WEBVTT - Is dark matter hot? 0:00:08.440 --> 0:00:11.080 Hey, Daniel, I have a question about dark matter. Oh, man, 0:00:11.200 --> 0:00:13.720 don't we all? I mean, I know that we don't 0:00:13.760 --> 0:00:16.520 know what it is, right, but what is it like? 0:00:16.880 --> 0:00:19.480 I mean, is it quishy? We don't know. What does 0:00:19.480 --> 0:00:21.919 it taste like? Well, you know, our tongues can't taste it, 0:00:22.040 --> 0:00:25.040 so again we don't really know. But is it fuzzy 0:00:25.400 --> 0:00:29.480 maybe we don't know, or scratchy? Probably not. But again 0:00:29.760 --> 0:00:32.920 we just don't know. You know, for such a hot topic, 0:00:33.400 --> 0:00:35.920 you would think you guys would know more about it. Well, 0:00:36.159 --> 0:00:38.800 that's one thing we do know, whether dark matter is 0:00:38.840 --> 0:00:57.440 hot or not. Hi am more Hammon, cartoonists and the 0:00:57.480 --> 0:01:01.600 creator of PhD comments. I'm Daniel. I'm a particle physicist, 0:01:01.640 --> 0:01:05.880 and I have no opinion about the attractiveness of dark matter. Well, 0:01:05.880 --> 0:01:11.480 it's definitely attractive, right, gravitationally speaking on a cosmological level. 0:01:12.040 --> 0:01:14.360 That's right. It is the great attractor from that point 0:01:14.360 --> 0:01:17.039 of view. But welcome to our podcast, Daniel and Jorge 0:01:17.120 --> 0:01:19.720 Explain the Universe, a production of I Heart Radio in 0:01:19.760 --> 0:01:22.319 which we talk about all the amazing and crazy things 0:01:22.360 --> 0:01:25.160 in our universe, the things that scientists have understood and 0:01:25.160 --> 0:01:28.760 the things that scientists are now working to understand. We 0:01:28.920 --> 0:01:31.160 break down all the crazy for you and explain it 0:01:31.200 --> 0:01:33.240 in a way that hopefully makes you smile. It's right, 0:01:33.319 --> 0:01:35.560 all the things that are hot in this universe and 0:01:35.600 --> 0:01:37.959 all the things that are not hot or cold or 0:01:38.000 --> 0:01:41.399 super cold, because the universe has a broad range. Right, 0:01:41.480 --> 0:01:43.880 things can be as hot as a million degrees or 0:01:44.000 --> 0:01:47.280 as cold as zero degree. That's right. Everything has a temperature, 0:01:47.360 --> 0:01:51.560 even black holes, we all have a rating. That's right. 0:01:51.640 --> 0:01:53.240 Most of the universe out there is at a very 0:01:53.280 --> 0:01:56.360 cold two point seven three degrees kelvin. But there are 0:01:56.400 --> 0:01:59.960 a few hot spots a place like Earth where hot 0:02:00.080 --> 0:02:03.800 little bits of temperature clue to make life an interesting podcast. 0:02:03.920 --> 0:02:05.920 And so we like to talk about in this podcast 0:02:06.000 --> 0:02:08.200 about dark matter a lot. And I feel like we 0:02:08.280 --> 0:02:11.200 talked about it a lot because it's such a huge mystery. 0:02:11.360 --> 0:02:14.520 I mean, it's of the universe and we don't know 0:02:14.560 --> 0:02:16.120 what it's made out of. I think it's one of 0:02:16.160 --> 0:02:19.760 the biggest open questions in science. You know, the person 0:02:19.880 --> 0:02:23.280 or the group that figures out, like what is dark matter? Anyway, 0:02:23.800 --> 0:02:26.560 that will be a historic moment, that will be a 0:02:26.560 --> 0:02:29.760 an understanding and achievement, a breakthrough that will go down 0:02:29.800 --> 0:02:32.240 in history for sure. Do you think a Nobel Prize 0:02:32.240 --> 0:02:34.520 would be enough for that discovery or they need to 0:02:34.560 --> 0:02:36.400 like stack him up or something, or maybe make up 0:02:36.520 --> 0:02:39.760 like a special Nobel Prize, the dark Nobel Prize. You know, 0:02:39.800 --> 0:02:42.280 they should have already given a Nobel Prize to Vera 0:02:42.360 --> 0:02:44.840 Reuben for the discovery that dark matter was out there. 0:02:44.880 --> 0:02:46.600 Even if we don't know what it is, we know 0:02:46.720 --> 0:02:50.040 it's there, we know it's matter, and Nobel Prize committee 0:02:50.080 --> 0:02:54.440 overlooked very Reuben. Some say because she's a woman, that's terrible. 0:02:54.560 --> 0:02:56.679 That's the dark history of the Nobel Prize. It's the 0:02:56.800 --> 0:03:00.560 dark history of dark matter. But we know something. There's 0:03:00.600 --> 0:03:02.880 a little bit about dark matter. Let it's there, and 0:03:02.960 --> 0:03:06.840 that it's affecting things gravitationally and keeping galaxies together. But 0:03:06.960 --> 0:03:09.640 the question is how much more do we know about it? 0:03:09.680 --> 0:03:12.680 What else do we know about this mysterious thing, if 0:03:12.720 --> 0:03:14.760 it even is a thing. That's right, We would love 0:03:14.800 --> 0:03:17.080 to know what dark matter is made out of. And 0:03:17.200 --> 0:03:20.400 particle physicists like me scratch their heads all day wondering 0:03:20.440 --> 0:03:22.120 what kind of particle is it made out of? For 0:03:22.240 --> 0:03:25.200 many particles or is it a particle at all? But 0:03:25.280 --> 0:03:28.040 along the way while we're looking for its particle nature, 0:03:28.040 --> 0:03:30.240 we have other ways to try to get clues as 0:03:30.280 --> 0:03:32.280 to what it might be. By looking at how it 0:03:32.360 --> 0:03:34.520 moves and how it clumps, and how it squishes and 0:03:34.560 --> 0:03:36.800 how it buzzes, we can try to get a handle 0:03:37.040 --> 0:03:39.200 on what it is or isn't. Yeah, and so to 0:03:39.240 --> 0:03:46.040 be on the program, we'll be asking the question is 0:03:46.120 --> 0:03:49.680 dark matter hot or not? Well, for those of you 0:03:49.680 --> 0:03:51.680 who are a little bit older, you might remember a 0:03:51.720 --> 0:03:55.320 popular website a few decades ago called hot or Not, 0:03:55.560 --> 0:04:00.720 which was probably inappropriate these days, totally inappropriate exact Yeah, 0:04:00.800 --> 0:04:03.560 rated people based on their hotness. And I'm guessing it 0:04:03.600 --> 0:04:05.360 was not the temperature. No, it was not the temperature, 0:04:05.360 --> 0:04:07.920 although maybe we should revive it in a physics version, 0:04:07.960 --> 0:04:11.080 like is the top cork hot or not? Our neutrinos 0:04:11.160 --> 0:04:14.200 hot or not? That might be or cold maybe based 0:04:14.240 --> 0:04:16.279 on how much fun thing you can get for it. 0:04:18.080 --> 0:04:20.800 That's right, And it's a weird combination of ideas, you know, 0:04:21.240 --> 0:04:24.159 dark matter, mysterious blobs of stuff out there in the universe, 0:04:24.279 --> 0:04:27.080 and temperature, But it turns out to be very important. 0:04:27.200 --> 0:04:29.719 It's one of the most powerful handles we have on 0:04:29.800 --> 0:04:31.800 the nature of dark matter and one of the most 0:04:32.040 --> 0:04:34.800 valuable clues we have that tells us what it is 0:04:34.880 --> 0:04:37.360 and what it can't be. Yeah, So, as usual, we 0:04:37.360 --> 0:04:39.599 were wondering how many people out there had thought about 0:04:39.640 --> 0:04:42.839 this question of whether dark matter is hot or cold, 0:04:43.440 --> 0:04:46.240 and so as usual Daniel went out there into the 0:04:46.240 --> 0:04:49.520 wilds of the Internet to ask people this question. That's right. 0:04:49.560 --> 0:04:51.960 So thank you to everybody who was willing to participate 0:04:52.000 --> 0:04:54.720 in our random person on the internet questions. And if 0:04:54.800 --> 0:04:58.680 you'd like to answer random questions from me in preparation 0:04:58.760 --> 0:05:01.160 for a future podcast, please is right to us two 0:05:01.240 --> 0:05:03.800 questions at Daniel and Jorge dot com. To think about 0:05:03.839 --> 0:05:05.920 it for a second, what would you answer if someone 0:05:05.960 --> 0:05:09.120 asked you, is dark matter hot or cold? Here's what 0:05:09.120 --> 0:05:11.599 people have to say. Yes, it seems most natural to 0:05:11.640 --> 0:05:16.560 me that dark matter would interact with itself, so it's 0:05:16.720 --> 0:05:19.240 doing so. It's reasonable think that it could have a 0:05:19.279 --> 0:05:23.919 temperature um it's relative to other dark matter, so I 0:05:23.920 --> 0:05:28.200 guess it will be hot. What I think it's that 0:05:29.000 --> 0:05:33.040 are parts that of the dark matter that can be 0:05:33.600 --> 0:05:37.839 hot and parts that are gonna be colder. I think 0:05:37.960 --> 0:05:41.120 dark matter is cold, or at least cooler than normal matter. 0:05:41.279 --> 0:05:44.920 On average average temperature of the universe is a few 0:05:45.000 --> 0:05:48.760 codings about zero. And since we have no idea, what 0:05:48.760 --> 0:05:52.039 what is what other constituent particles of dark mastra? I 0:05:52.080 --> 0:05:55.120 think the answer is we have no idea. I don't 0:05:55.160 --> 0:05:59.240 know a lot about dark matter, but I don't usually 0:06:00.000 --> 0:06:02.800 ink of matter having a specific temperature. I'd say we 0:06:02.800 --> 0:06:04.800 don't know, because we don't even know what it is. 0:06:05.080 --> 0:06:08.479 I would say that it's probably not hot. Well, hot 0:06:08.520 --> 0:06:11.640 and cold are relative terms, So if what you mean 0:06:11.760 --> 0:06:14.240 is does dark matter have a temperature, then I would 0:06:14.279 --> 0:06:17.880 say probably not, because everything with the temperature gives off 0:06:18.120 --> 0:06:21.839 infrared radiation. I had to consult my eleven year old, 0:06:21.960 --> 0:06:25.440 who is the cosmologist in our family. So we think 0:06:25.480 --> 0:06:28.880 that dark matter is cold. The only reason we know 0:06:28.960 --> 0:06:32.800 it exists is because it reacts with gravity, and I 0:06:32.839 --> 0:06:36.560 don't think it will react with anything on the electromagnetic spectrum, 0:06:36.680 --> 0:06:42.640 so it wouldn't be hot or cold. Both knowing scientists, 0:06:43.440 --> 0:06:46.960 they fancim intrinsic property of dark matter and named it 0:06:47.080 --> 0:06:49.680 hot and cold, even though it doesn't mean anything like 0:06:49.839 --> 0:06:52.880 hot or cold. All right, I like how people evaded 0:06:52.920 --> 0:06:57.000 the question very expertise. You're impressed by that are you disappointed? 0:06:57.440 --> 0:07:01.640 I'm impressed. They're like, Oh, they're thinking, like physics avoid 0:07:01.680 --> 0:07:04.760 answering the question. It's like what is hot and cold? 0:07:05.240 --> 0:07:07.400 Let's divert into that discussion. Well, we do this a 0:07:07.400 --> 0:07:10.840 lot in physics. We apply weird sounding characteristics to things, 0:07:10.880 --> 0:07:13.120 you know, like when we're talking about particles, we're talking 0:07:13.160 --> 0:07:15.640 about their spin that's not really spin, and we're talking 0:07:15.720 --> 0:07:17.920 about their mass, but they don't have any stuff to them. 0:07:18.320 --> 0:07:20.960 And so I understand why people are a little wary 0:07:21.000 --> 0:07:24.640 of interpreting like the temperature of dark matter, Like what 0:07:24.680 --> 0:07:28.080 does that actually mean? What are we really talking about? Yeah, Like, 0:07:28.120 --> 0:07:29.840 we don't even know if it's a thing, So how 0:07:29.880 --> 0:07:33.440 can I not thing have temperature? That's right? It feels 0:07:33.440 --> 0:07:35.480 like a detail, Like are you worried about what color 0:07:35.520 --> 0:07:37.000 it is? You don't even know if it exists. Why 0:07:37.000 --> 0:07:39.560 do you care if it's purple or brown? Right? Yeah? Yeah? 0:07:39.560 --> 0:07:43.320 What color is dark matter? Then you it's dark? Alright, 0:07:43.360 --> 0:07:45.680 So let's break it down for folks. And first of all, 0:07:45.720 --> 0:07:48.000 I guess the question is, how can dark matter even 0:07:48.080 --> 0:07:50.480 have a temperature if we don't know what it is? Right? Well, 0:07:50.560 --> 0:07:54.680 let's remember what temperature really means. For us, temperature is 0:07:54.720 --> 0:07:58.080 a macroscopic quantity. Right, you touch something, it feels hot 0:07:58.160 --> 0:08:00.720 or it feels cold, and that's really actually about the 0:08:00.760 --> 0:08:03.520 heat difference, Like if something has more energy in it 0:08:03.800 --> 0:08:06.160 than you do, then the heat flows from it into 0:08:06.160 --> 0:08:08.720 your finger, like when you touch a hot burner, and 0:08:08.760 --> 0:08:11.480 that's what you're feeling. So you don't actually measure temperature 0:08:11.480 --> 0:08:13.800 with your finger. You measure like a relative heat. But 0:08:13.840 --> 0:08:16.800 when we think about temperature, like microscopically, we try to 0:08:16.880 --> 0:08:20.360 understand how that experience of feeling things being hot or 0:08:20.360 --> 0:08:24.560 cold translates to like the motion of the particles inside it. 0:08:25.240 --> 0:08:28.400 And so most loosely, we think about temperatures relating to 0:08:28.760 --> 0:08:33.720 how fast those particles inside something are moving. Like a gas. 0:08:33.800 --> 0:08:36.280 If it's a hot gas, then the particles in it 0:08:36.400 --> 0:08:39.120 are moving really fast, that's right, and that's in fact 0:08:39.200 --> 0:08:42.319 what's happening. But also for liquids and for solids, and 0:08:42.320 --> 0:08:45.440 in fact that's why liquids and solids are more solid 0:08:45.440 --> 0:08:48.160 than gases, right, because their particles are not moving as much, 0:08:48.200 --> 0:08:51.840 they're more easily trapped by all the bonds, and solid 0:08:51.880 --> 0:08:54.680 has various temperatures because the atoms and it can wiggle 0:08:54.960 --> 0:08:57.400 more or less they can shake and vibrate in that 0:08:57.559 --> 0:08:59.839 kind of stuff. So it's all about the energy stored 0:08:59.840 --> 0:09:02.200 in those parts, like the motion of the particles inside, 0:09:02.400 --> 0:09:04.920 like the speed almost. Yeah, if you're talking about a gas, 0:09:04.920 --> 0:09:07.400 then it's mostly about the speed. And I think this 0:09:07.440 --> 0:09:10.679 is really interesting stuff to like take something that's macroscopic 0:09:10.720 --> 0:09:13.439 and kind of qualitative, you know, this feeling of temperature, 0:09:13.760 --> 0:09:16.840 and try to understand it on the microscopic scale, and 0:09:16.920 --> 0:09:19.840 it sometimes works and it doesn't always work. And we 0:09:19.880 --> 0:09:21.640 had a whole podcast where we talked about like the 0:09:21.679 --> 0:09:24.400 hottest things in the universe, and some of these things 0:09:24.440 --> 0:09:27.040 are counterintuitive. Like some of the hottest stuff in the 0:09:27.120 --> 0:09:31.040 universe is the interstellar plasma, which is like some crazy 0:09:31.160 --> 0:09:34.800 high temperature like three thousand degrees kelvin. But if we 0:09:34.960 --> 0:09:37.800 dropped you in it, you would freeze to death immediately. 0:09:38.320 --> 0:09:41.160 And that feels counterintuitive because there isn't much of it 0:09:41.360 --> 0:09:44.640 out there that's right of this plasma. It's very hot, 0:09:44.640 --> 0:09:46.760 but it's very dilute, so it doesn't contain a lot 0:09:46.800 --> 0:09:49.600 of heat, and so you're much denser blob of heat. 0:09:49.600 --> 0:09:51.520 If we dropped doing it, most of your heat would 0:09:51.600 --> 0:09:54.600 leak out but the particles of that plasma individually are 0:09:54.600 --> 0:09:57.319 moving super duper fast, and so you can still call 0:09:57.400 --> 0:10:00.040 it hot, right, So it's related to the speed and 0:10:00.120 --> 0:10:02.800 the get or the vibration or like the kinetic energy 0:10:02.920 --> 0:10:07.000 of the molecules and particles in something. But how does 0:10:07.040 --> 0:10:09.000 that apply to dark matter, because we don't really know 0:10:09.040 --> 0:10:11.240 if dark matter is made out of particles or not. 0:10:11.400 --> 0:10:13.720 We don't really know. Well, we know that something is 0:10:13.760 --> 0:10:17.079 out there creating gravity. We know there's a kind of matter, 0:10:17.360 --> 0:10:20.199 and that's really about it. We know sort of where 0:10:20.240 --> 0:10:22.439 it is in the universe, but you're right, we don't 0:10:22.520 --> 0:10:24.440 know that it's a particle that could turn out to 0:10:24.440 --> 0:10:26.800 be something else. And you know, all the matter that 0:10:26.840 --> 0:10:29.319 we've ever seen in the universe so far has been 0:10:29.360 --> 0:10:32.480 made out of particles. So it seems tempting to say, well, 0:10:32.520 --> 0:10:35.080 then the dark matter must also be made out of particles. 0:10:35.120 --> 0:10:37.600 But you know, remember that dark matter is most of 0:10:37.600 --> 0:10:40.320 the stuff in the universe. We've only seen a little slice. 0:10:40.320 --> 0:10:43.560 We've seen five percent of the universe, so it's dangerous 0:10:43.600 --> 0:10:47.560 to extrapolate to like a full and say the rest 0:10:47.559 --> 0:10:49.920 of it must also be made out of particles. But 0:10:50.000 --> 0:10:53.160 we don't really have better ideas, and so we typically 0:10:53.200 --> 0:10:55.920 just assume dark matters made out of particles. So that's 0:10:55.960 --> 0:10:58.640 kind of like the working hypothesis. Yeah, it's like, let's 0:10:58.679 --> 0:11:01.160 try this, let's see if it works. If it breaks, 0:11:01.200 --> 0:11:03.760 then we'll go back and examine all the assumptions we made. 0:11:03.800 --> 0:11:06.040 But when you're exploring the unknown, you've got to make 0:11:06.120 --> 0:11:09.400 some assumptions just to like have something to do, because 0:11:09.400 --> 0:11:10.920 you can't just sit at home and go like, I 0:11:10.960 --> 0:11:13.160 don't know what dark matter is. You know, it sort 0:11:13.200 --> 0:11:15.920 of ends there. So we say, maybe dark matters of 0:11:16.000 --> 0:11:18.599 particle and then we can ask if dark matter is 0:11:18.640 --> 0:11:21.959 made of particles, are those particles moving fast or are 0:11:22.000 --> 0:11:24.040 they moving slow? Right? Are they hot or not? Are 0:11:24.080 --> 0:11:26.120 they hot or not? That's exactly what that really means. 0:11:26.120 --> 0:11:28.959 It means is dark matter made out of super fast, 0:11:29.040 --> 0:11:32.679 zippie particles moving relativistic speeds or is it made of 0:11:32.720 --> 0:11:35.520 like heavier, slower moving particles that just sort of like 0:11:35.760 --> 0:11:38.640 float around its slower speeds. I guess it's kind of 0:11:38.679 --> 0:11:42.240 weird to think of something being hot but not being 0:11:42.280 --> 0:11:44.040 able to touch it, you know what I mean? Like, 0:11:44.120 --> 0:11:47.000 that's weird, right, I was just thinking, do neutrinos have 0:11:47.040 --> 0:11:49.800 a temperature, like in a neutrino who we can't interact 0:11:49.880 --> 0:11:54.120 with through electromagnetism, Can that have a temperature? Yes, absolutely, 0:11:54.160 --> 0:11:57.920 neutrinos are very hot, and the reason is that neutrinos 0:11:57.960 --> 0:12:00.680 have almost no mass, and so these through the universe 0:12:00.760 --> 0:12:03.760 at very very high speeds, and so they contain a 0:12:03.760 --> 0:12:06.040 lot of energy. You would say they have a high temperature, 0:12:06.120 --> 0:12:08.120 but you're right that you can't feel them, and the 0:12:08.120 --> 0:12:10.760 reason is that you have no interaction in common with them, 0:12:10.840 --> 0:12:13.600 or almost none, because all they feel is the weak force. 0:12:13.960 --> 0:12:15.720 So they have all this energy, but they have no 0:12:15.760 --> 0:12:18.240 way to transmit it to you. So it's like you 0:12:18.280 --> 0:12:21.200 pass right through each other. And so they can have 0:12:21.280 --> 0:12:23.599 that high temperature, they can have that high energy, but 0:12:23.640 --> 0:12:27.080 if there's no common interaction, no way to communicate, there's 0:12:27.080 --> 0:12:29.480 no way for that energy to flow to you, and 0:12:29.520 --> 0:12:32.280 so you won't feel them being hot. What about like 0:12:32.320 --> 0:12:34.760 if what if dark matter is not a particle? Can 0:12:34.760 --> 0:12:37.240 it still have a temperature and something that's not a 0:12:37.280 --> 0:12:40.640 particle still be hot? WHOA? You just blew my mind. 0:12:40.760 --> 0:12:44.400 Could something that's not made of particles have a temperature? 0:12:44.760 --> 0:12:48.600 We've never seen anything that's not made of particles, So 0:12:48.679 --> 0:12:52.679 that's quite a reach. But I guess macroscopically you could 0:12:52.720 --> 0:12:55.520 like see if it emits light, and everything in the 0:12:55.600 --> 0:12:58.680 universe that does emit light has a temperature, it's black 0:12:58.720 --> 0:13:01.760 body radiation. But I don't know. That would be an 0:13:01.800 --> 0:13:04.880 amazing thing to explore if we discover the dark matter 0:13:05.040 --> 0:13:07.880 wasn't made of particles, because we do know something about 0:13:07.920 --> 0:13:10.080 its temperature, which is what we're going to talk about today. 0:13:10.160 --> 0:13:12.120 I see, so that it's made out of particles is 0:13:12.160 --> 0:13:15.520 not just a working hypothesis. It's it's like your only hypothesis. 0:13:16.080 --> 0:13:17.560 It's all we got at this point. It's like the 0:13:17.559 --> 0:13:20.320 one idea we've been using for a hundred years, or 0:13:20.679 --> 0:13:24.679 you know, empty box for crazy new ideas somebody should 0:13:24.720 --> 0:13:27.320 come up with. Really, like, could it be something that's 0:13:27.320 --> 0:13:29.640 not a particle? It certainly could be. I mean, we 0:13:29.880 --> 0:13:32.520 have no concrete evidence that it is a particle other 0:13:32.559 --> 0:13:35.600 than all matter so far discovered is made of particles, 0:13:36.000 --> 0:13:38.360 but it certainly could be. We're open to surprises. I mean, 0:13:38.440 --> 0:13:42.439 dark matter itself is a surprise. Its existence was a surprise, 0:13:43.040 --> 0:13:47.079 and there have been some ideas about un particles matter 0:13:47.160 --> 0:13:49.160 made out of things that are not quite particles that 0:13:49.200 --> 0:13:52.240 you know, don't have a definitive size, but it's a 0:13:52.240 --> 0:13:54.880 bit fuzzy, and nobody's really worked out the math for 0:13:55.040 --> 0:13:56.960 how it could be dark matter, So they're just sort 0:13:56.960 --> 0:13:59.319 of like the beginnings of ideas, I guess. I mean, 0:13:59.360 --> 0:14:03.839 you know, like energy, is energy also particle based, because 0:14:03.840 --> 0:14:06.680 you know, energy can have gravity or exert gravity or 0:14:06.760 --> 0:14:10.800 effect gravity. Energy density certainly has gravity, and some energy 0:14:10.880 --> 0:14:14.800 is particle based, like photons, right, Photons are basically just energy. 0:14:15.200 --> 0:14:18.240 They have no mass to them, and photons contribute to 0:14:18.240 --> 0:14:21.520 the energy density of the universe, and therefore it's curvature. 0:14:21.840 --> 0:14:26.760 So certainly, all right, Well, I guess there's no maybe 0:14:26.880 --> 0:14:30.320 room and your equations so far to account for something 0:14:30.360 --> 0:14:32.240 that's not a particle, Is that kind of what you're saying. 0:14:32.240 --> 0:14:34.440 That's right, yeah, But I would love to blow up 0:14:34.480 --> 0:14:36.640 those equations. I would love if we found something about 0:14:36.720 --> 0:14:39.320 dark matter that proved that it wasn't the particle and 0:14:39.320 --> 0:14:40.880 then we have to go back to the drawing board 0:14:40.920 --> 0:14:44.320 and think from scratch. That would be a tremendous breakthrough, 0:14:44.360 --> 0:14:47.520 an intellectual crack in the very foundations of physics, which 0:14:47.560 --> 0:14:49.440 is the kind of thing we're all hoping will happen, 0:14:49.560 --> 0:14:51.400 you know, because those are the moment you get, like 0:14:51.440 --> 0:14:54.200 the real insights. It pulled back the curtain and discover 0:14:54.360 --> 0:14:57.760 something surprising and fascinating about the universe. So yeah, this 0:14:57.800 --> 0:14:59.400 is all we got so far, and I would love 0:14:59.440 --> 0:15:01.680 to see it break into pieces. You'd love to prove 0:15:01.720 --> 0:15:06.040 that they're not so hot, all these theories precisely. All right, Well, 0:15:06.120 --> 0:15:09.200 let's get into how we could tell whether or not 0:15:09.320 --> 0:15:13.040 dark matter has a temperature besides like I guess, feeling 0:15:13.080 --> 0:15:18.160 its forehead. Daniel, that's right, These days were very sensitive 0:15:18.200 --> 0:15:21.440 to high temperatures. But let's get into how we could 0:15:21.480 --> 0:15:24.320 tell and what it tells us about dark matter. But first, 0:15:24.400 --> 0:15:37.600 let's take a quick break, all right, Daniel, we're talking 0:15:37.600 --> 0:15:40.840 about whether dark matter is hot or cold, and so 0:15:40.880 --> 0:15:42.720 we talked about how we have to kind of assume 0:15:42.840 --> 0:15:45.560 that it's a particle because that's the only idea that 0:15:45.600 --> 0:15:48.120 we have. And so if it's a particle, then you 0:15:48.160 --> 0:15:51.760 can talk about whether those dark matter particles are moving 0:15:51.800 --> 0:15:55.000 a lot or vibrating a lot, which case wouldn't make 0:15:55.040 --> 0:15:57.760 them technically hot even though we can't feel it. That's right, 0:15:57.800 --> 0:16:01.440 and we're really interested in whether dark matter is fast 0:16:01.480 --> 0:16:04.320 moving or slow moving, because it tells us also whether 0:16:04.440 --> 0:16:06.560 the particle is heavy, in which case it is more 0:16:06.600 --> 0:16:09.640 likely slow moving and cold, or very very low mass, 0:16:09.720 --> 0:16:12.560 in which case is probably faster moving and hot. So 0:16:12.600 --> 0:16:14.320 we're we're using this as a way to sort of 0:16:14.320 --> 0:16:16.480 get a clue as to the nature of dark matter. 0:16:16.520 --> 0:16:19.520 It's but it could dark matter be both, like I mean, 0:16:19.560 --> 0:16:21.600 it could it be like regular matter that some of 0:16:21.640 --> 0:16:24.240 it is hot and some of it is cold. Totally absolutely, 0:16:24.320 --> 0:16:26.920 dark matter could be lots of different particles, some of 0:16:26.920 --> 0:16:29.040 which are very heavy and some of which are very light. 0:16:29.240 --> 0:16:31.160 But we know that dark matter sticks around for a 0:16:31.280 --> 0:16:34.440 very very long time. It's like cosmologically stable. It's been 0:16:34.520 --> 0:16:37.480 here since the beginning. It's affected the structure of the universe. 0:16:37.560 --> 0:16:40.160 We've seen it put its imprint on the whole history 0:16:40.160 --> 0:16:43.480 of the universe. And so that suggests that it's probably stable, 0:16:43.560 --> 0:16:46.000 that it's not changing a lot from one kind of 0:16:46.000 --> 0:16:48.600 mass to another. But you know, we really just don't know. 0:16:48.680 --> 0:16:51.480 All right, Well, let's get into now how we could 0:16:51.480 --> 0:16:53.840 tell whether dark matter has the temperature or not, Like, 0:16:53.840 --> 0:16:55.760 how do you how would you even measure the temperature? 0:16:55.840 --> 0:16:57.800 Of dark matter. If a particle of dark matter is 0:16:57.800 --> 0:17:00.680 moving a lot or vibrating a lot, or could we 0:17:00.720 --> 0:17:03.080 even tell the different We can actually tell the difference, 0:17:03.120 --> 0:17:04.920 and I think this is really clever. It's one of 0:17:04.960 --> 0:17:07.960 the most elegant pieces of science that I've seen recently. 0:17:08.480 --> 0:17:11.280 We can tell whether dark matter is moving fast or 0:17:11.359 --> 0:17:14.240 slow because of the way it makes an imprint on 0:17:14.280 --> 0:17:17.400 the growth of the universe. You know, the universe started 0:17:17.480 --> 0:17:20.800 from like the Big Bang, and back then things were 0:17:20.840 --> 0:17:24.200 hot and dense and mostly uniform, and then you've got 0:17:24.280 --> 0:17:28.160 little quantum fluctuations, little pockets of density here and less 0:17:28.200 --> 0:17:31.119 density there, and those pockets are critical because that's what 0:17:31.240 --> 0:17:34.040 seeds the whole structure of the universe. Like the reason 0:17:34.080 --> 0:17:36.240 we have a galaxy here and not over there is 0:17:36.280 --> 0:17:39.399 because some initial fluctuation made things a little dense, and 0:17:39.400 --> 0:17:42.560 then gravity clumped them together and clumped them together even further. 0:17:43.040 --> 0:17:45.960 So you've got these little fluctuations in the early universe, 0:17:46.200 --> 0:17:49.480 which see the structure of the universe, right, because gravity 0:17:49.520 --> 0:17:53.240 takes over from these little wrinkles. But dark matter plays 0:17:53.240 --> 0:17:56.040 a really big role in that because dark matter basically 0:17:56.240 --> 0:17:58.960 is gravity, right, It's the biggest source of gravity in 0:17:59.000 --> 0:18:02.760 the universe, and so where dark matter is and how 0:18:02.800 --> 0:18:06.080 it's distributed determines the shape and the structure of the 0:18:06.080 --> 0:18:08.920 whole universe. And so we can tell from like pictures 0:18:08.920 --> 0:18:11.480 of the Big Bang until the temperature of dark matter 0:18:11.840 --> 0:18:13.720 at the beginning of time or right now. Well, we 0:18:13.720 --> 0:18:16.200 can tell the temperature of dark matter over the history 0:18:16.240 --> 0:18:19.000 of the universe. Everything is cooling down, but we can 0:18:19.000 --> 0:18:22.439 tell whether dark matter was made hot or made cold. 0:18:22.640 --> 0:18:25.200 Everything is getting colder over time, but we can tell 0:18:25.240 --> 0:18:28.879 whether dark matter started out hotter or colder. And we 0:18:28.920 --> 0:18:31.160 can do that by seeing whether or not it's moved 0:18:31.160 --> 0:18:34.440 around a lot, whether or not it's been wiggling around 0:18:34.720 --> 0:18:36.960 and that's affecting the structure universe, or whether it's been 0:18:36.960 --> 0:18:39.320 mostly staying in the places it was made. I see 0:18:39.359 --> 0:18:41.000 because I guess you assume that it's kind of like 0:18:41.000 --> 0:18:43.879 a gas, right Like you don't assume it's a solid. 0:18:44.119 --> 0:18:48.359 You assume that it's you know, kind of moving around freely. 0:18:48.520 --> 0:18:51.359 It's not tied together to itself except with gravity. That's right, 0:18:51.440 --> 0:18:53.480 only held together with gravity. And so we think of 0:18:53.520 --> 0:18:56.200 it like a diffuse gas, like a pressureless gas that 0:18:56.480 --> 0:18:59.480 doesn't even bounce against itself, and so basically it just 0:18:59.520 --> 0:19:01.959 has gravity tational effects. And so we can sort of 0:19:02.000 --> 0:19:04.719 walk through the history of the universe with a cold 0:19:04.840 --> 0:19:06.879 version of dark matter, a version where dark matter is 0:19:06.920 --> 0:19:09.280 mostly staying where it was, and then we can walk 0:19:09.320 --> 0:19:11.200 through a version of the universe where dark matter is hot, 0:19:11.200 --> 0:19:13.800 where it's zipping around really fast, and we see that 0:19:13.880 --> 0:19:17.040 those two things predict different shapes of the universe that 0:19:17.080 --> 0:19:19.880 we see today and also different histories of the universe, 0:19:20.160 --> 0:19:22.159 and then we can compare those histories to what we 0:19:22.240 --> 0:19:24.879 actually see because like, if the dark matter at the 0:19:24.880 --> 0:19:30.520 beginning of time was super cold, then I guess it 0:19:31.160 --> 0:19:34.240 particles themselves don't have enough speed to like go off 0:19:34.400 --> 0:19:37.119 and spread out. They would sort of stay clumped together. 0:19:37.400 --> 0:19:40.080 That's exactly right. So if dark matter is very cold, 0:19:40.560 --> 0:19:43.119 then the structure of the universe forms sort of bottom up. 0:19:43.400 --> 0:19:45.800 Everything is where it was and it's not zipping around 0:19:45.920 --> 0:19:48.000 very much, and so you get these little clumps of 0:19:48.040 --> 0:19:50.919 density from those initial wrinkles, and that's what seeds like 0:19:51.080 --> 0:19:54.639 the formation of stars, and then stars get together and 0:19:54.680 --> 0:19:58.560 they form galaxies, and galaxies pull themselves together to form 0:19:58.600 --> 0:20:01.679 galaxy clusters. You get this structure formation that's sort of 0:20:01.840 --> 0:20:05.560 bottom up. Everything starts clumping where it was and then 0:20:05.800 --> 0:20:10.399 pulls together, so you get, for example, galaxies forming before 0:20:10.440 --> 0:20:14.720 galaxy clusters. You get stars forming, then galaxies, then galaxy 0:20:14.760 --> 0:20:17.680 clusters in that order. And we can look back through 0:20:17.680 --> 0:20:19.960 the history of time because remember as we look out 0:20:20.000 --> 0:20:22.720 through space, we're looking backwards in times, so we can 0:20:22.760 --> 0:20:26.199 see where their galaxies a billion years after the universe started, 0:20:26.400 --> 0:20:29.320 where their stars. Which order did things get made? We 0:20:29.320 --> 0:20:31.480 can tell by looking deep into the history of the 0:20:31.560 --> 0:20:34.480