WEBVTT - Do we know where dark matter is? 0:00:08.600 --> 0:00:10.719 Hey, or hey, I think I may have lost something. 0:00:11.160 --> 0:00:14.159 Oh did you lose your mind again? I happened a 0:00:14.280 --> 0:00:17.480 long time ago. This is something new? Is it important? 0:00:17.920 --> 0:00:21.239 It's actually kind of a big deal. Is it like 0:00:21.360 --> 0:00:23.759 really small and easy to lose, like your keys or 0:00:23.880 --> 0:00:26.920 your wedding band. No, it's much more embarrassing because it's 0:00:27.080 --> 0:00:31.880 really enormous, like cosmically large, like most of the stuff 0:00:31.920 --> 0:00:34.919 in the universe, and you can't find it. I've been 0:00:34.960 --> 0:00:37.839 looking everywhere for it. Well, I guess you know what 0:00:37.880 --> 0:00:39.920 to do. Oh yeah, what's that? You know? Give it 0:00:39.920 --> 0:00:42.479 a cool sounding name, and then ask the federal government 0:00:42.479 --> 0:00:44.800 to help you find it. Do you think anybody would 0:00:44.840 --> 0:01:03.360 actually fall for that? I think they already have. Hi 0:01:03.440 --> 0:01:06.560 am jorhammy cartoonists and the creator of PhD comics. Hi. 0:01:06.680 --> 0:01:09.200 I'm Daniel. I'm a particle physicist and a professor you 0:01:09.280 --> 0:01:11.960 see Irvine, and I am actually paid to hunt for 0:01:12.080 --> 0:01:15.559 missing things. M interesting, You're like the Lost and Found 0:01:15.600 --> 0:01:19.160 of the universe department. That sounds very unglamorous. I like 0:01:19.240 --> 0:01:21.600 to think of myself more of the Sherlock Holmes of 0:01:21.640 --> 0:01:25.840 the universe. I see you're not in like in the 0:01:25.880 --> 0:01:29.160 basement office with a little window that people go there 0:01:29.160 --> 0:01:31.479 and claim lost things. That's right. I'm not sitting here 0:01:31.480 --> 0:01:35.479 surrounded by people's lost purses or socks. Where do all 0:01:35.520 --> 0:01:37.520 the socks and the dryers go? That's what I want 0:01:37.520 --> 0:01:39.399 to know. We should have a whole episode about that. 0:01:39.520 --> 0:01:41.720 They're orbiting the Earth in the hose zone layer. Well, 0:01:41.760 --> 0:01:45.760 that socks made. They're all connected by wormholes. You think dryers, Well, 0:01:45.760 --> 0:01:48.320 that's spinning and that he could that create a warmhole. 0:01:48.520 --> 0:01:50.960 Keep running your dryer let us know. But welcome to 0:01:50.960 --> 0:01:53.920 our podcast Daniel and Jorge Explain the Universe, a production 0:01:53.960 --> 0:01:56.240 of I Heart Radio, which we operated as a sort 0:01:56.280 --> 0:01:59.840 of lost and found of ideas about the universe. Everything 0:01:59.840 --> 0:02:02.280 we have found about the universe and all the ideas 0:02:02.320 --> 0:02:05.320 we have lost. Everything we do know about what's going 0:02:05.400 --> 0:02:08.120 on out there, the size of the universe, the number 0:02:08.120 --> 0:02:11.440 of dimensions of space, whether or not they're alien creators 0:02:11.440 --> 0:02:15.040 on other planets looking through their telescopes at us. Every 0:02:15.160 --> 0:02:18.120 deep and enormous question about the universe you might consider 0:02:18.240 --> 0:02:20.960 we talk about here on the podcast. Yeah, because it 0:02:21.080 --> 0:02:23.639 is a huge universe full of amazing and wonderful things 0:02:23.680 --> 0:02:25.680 that seem to be screaming at us for us to 0:02:25.880 --> 0:02:28.560 find them. We get light from distant stars all the 0:02:28.560 --> 0:02:32.200 time at all times, radiation, cosmic rays, and it's all 0:02:32.280 --> 0:02:34.720 coming to us sort of one thing for us to 0:02:34.800 --> 0:02:38.280 find out and what's out there and to discover and 0:02:38.440 --> 0:02:41.160 learn about how it all works. You make the universe 0:02:41.200 --> 0:02:43.480 sounds sort of cooperative, like it wants to play a 0:02:43.600 --> 0:02:46.959 role in our science and be helpful. Universe, right, I mean, 0:02:47.120 --> 0:02:49.400 it's constantly shouting at us, isn't it. It is? But 0:02:49.520 --> 0:02:53.120 sometimes these clues are very, very subtle and frustratingly difficult 0:02:53.120 --> 0:02:55.560 to grasp. I feel like the universe is sort of 0:02:55.560 --> 0:02:58.720 playing cat and mouse with us, sometimes hiding all the 0:02:58.760 --> 0:03:02.240 best bits. It's like coy universe, mean, like it wants 0:03:02.280 --> 0:03:04.800 us to find it, but it's not telling us everything 0:03:04.840 --> 0:03:07.040 that it can about itself. Yeah, it doesn't just come 0:03:07.120 --> 0:03:10.000 right out and tell us it's deep nature. It sometimes 0:03:10.040 --> 0:03:11.720 seems to make sense, and then you dig deeper and 0:03:11.760 --> 0:03:15.080 discover oh my gosh, it's bonkers underneath. But there is 0:03:15.120 --> 0:03:18.800 a lot of fascinating information in those little hints that 0:03:18.880 --> 0:03:21.960 do arrive here on Earth. Yeah, and it's pretty amazing that, 0:03:22.000 --> 0:03:23.440 you know, if you think about it, that we're sitting 0:03:23.440 --> 0:03:25.760 on this little rock floating through space in a corner 0:03:25.800 --> 0:03:27.920 of the galaxy, in a little corner of the universe. 0:03:27.919 --> 0:03:31.359 And somehow, from this light that we're getting from distant stars, 0:03:31.400 --> 0:03:34.840 we can somehow piece together the whole structure almost of 0:03:34.920 --> 0:03:37.520 the universe. I like how you just say somehow, you know, 0:03:37.560 --> 0:03:40.000 It's like the information comes and dot dot dot, we 0:03:40.080 --> 0:03:43.200 know these things. You just like, somehow my entire career 0:03:43.280 --> 0:03:45.520 right there. Yeah, somehow it goes to the lost and 0:03:45.520 --> 0:03:48.880 found department in the basement and we get a little 0:03:48.880 --> 0:03:53.160 memo about what happened. That's right. Somehow, because the government 0:03:53.200 --> 0:03:56.640 decides to fund basic science research, we have uncovered some 0:03:56.760 --> 0:03:59.560 truths about the universe. I guess what I mean by 0:03:59.600 --> 0:04:02.400 somehow thought dot dot is you know, mostly engineering and 0:04:02.400 --> 0:04:07.400 then some science, some happy collaboration between scientists and engineers. 0:04:07.760 --> 0:04:10.000 But yeah, we are learning more and more about the universe. 0:04:10.120 --> 0:04:13.520 And there's a big question, especially a big question about 0:04:13.520 --> 0:04:16.120 the universe, about what it's made out of and how 0:04:16.160 --> 0:04:19.360 it's what structured it has out there in the galaxies 0:04:19.400 --> 0:04:22.280 and also in between galaxies. What does it all look like, 0:04:22.520 --> 0:04:24.400 how's it all put together? It's really one of the 0:04:24.440 --> 0:04:26.640 biggest questions in the universe, and one of the first 0:04:26.720 --> 0:04:30.840 questions you might ask, which is what is in the universe? 0:04:31.040 --> 0:04:33.839 What's the universe made out of? Where is all the 0:04:34.000 --> 0:04:37.479 stuff out there? And what kind of stuff is it? Really, 0:04:37.520 --> 0:04:40.760 it's sort of shocking and amazing that we don't know 0:04:40.839 --> 0:04:44.320 the answer to that very basic question about the nature 0:04:44.320 --> 0:04:47.120 of our own reality. Yeah, because the universe sort of 0:04:47.120 --> 0:04:48.839 pulled a fast one on us, right Like it did 0:04:48.880 --> 0:04:52.039 this big reveal twist somewhere a few decades ago where 0:04:52.040 --> 0:04:54.080 we thought we knew what the universe was made out of. 0:04:54.120 --> 0:04:57.640 It was stars and matter and atoms and things like that, 0:04:57.720 --> 0:05:00.839 But suddenly we learned that that was only like five 0:05:00.839 --> 0:05:02.800 percent of it. You know. It's kind of like when 0:05:02.839 --> 0:05:05.200 you're watching a show and Sony you realize that you 0:05:05.240 --> 0:05:08.680 still have like ninety five episodes ago. It's sort of 0:05:08.680 --> 0:05:11.240 like the universe was wearing a mask and then somebody 0:05:11.279 --> 0:05:13.400 pulled it off and we discovered, oh my gosh, there's 0:05:13.440 --> 0:05:16.000 a lot more to it. So much for the universe 0:05:16.040 --> 0:05:18.880 being helpful and revealing, right, well, I think it just 0:05:18.920 --> 0:05:21.400 wants you to keep watching, I guess, trying to parse 0:05:21.480 --> 0:05:24.359 out the information, trying to keep it interesting. Maybe the 0:05:24.400 --> 0:05:26.719 show runners of the Universe are just trying to dole 0:05:26.760 --> 0:05:30.680 out the reveals a little bit at a time. Show runners, Huh, 0:05:30.800 --> 0:05:34.680 so you're a multitheist. I think it's got to be 0:05:34.720 --> 0:05:37.279 a committee. I mean, who, there's so much to do. 0:05:38.560 --> 0:05:41.520 That's not a flattering I guess description of it looks 0:05:41.520 --> 0:05:43.880 like it was made by a committee. The universe looks 0:05:43.920 --> 0:05:46.040 like it had too many writers. Well, it does sometimes 0:05:46.080 --> 0:05:48.839 seem inconsistent, you know. Well, it is sort of a 0:05:48.960 --> 0:05:51.200 mysterious question. What is the universe made out of? And 0:05:51.240 --> 0:05:54.040 as we've learned in the last few decades, it's not 0:05:54.120 --> 0:05:57.560 made out of just stars and atoms and elements. It's 0:05:57.600 --> 0:06:00.839 mostly made out of other things, namely dark matter and 0:06:00.960 --> 0:06:03.359 dark energy. The kind of stuff that makes up you 0:06:03.640 --> 0:06:06.960 and me and hamsters and ice cream and bologna sandwiches 0:06:07.160 --> 0:06:11.359 is actually quite unusual in the universe. Most of the 0:06:11.400 --> 0:06:14.520 stuff that's out there is not stars and gas and 0:06:14.640 --> 0:06:18.400 dust and all the visible stuff. It's something else, something dark, 0:06:18.520 --> 0:06:23.159 something we've only recently discovered exists out there. Yeah, and 0:06:23.200 --> 0:06:25.760 so dark energy is this sort of the phenomenon where 0:06:25.800 --> 0:06:29.160 the universe's expanding faster and faster every second, but dark 0:06:29.200 --> 0:06:32.799 matter is especially sort of weird and concerning because it's matter, 0:06:32.920 --> 0:06:35.720 it's stuff. It's like exerting gravity. You can feel it, 0:06:35.800 --> 0:06:38.120 but we can't see it, which means we don't know 0:06:38.160 --> 0:06:40.960 where it is exactly. We know that dark matter is 0:06:41.040 --> 0:06:45.640 some kind of stuff. There's something out there exerting gravity 0:06:45.680 --> 0:06:47.719 on the rest of the universe, and we can sort 0:06:47.760 --> 0:06:50.760 of tell that it exists, but we don't know what 0:06:50.800 --> 0:06:53.200 it is. And we've done a lot of podcasts digging 0:06:53.200 --> 0:06:55.719 into what it might be. Is it axons? Is it 0:06:55.800 --> 0:06:59.200 black holes? Is it whimps? Is this something else? Something 0:06:59.240 --> 0:07:02.760 weird in there? Even more crazy ideas we haven't yet covered. 0:07:02.920 --> 0:07:04.479 It's sort of like we know there's an elephant in 0:07:04.480 --> 0:07:06.960 the room or in the in the galaxy or the universe, 0:07:07.040 --> 0:07:09.320 but we don't know what it's. What this elephant is doing, 0:07:09.760 --> 0:07:12.400 or you know, what is it like striking a post? 0:07:12.480 --> 0:07:15.400 Is it jogging? Is it sleeping. It's kind of a 0:07:15.400 --> 0:07:17.920 big mystery, like we know it's there, but we sort 0:07:17.920 --> 0:07:21.440 of don't know what it's doing or what structure it has. Yeah, 0:07:21.480 --> 0:07:24.120 you can ask so many fascinating questions. So far, we've 0:07:24.160 --> 0:07:27.240 mostly focused on what is dark matter? Is it this 0:07:27.320 --> 0:07:29.520 kind of particle. Is it that kind of particle but 0:07:29.680 --> 0:07:32.560 equally interesting without even knowing what it's made out of, 0:07:32.760 --> 0:07:36.160 is just wondering, like where is the dark matter? Is 0:07:36.200 --> 0:07:38.440 it here with us in this room? Is it out 0:07:38.440 --> 0:07:41.320 there in deep space? Does it form planets and other 0:07:41.360 --> 0:07:44.720 kinds of structure? Is it smoothly spread out throughout the universe? 0:07:44.960 --> 0:07:48.040 Where in the universe is all of this stuff? So 0:07:48.080 --> 0:07:55.360 today on the podcast, we'll be asking the question, how 0:07:55.400 --> 0:07:59.360 do we know where dark matter is? It sounds like 0:07:59.400 --> 0:08:01.960 he lost you lost it again? Then I feel like 0:08:02.000 --> 0:08:03.520 we said a new word. It was, but now we 0:08:03.560 --> 0:08:05.760 don't know where it is. It is a really important 0:08:05.840 --> 0:08:09.040 question because knowing where the dark matter is can help 0:08:09.120 --> 0:08:12.480 us get clues about what it might be, because certain 0:08:12.600 --> 0:08:15.360 kinds of dark matter might clump up in different ways, 0:08:15.400 --> 0:08:18.280 and other kinds of dark matter might not. Yeah, is 0:08:18.320 --> 0:08:20.440 it is it chunky or smooth? I guess it's a 0:08:20.480 --> 0:08:24.560 big question about the peanut butter dark matter of the universe. Well, 0:08:24.560 --> 0:08:26.840 I hope the showrunners don't disagree about that, because then 0:08:26.880 --> 0:08:29.320 you might get chunky parts of the universe and creamy 0:08:29.360 --> 0:08:32.839 parts of the universe. You're not a creamy peanut butter fan. 0:08:33.000 --> 0:08:36.280 It's all about the chunks, man, It's all about the chunks. Well, 0:08:36.360 --> 0:08:38.439 to each their own may there's a different flavor of 0:08:38.520 --> 0:08:40.680 dark matter for every tape. New Tella there you go, 0:08:40.760 --> 0:08:45.600 New Tella is the best flavor of dark rutter and 0:08:45.640 --> 0:08:48.720 everyone gets a heart attack from being surrounded by all 0:08:48.800 --> 0:08:51.440 this saturated fat. But yeah, it's a big question. Where 0:08:51.600 --> 0:08:54.439 is dark matter and sort of like what structure it 0:08:54.480 --> 0:08:57.240 has in the universe. Is it sort of smoothed out 0:08:57.240 --> 0:09:00.560 there like a big cloud or is it is it chunky? 0:09:00.640 --> 0:09:03.440 Is it in like strands? Is it in clumps? What's 0:09:03.480 --> 0:09:05.760 it doing out there? And this is something that scientists 0:09:05.760 --> 0:09:08.720 are eager to figure out because they just want to 0:09:08.720 --> 0:09:10.960 know where all of this stuff is. They're trying to 0:09:11.000 --> 0:09:15.760 develop a picture of the universe both visible and invisible, 0:09:15.880 --> 0:09:18.559 and of course the invisible stuff much harder to see, 0:09:18.600 --> 0:09:20.760 but since there's much more of it than there is 0:09:20.800 --> 0:09:24.079 the visible stuff, it's a very important question. So, as usual, 0:09:24.120 --> 0:09:26.040 we were wondering how many people out there had thought 0:09:26.080 --> 0:09:29.640 about this question of the location and structure of dark matter. 0:09:30.000 --> 0:09:32.800 So Daniel went out there to ask people on the internet, 0:09:32.960 --> 0:09:35.000 how do we know where dark matter is? So if 0:09:35.040 --> 0:09:37.000 you'd like to put your brain in the test and 0:09:37.080 --> 0:09:41.040 answer questions that leading physicists don't know the answer to, 0:09:41.440 --> 0:09:44.720 please write to me two questions at Daniel and Jorge 0:09:45.000 --> 0:09:48.000 dot com. Here's what people had to say. Well, doc 0:09:48.040 --> 0:09:53.280 matter reacts with gravity, so by looking out into the universe, Um, 0:09:53.320 --> 0:09:55.840 we can sort of detect where the doc matter is 0:09:55.880 --> 0:09:58.760 because of its effect on gravity. I know that it's 0:09:58.800 --> 0:10:02.800 by gravity. Is the gravitational lensing that makes it possible 0:10:02.840 --> 0:10:05.680 to have like some kind of ideas where it could 0:10:05.679 --> 0:10:09.920 be dark matter in the universe. The distortion of light 0:10:10.000 --> 0:10:13.320 suggests that there are more matter out there that we 0:10:13.360 --> 0:10:17.319 can actually say whenever it's like bands. Well, once again, 0:10:17.360 --> 0:10:21.560 I assume this has to do with the pot with 0:10:21.679 --> 0:10:24.640 the examples and instance, as you were talking of these 0:10:24.720 --> 0:10:28.679 massive tubs of argone if I'm not mistaken, that are 0:10:28.760 --> 0:10:32.320 deep underground. We cannot see what dark matter itself is. 0:10:32.640 --> 0:10:35.960 Light travels straight through it, but we can see other 0:10:36.000 --> 0:10:40.080 planets and we can see light being distorted by the 0:10:40.160 --> 0:10:43.480 gravitational effects of what dark matter is itself. I'd say 0:10:43.520 --> 0:10:45.439 we know where the dark matter is because we can 0:10:45.480 --> 0:10:49.199 see the gravitational force it exerts on the visible matter 0:10:49.240 --> 0:10:54.280 around it. Next to that, it also bends light from 0:10:54.320 --> 0:10:59.120 distant galaxies when it comes towards us. So I'd say 0:10:59.400 --> 0:11:03.320 gravity is the usual suspect for us knowing where dark 0:11:03.360 --> 0:11:06.680 matter is. Who is d M? I think I've heard 0:11:06.720 --> 0:11:10.559 that dark matter is everywhere, that it just kind of 0:11:10.600 --> 0:11:15.599 permeates the universe, so all over the place. So my 0:11:15.640 --> 0:11:18.160 guess is for what we know dark matter is, or 0:11:18.200 --> 0:11:21.400 how we know it is, if it gives off gravitational 0:11:21.400 --> 0:11:26.280 waves because it interacts with stuff through gravity, then we 0:11:26.320 --> 0:11:30.160 would use something that detects gravitational waves to see either 0:11:30.200 --> 0:11:34.200 how far away it is or where it is in 0:11:34.280 --> 0:11:37.000 respect to something else. I think we know where dark 0:11:37.080 --> 0:11:41.840 matter is by looking for localized gravitational effects like lensing 0:11:41.960 --> 0:11:46.480 or relative velocities that aren't completely explained by matter we 0:11:46.520 --> 0:11:50.920 can observe in other ways. Right, A pretty interesting answer 0:11:50.920 --> 0:11:54.640 is a lot of people went with gravity, which is true, right, Like, 0:11:54.640 --> 0:11:58.600 like that's how we initially discovered dark matter is through gravity. Yeah, 0:11:58.640 --> 0:12:01.880 that's basically the only way we can sense dark matter, 0:12:02.280 --> 0:12:05.080 and so gravity is basically the answered. Gravity is the 0:12:05.120 --> 0:12:07.880 reason we know dark matter exists, and dark matter is 0:12:07.920 --> 0:12:13.360 basically an explanation for otherwise unexplainable gravity. So yeah, gravity 0:12:13.440 --> 0:12:16.600 is basically our portal into the dark universe. And I 0:12:16.640 --> 0:12:20.760 like this person who said who is dark matter? It's 0:12:20.800 --> 0:12:23.920 like who it this new phone? I think that's because 0:12:24.000 --> 0:12:26.520 in the question I wrote d M instead of dark matter, 0:12:26.760 --> 0:12:31.040 assuming that everybody would know what DM meant, right, who 0:12:31.120 --> 0:12:34.559 doesn't know what DM is? I was sliding into some 0:12:34.640 --> 0:12:36.160 of these d M s with that one. I guess 0:12:37.120 --> 0:12:40.959 you're being a physicism using acronyms on people who had 0:12:40.960 --> 0:12:43.720 no idea what those acronyms are. But yeah, a lot 0:12:43.760 --> 0:12:46.080 of people seem to set and have a basic idea 0:12:46.120 --> 0:12:48.440 that it's through gravity. But the picture is a little 0:12:48.480 --> 0:12:50.880 bit more complicated than that, right, I mean, we sort 0:12:50.880 --> 0:12:54.120 of know that it's there because of gravity, but sort 0:12:54.160 --> 0:12:56.719 of finding out exactly where it is or whether it 0:12:56.840 --> 0:13:00.440 clumps or strands or smooth, that's much harder because we 0:13:00.480 --> 0:13:02.840 can't see it right exactly. We can't see it in 0:13:02.880 --> 0:13:05.000 the way that we can see the other kinds of matter. 0:13:05.280 --> 0:13:08.080 It doesn't give off light, it doesn't reflect light. Remember 0:13:08.080 --> 0:13:10.240 that dark matter is a bit of a confusing name 0:13:10.600 --> 0:13:14.920 because dark matter is not actually dark. It's transparent, it's invisible. 0:13:15.040 --> 0:13:17.360 It's not like a cloud of dark matter between you 0:13:17.440 --> 0:13:20.680 and another star would block your view of that star. 0:13:21.160 --> 0:13:23.080 If that were tue would be much easier to see 0:13:23.160 --> 0:13:26.440 dark batter than it is today. Instead, light passes right 0:13:26.480 --> 0:13:29.040 through dark matter. So give us, maybe start us off 0:13:29.040 --> 0:13:32.040 with a refresher of dark matter or d M as 0:13:32.080 --> 0:13:34.080 the physicist call it. You know, what do we know 0:13:34.120 --> 0:13:36.520 about it? Well, we know that dark matter is about 0:13:36.600 --> 0:13:40.280 twenty five of the energy budget of the universe. That 0:13:40.360 --> 0:13:42.839 means if you take like a cubic light year of space, 0:13:42.920 --> 0:13:46.560 or any volume of space, then of the energy in 0:13:46.640 --> 0:13:50.000 that space is devoted to the mass of dark matter, 0:13:50.240 --> 0:13:53.840 whereas five percent of the energy budget of any chunk 0:13:53.880 --> 0:13:57.280 of space on average, you know, averaging over big distances, 0:13:57.600 --> 0:14:00.400 is devoted to making things like stars and gala seas 0:14:00.440 --> 0:14:02.600 and dust and giraffes and all of that kind of 0:14:02.800 --> 0:14:05.400 normal matter made out of atoms. And that means that 0:14:05.440 --> 0:14:08.480 there's a huge amount of dark matter. That a galaxy, 0:14:08.520 --> 0:14:11.959 for example, is mostly dark matter. That the universe, the 0:14:12.080 --> 0:14:15.040 stuff in the universe, the matter, you know, the physical 0:14:15.080 --> 0:14:18.360 form of the universe is mostly dark matter. So we've 0:14:18.400 --> 0:14:20.960 been studying the universe for thousands of years looking up 0:14:20.960 --> 0:14:24.080 at the sky wondering how things work, and only recently 0:14:24.120 --> 0:14:26.880 have we discovered that we've been missing most of it. 0:14:27.080 --> 0:14:29.800 So that's pretty exciting. And we know that dark matter 0:14:29.960 --> 0:14:32.640 is not made out of atoms, not made out of 0:14:32.640 --> 0:14:34.600 the kind of stuff that you and I are made 0:14:34.600 --> 0:14:36.960 out of. If it were, then it would probably interact 0:14:37.040 --> 0:14:39.600 with light. And we can also do some careful accounting 0:14:39.600 --> 0:14:41.920 from the very beginning of the universe, where we know 0:14:42.040 --> 0:14:44.840 something about how much material there was to make atoms, 0:14:45.040 --> 0:14:47.080 we can kind of account for where all of that went. 0:14:47.280 --> 0:14:49.920 So we're pretty sure. We're almost certain that dark matter 0:14:49.960 --> 0:14:52.600 is some kind of matter that doesn't give off light 0:14:52.680 --> 0:14:55.480 or reflect light. It must be made out of something else. 0:14:55.720 --> 0:14:57.800 And we know that it doesn't move very fast. We 0:14:57.880 --> 0:15:00.480 call it cold dark matter, the as if it did, 0:15:00.520 --> 0:15:03.600 it would spread out much more throughout the universe. Yeah, 0:15:03.600 --> 0:15:05.720 and it's kind of interesting because I think I almost 0:15:05.720 --> 0:15:08.320 feel like like, in a way, physicists called this thing 0:15:08.560 --> 0:15:10.760 or named it a little bit too early, you know 0:15:10.840 --> 0:15:12.360 what I mean, Like we gave it a name like 0:15:12.400 --> 0:15:14.400 a dark matter, maybe a little too early, Like maybe 0:15:14.400 --> 0:15:16.280 you should have kept going and say that you know 0:15:17.120 --> 0:15:20.040 of the universe is just something that we don't understand 0:15:20.200 --> 0:15:22.560 or something that is not like the rest of the 0:15:22.640 --> 0:15:25.000 stuff in the universe. Well, that doesn't work as well 0:15:25.000 --> 0:15:28.800 in grand proposals as a nice zingy phrase like dark matter. 0:15:31.280 --> 0:15:33.880 But yeah, but I know what, I guess the point 0:15:33.920 --> 0:15:35.640 is it really we don't know that much about it. 0:15:35.680 --> 0:15:37.680 I mean, we sort of know it's presence, or at 0:15:37.760 --> 0:15:39.480 least we know it's the effect on the rest of 0:15:39.480 --> 0:15:41.720 the universe, but we don't even know if it's matter. Right, Well, 0:15:41.760 --> 0:15:44.720 we know that it generates gravity, which suggests that it's 0:15:44.800 --> 0:15:49.320 curves space according to general relativity. And so either our 0:15:49.440 --> 0:15:53.320 understanding of how space curves is wrong, or it's some 0:15:53.440 --> 0:15:56.520 new kind of energy and matter that does curve space. 0:15:57.000 --> 0:16:00.600 And it's possible that we don't understand gravity. It's certain 0:16:00.800 --> 0:16:03.840 that we don't have complete understanding of gravity. There are 0:16:03.960 --> 0:16:08.480 alternative ideas to explain dark matter using variations on gravitational theory, 0:16:08.600 --> 0:16:11.520 but none of them can really explain everything that we see. 0:16:12.000 --> 0:16:15.160 And so you're right that we're not certain that it's matter. 0:16:15.360 --> 0:16:18.200 But it's the simplest explanation that fits all of the data. 0:16:18.360 --> 0:16:21.800 A new kind of particle that only interacts gravitationally, explains 0:16:21.880 --> 0:16:24.840 basically everything that we see out there in the universe, 0:16:25.120 --> 0:16:28.280 from the ripples in the earliest light to the structure 0:16:28.280 --> 0:16:31.480 of the universe today to the rotations of galaxies. So 0:16:31.520 --> 0:16:34.800 we're not certain, but it's the best candidate. Maybe should 0:16:34.840 --> 0:16:38.760 have called it dark probably matter or dark most likely 0:16:38.760 --> 0:16:42.760 matter d M M L D d M l M. 0:16:43.160 --> 0:16:45.160 But we, as you said, so far, we only know 0:16:45.200 --> 0:16:48.080 about it because of its gravitational effects, right, But we 0:16:48.120 --> 0:16:50.640 sort of know quite a few things about sort of 0:16:50.840 --> 0:16:53.160 generally where it is. We do have a good idea 0:16:53.320 --> 0:16:57.280 of where it might be because of its gravitational effects. Right, 0:16:57.360 --> 0:17:00.440 it is invisible. It doesn't give off light or interact 0:17:00.480 --> 0:17:03.280 with any other kind of force. But gravity is local. Right, 0:17:03.320 --> 0:17:05.920 if you are close to something, it tugs on you 0:17:06.119 --> 0:17:08.720 more strongly than if you're far from something. So if 0:17:08.760 --> 0:17:10.840 you're measuring the gravity of an object, if you can 0:17:10.920 --> 0:17:13.640 only tell if something is there because of its gravity, 0:17:13.920 --> 0:17:16.240 you can get an idea for where it is based 0:17:16.240 --> 0:17:18.880 on what it tugs on. For example, we can see 0:17:18.880 --> 0:17:21.080 the black holes are there because of the way the 0:17:21.119 --> 0:17:25.600 stars move around them without actually directly seeing black holes, 0:17:25.800 --> 0:17:28.320 and so gravity definitely can give you a picture as 0:17:28.320 --> 0:17:30.840 to where things are in the universe, and we have 0:17:30.880 --> 0:17:33.480 a rough idea for where dark matter is. We think 0:17:33.520 --> 0:17:37.280 that dark matter is mostly lined up with the normal matter. 0:17:37.440 --> 0:17:40.000 That where you see a galaxy is where there's a 0:17:40.080 --> 0:17:43.520 huge clump of dark matter. So every galaxy, we think, 0:17:43.560 --> 0:17:46.600 for example, is embedded in a huge cloud. We call 0:17:46.640 --> 0:17:50.400 it a dark matter halo for every galaxy. Yeah, it's 0:17:50.400 --> 0:17:53.880 like where you see regular stars and planets, you see 0:17:54.000 --> 0:17:56.840 dark matter. Or it's almost like the opposite, right, it's 0:17:56.880 --> 0:17:59.119 like where you see dark matter is where all the 0:17:59.160 --> 0:18:01.880 stars and planets formed. In a way, yes, stars are 0:18:01.880 --> 0:18:04.479 more like the tracers for the rest of stuff, right, 0:18:04.560 --> 0:18:07.200 dark matter leads the way. There's more dark matter than 0:18:07.240 --> 0:18:10.160 everything else. And so it's actually like where the dark 0:18:10.200 --> 0:18:13.920 matter started clumping is where the normal matter fell into 0:18:14.000 --> 0:18:17.320 it because of its gravity and then formed galaxies and 0:18:17.440 --> 0:18:19.840 stars and all kinds of stuff that we can see. 0:18:20.040 --> 0:18:22.320 So you know how when the military is fighting at night, 0:18:22.359 --> 0:18:24.280 they shoot bullets and then occasionally, like one out of 0:18:24.320 --> 0:18:27.320 every thousand bullets is a tracer. It's like glows, so 0:18:27.359 --> 0:18:29.840 they can see where they're shooting. Stars are sort of 0:18:29.880 --> 0:18:32.000 like that. They follow the dark matter, and they give 0:18:32.040 --> 0:18:34.960 us a clue as to where that dark matter is. 0:18:35.520 --> 0:18:38.640 And that's why we think that most galaxies are embedded 0:18:38.640 --> 0:18:42.639 in this cloud, this halo of sort of spherical, sort 0:18:42.680 --> 0:18:46.040 of a little bit elliptical dark matter that goes well 0:18:46.119 --> 0:18:49.080 beyond actually where the stars are. Yeah, I've heard this 0:18:49.240 --> 0:18:52.359 sort of analogy that regular matter like planets and stars. 0:18:52.400 --> 0:18:54.760 It's sort of like the sprinkling on the icing of 0:18:54.800 --> 0:18:57.720 a cupcake. Like, not just like in terms of our 0:18:57.800 --> 0:19:00.840 relative importance to that and this the eye of the universe, 0:19:00.880 --> 0:19:02.760 but also kind of like you know, sprinkles stick to 0:19:02.800 --> 0:19:05.120 the icing in a cupcake. You know, you can't sort 0:19:05.160 --> 0:19:08.159 of have sprinkles anywhere else. They're like, you know, the 0:19:08.200 --> 0:19:12.640 sprinkles sort of tell you where the icing is. Yeah, 0:19:12.640 --> 0:19:15.240 the sprinkles are sort of the stars, and the icing 0:19:15.280 --> 0:19:17.600 is sort of dark matter, and the cupcake itself is 0:19:17.680 --> 0:19:20.359 dark energy. That gives you sort of a sense of 0:19:20.400 --> 0:19:24.080 the relative fractions of the energy budget of the universe. Yeah, 0:19:24.240 --> 0:19:26.439 so I guess we are just the hangar ons of 0:19:26.480 --> 0:19:29.879 the universe. We're just hanging onto dark matter. And so 0:19:30.000 --> 0:19:32.239 let's get into a little bit more detail about what 0:19:32.320 --> 0:19:35.600 we know about this halo around galaxies and also how 0:19:35.680 --> 0:19:38.399 we know where it is, but first let's take a 0:19:38.480 --> 0:19:53.760 quick break. All right, we're talking about dark matter and 0:19:54.119 --> 0:19:57.359 where exactly it is, because I guess we know it's there, 0:19:57.359 --> 0:20:00.520 but physicists can't find it. It's pretty tricky to nail 0:20:00.560 --> 0:20:04.159 down an individual piece of dark matter, for example, because 0:20:04.160 --> 0:20:07.719 it only interacts gravitationally, imagine trying to find like an 0:20:07.760 --> 0:20:10.840 invisible piece of sand in your room. How would you 0:20:10.880 --> 0:20:14.440 detect it? It's gravity is essentially nothing because gravity is 0:20:14.440 --> 0:20:17.359 a really really weak force. All of the other forces 0:20:17.400 --> 0:20:21.760 of electromagnetism, even the weak force, is much stronger than gravity. 0:20:22.119 --> 0:20:24.800 So in order to detect something through gravity, you need 0:20:24.840 --> 0:20:26.760 to have a huge force. You need to have like 0:20:26.920 --> 0:20:30.160 a planet sized force or a solar system sized force 0:20:30.520 --> 0:20:33.680 because gravity is so weak. So when we use gravity 0:20:33.720 --> 0:20:36.040 to look for dark matter, we can only sort of 0:20:36.080 --> 0:20:39.360 tell the large scale structure. It's very difficult to get 0:20:39.359 --> 0:20:42.480 a fine grained picture of where things are. But even 0:20:42.520 --> 0:20:45.119 though it's very weak gravitation, and we do have a 0:20:45.200 --> 0:20:47.600 kind of a pretty good idea of what shape it 0:20:47.680 --> 0:20:50.280 has in the galaxy, right like this halo, it's not 0:20:50.440 --> 0:20:53.240 just like a blob. It has some sort of shaped 0:20:53.280 --> 0:20:55.720 to it. That's right. It tends to be denser at 0:20:55.720 --> 0:20:59.040 the core and thinner further out, much like the visible 0:20:59.080 --> 0:21:01.320 matter in the galaxy. See, and we can tell where 0:21:01.320 --> 0:21:03.760 it is because it has an effect on how the 0:21:03.800 --> 0:21:07.040