these experiments have turned up any evidence for

733
00:37:12,480 --> 00:37:15,719
Speaker 1: dark matter that anybody believes, and so we're still in

734
00:37:15,719 --> 00:37:18,400
Speaker 1: the hunt. But you know, even though we don't know

735
00:37:18,480 --> 00:37:20,279
Speaker 1: what dark matter is, we're able to say some things

736
00:37:20,280 --> 00:37:22,719
Speaker 1: about what it isn't right, is it weird that you

737
00:37:22,800 --> 00:37:26,200
Speaker 1: haven't found dark matter in these colliders. I mean, like

738
00:37:26,239 --> 00:37:29,040
Speaker 1: in the universe there's five times more dark matter than

739
00:37:29,080 --> 00:37:32,120
Speaker 1: regular matter, which might make you think that it's like

740
00:37:32,520 --> 00:37:35,440
Speaker 1: it's more likely to happen, but in our colliders you

741
00:37:35,480 --> 00:37:37,279
Speaker 1: can't seem to make even a little bit of it.

742
00:37:37,360 --> 00:37:40,279
Speaker 1: That's right. It is a little weird. Now. On one hand,

743
00:37:40,520 --> 00:37:43,080
Speaker 1: it may be the dark matters everywhere, but we can't

744
00:37:43,120 --> 00:37:45,600
Speaker 1: make it because we're playing with our kind of matter,

745
00:37:45,880 --> 00:37:48,600
Speaker 1: like our kind of matter might not interact with dark matter,

746
00:37:48,640 --> 00:37:52,040
Speaker 1: which means that we can't use our matter to look

747
00:37:52,040 --> 00:37:54,279
Speaker 1: for dark matter, and we can't use our matter to

748
00:37:54,400 --> 00:37:56,879
Speaker 1: make dark matter like for that to work for any

749
00:37:56,920 --> 00:37:59,759
Speaker 1: of the experiments I just described to work to this

750
00:38:00,040 --> 00:38:02,440
Speaker 1: govern the particle nature of dark matter means there has

751
00:38:02,480 --> 00:38:05,560
Speaker 1: to be some way for our particles to talk to

752
00:38:05,560 --> 00:38:08,479
Speaker 1: the dark matter particles, to share some sort of new

753
00:38:08,560 --> 00:38:12,080
Speaker 1: dark photon, or some new force has to exist that

754
00:38:12,160 --> 00:38:15,440
Speaker 1: works on both particles. And it could be that it

755
00:38:15,520 --> 00:38:17,560
Speaker 1: just doesn't. It could be the dark matters out there.

756
00:38:17,800 --> 00:38:21,240
Speaker 1: It's a particle and it just feels nothing except for gravity,

757
00:38:21,239 --> 00:38:24,400
Speaker 1: in which case it's basically hopeless for us to discover

758
00:38:24,480 --> 00:38:27,719
Speaker 1: its particle nature because gravity is so weak that we

759
00:38:27,719 --> 00:38:31,200
Speaker 1: can only detect dark matter when you have enormous, like

760
00:38:31,320 --> 00:38:34,319
Speaker 1: galaxy sized blobs of it. Which makes it pretty hard

761
00:38:34,360 --> 00:38:37,080
Speaker 1: to do particle experiments. But I thought when you smash particles,

762
00:38:37,120 --> 00:38:39,759
Speaker 1: it turns into like pure energy, and then anything can

763
00:38:39,800 --> 00:38:41,880
Speaker 1: come out of it. You're saying that maybe it's possible

764
00:38:41,920 --> 00:38:43,839
Speaker 1: that not even dark matter can come out of that.

765
00:38:43,840 --> 00:38:46,400
Speaker 1: That's right. When you smash particles together, it's not exactly

766
00:38:46,480 --> 00:38:49,800
Speaker 1: pure energy. It turns into one of the bosons of

767
00:38:49,880 --> 00:38:52,800
Speaker 1: the forces that can interact with those particles. So, for example,

768
00:38:52,960 --> 00:38:55,520
Speaker 1: when you smash a quark and an antiquark together, you

769
00:38:55,560 --> 00:38:57,840
Speaker 1: can get a glue on, or you can get a photon,

770
00:38:58,239 --> 00:39:01,440
Speaker 1: or you can get a w boson. But if those forces,

771
00:39:01,560 --> 00:39:04,640
Speaker 1: the weak and the strong force and electromagnetism don't interact

772
00:39:04,680 --> 00:39:08,120
Speaker 1: with dark matter, then those bosons which represent that energy

773
00:39:08,560 --> 00:39:11,360
Speaker 1: can't then turn into dark matter. And so that is

774
00:39:11,440 --> 00:39:13,360
Speaker 1: one limitation I know that I like to say in

775
00:39:13,400 --> 00:39:15,759
Speaker 1: this podcast that we can use colliders to explore the

776
00:39:15,840 --> 00:39:18,479
Speaker 1: universe because anything that can be made will be made.

777
00:39:18,640 --> 00:39:21,960
Speaker 1: But there is an important caveat there that whatever can

778
00:39:22,000 --> 00:39:24,960
Speaker 1: be made has to somehow interact with the particles that

779
00:39:25,000 --> 00:39:28,000
Speaker 1: were smashing. If there's no way to interact, then you

780
00:39:28,080 --> 00:39:31,400
Speaker 1: just can't make it. You need a dark matter collider, Daniel. Obviously,

781
00:39:32,480 --> 00:39:35,000
Speaker 1: to discover dark matter, you have to build a dark

782
00:39:35,000 --> 00:39:38,880
Speaker 1: matter collider. All right, Well, um, it sounds like we

783
00:39:38,960 --> 00:39:41,480
Speaker 1: don't know what dark matter is still, but we know

784
00:39:41,520 --> 00:39:44,840
Speaker 1: that it's pretty cool. It's a pretty cool thing in

785
00:39:44,880 --> 00:39:47,799
Speaker 1: the universe. It's cool, that's right. Dark matter is pretty chill.

786
00:39:48,480 --> 00:39:50,440
Speaker 1: You know. It wants to come over and watch Netflix

787
00:39:50,480 --> 00:39:54,400
Speaker 1: with you, even if you don't think it's hot. Yeah,

788
00:39:54,520 --> 00:39:57,000
Speaker 1: all right, Well again, just makes me think about all

789
00:39:57,040 --> 00:39:59,239
Speaker 1: the crazy things we don't know, you know, and all

790
00:39:59,280 --> 00:40:02,680
Speaker 1: the sort of fun and clever ways we can tell

791
00:40:02,719 --> 00:40:05,279
Speaker 1: about things we don't know even though we don't know

792
00:40:05,320 --> 00:40:08,319
Speaker 1: anything about it. Yeah, And this is what science does,

793
00:40:08,440 --> 00:40:11,080
Speaker 1: is we probe things from every direction. We're trying to

794
00:40:11,160 --> 00:40:13,600
Speaker 1: uncover a real truth about the universe, and that has

795
00:40:13,680 --> 00:40:16,640
Speaker 1: lots of facets. And so if we get stumped in

796
00:40:16,640 --> 00:40:18,440
Speaker 1: one direction, like we can't seem to find it in

797
00:40:18,480 --> 00:40:20,719
Speaker 1: our detectors, then we go another route and say, well,

798
00:40:20,719 --> 00:40:22,719
Speaker 1: can we say anything about it from this perspective or

799
00:40:22,760 --> 00:40:25,239
Speaker 1: from that perspective? And we're trying to be clever in

800
00:40:25,280 --> 00:40:28,160
Speaker 1: the field of particle physics, and science in general is

801
00:40:28,239 --> 00:40:31,239
Speaker 1: filled with clever people having new ideas about ways to

802
00:40:31,280 --> 00:40:33,640
Speaker 1: answer these questions, and so to me, this is one

803
00:40:33,640 --> 00:40:36,480
Speaker 1: of the most elegant ways to put a really important,

804
00:40:36,880 --> 00:40:39,959
Speaker 1: really insightful constraint on what dark matter is and isn't.

805
00:40:40,640 --> 00:40:43,279
Speaker 1: All Right, Well, I think we answered that question pretty good,

806
00:40:43,400 --> 00:40:44,960
Speaker 1: and I think we can all learn a little bit

807
00:40:44,960 --> 00:40:48,720
Speaker 1: from dark matter to just be cool. Don't get too excited.

808
00:40:49,040 --> 00:41:00,120
Speaker 1: Thanks for joining us, See you next time. Thanks for listening,

809
00:41:00,120 --> 00:41:02,840
Speaker 1: and remember that Daniel and Jorge Explain the Universe is

810
00:41:02,880 --> 00:41:06,279
Speaker 1: a production of I Heart Radio. For More podcast. For

811
00:41:06,400 --> 00:41:10,160
Speaker 1: my heart Radio, visit the I heart Radio app, Apple Podcasts,

812
00:41:10,280 --> 00:41:13,560
Speaker 1: or wherever you listen to your favorite shows. H