WEBVTT - Could dark matter be a superfluid? 0:00:08.800 --> 0:00:12.080 Jorge, I have an idea for our Daniel and Jorge 0:00:12.119 --> 0:00:15.320 explain the universe food truck. Oh nice, wait, are reactually 0:00:15.360 --> 0:00:16.920 doing that? I thought it was a joke. Well, it 0:00:17.000 --> 0:00:19.720 was a joke, but then our listener Tim Lazarov road 0:00:19.760 --> 0:00:21.880 in to ask when our food truck is going to 0:00:21.960 --> 0:00:24.319 be in his neighborhood. May should be more like a 0:00:24.400 --> 0:00:27.120 food spaceship it would be more appropriate. Well, that goes 0:00:27.280 --> 0:00:30.920 perfectly well with my idea. You see, normal food trucks 0:00:31.080 --> 0:00:34.120 sort of drinks right fluids to wash down their taste treats. 0:00:34.880 --> 0:00:39.400 But our food spaceship should serve super fluids like zero 0:00:39.560 --> 0:00:43.280 viscosity beverages nice, so they go down easier. Or do 0:00:43.320 --> 0:00:47.280 they have Bose Einstein condensation on the outside. I'm thinking 0:00:47.320 --> 0:00:49.479 we should test it on some undergrads, you know, make 0:00:49.479 --> 0:00:51.200 sure it's safe before we sell it to the public. 0:00:51.200 --> 0:00:53.199 I'm sure the FDA is all over that. I don't 0:00:53.240 --> 0:00:55.760 think they have a physics food division yet. That's the 0:00:55.840 --> 0:01:14.680 p d A Physics and Drugs Administration. Hi, I'm or 0:01:14.720 --> 0:01:16.760 hand me a a cartoonists and the co author of Frequently 0:01:16.800 --> 0:01:20.160 Asked Questions about the Universe. Hi I'm Daniel. I'm a 0:01:20.200 --> 0:01:24.000 physics professor and a particle physicist who does research at CERN. 0:01:24.319 --> 0:01:26.560 And every time I'm in a lab, I'm always tempted 0:01:26.640 --> 0:01:31.480 to taste everything that sounds like a terrible Daniel, especially 0:01:31.560 --> 0:01:34.480 if you visit a virology lab. Yeah, but you know 0:01:34.520 --> 0:01:37.039 they got like weird glowing goo. You're like, m I 0:01:37.120 --> 0:01:40.000 wonder what flavor that is. I'm never gonna do it, 0:01:40.080 --> 0:01:44.240 of course, I hope, but you know, the curious mind wonders, right, 0:01:44.280 --> 0:01:46.720 Remind me never to invite you to my lab, otherwise 0:01:46.760 --> 0:01:50.600 you'll be licking everything like a dog or a two 0:01:50.680 --> 0:01:52.560 year old, like that kid in the movie that licks 0:01:52.560 --> 0:01:54.920 the flagpole and wonders if his tongue is really going 0:01:55.000 --> 0:01:57.440 to stick to it. We should get into the physics 0:01:57.520 --> 0:02:01.720 of a Christmas story will shoot your eyes out. But 0:02:01.800 --> 0:02:04.520 welcome to our podcast Daniel and Jorge Explain the Universe, 0:02:04.600 --> 0:02:07.080 a production of I Heart Radio in which we invite 0:02:07.120 --> 0:02:10.919 you to taste the entire universe to enjoy the flavor 0:02:10.960 --> 0:02:14.240 of knowledge as well as ignorance, to take a bite 0:02:14.240 --> 0:02:16.760 out of everything that we do and do not know 0:02:16.919 --> 0:02:20.360 about the universe. We don't shy away from the big mysteries. 0:02:20.400 --> 0:02:22.720 On this podcast. We talk about the smallest things, the 0:02:22.840 --> 0:02:26.160 medium sized things, and even the biggest things, and we 0:02:26.240 --> 0:02:29.359 attack all of them and try to explain them to you. Yeah, 0:02:29.400 --> 0:02:32.680 it is a delicious looking universe, like a giant cosmic 0:02:32.720 --> 0:02:36.400 bouffet of amazing ideas and incredible phenomena that are there 0:02:36.480 --> 0:02:40.040 for us to dig into and fill our bellies with 0:02:40.160 --> 0:02:42.919 amazing knowledge. Yeah. And if you're not a curious person, 0:02:43.000 --> 0:02:45.200 then you wouldn't take a bite to these weird things. 0:02:45.240 --> 0:02:47.440 And I think that's why when I'm in some kind 0:02:47.440 --> 0:02:49.799 of laboratory and I see something weird bubbling in a 0:02:49.840 --> 0:02:52.120 flask and I wonder, I wonder if that would make 0:02:52.120 --> 0:02:54.960 a good soda for our food truck, It's that same 0:02:55.040 --> 0:02:57.600 curiosity that inspires me to try to take a bite 0:02:57.600 --> 0:03:00.160 out of the whole universe because I want to know. 0:03:00.280 --> 0:03:02.800 I want to understand. It's not enough to just say 0:03:02.919 --> 0:03:05.880 I bet that green bubbling thing tastes something like lemon 0:03:05.919 --> 0:03:08.679 lime soda. I want to actually know the truth. I 0:03:08.720 --> 0:03:11.200 think there are easier ways to find out what something is, Daniel, 0:03:11.280 --> 0:03:14.000 rather than bringing it in your mouth and in your body, 0:03:14.120 --> 0:03:16.600 Have you tried asking what it is? It seems like 0:03:16.639 --> 0:03:18.360 the polite thing to do. But then how would they know, 0:03:18.560 --> 0:03:21.000 right if they haven't tasted it. Maybe some questions are 0:03:21.000 --> 0:03:23.480 not meant to be answered, What like, are you curious 0:03:23.480 --> 0:03:27.040 about how sanide tastes? I am curious? Actually, you know, 0:03:27.080 --> 0:03:29.560 I like almond pastries and so hey, maybe you know 0:03:29.600 --> 0:03:32.000 a nice cyanide after flavor is not the worst thing 0:03:32.040 --> 0:03:34.080 in the world. But yeah, I'm curious about all this stuff, 0:03:34.120 --> 0:03:36.400 even the stuff that might kill you to find out 0:03:36.440 --> 0:03:40.200 the answer. It's just this deep desire to know these truths. Well, 0:03:40.240 --> 0:03:44.680 speaking of dark matters like tasting poison, there are amazing 0:03:44.720 --> 0:03:47.280 mysteries out there, including one that is maybe one of 0:03:47.320 --> 0:03:49.280 the biggest mysteries in the universe. At least it's the 0:03:49.280 --> 0:03:52.360 second biggest mystery in the universe by percentage. Boys, right, 0:03:52.440 --> 0:03:54.480 that's right. If the universe was laid out as a buffet, 0:03:54.560 --> 0:03:56.920 most of it would be dark energy, but a huge, 0:03:57.200 --> 0:04:01.280 heaping pile of it would be dark arc matter. Most 0:04:01.360 --> 0:04:05.200 of the actual stuff in the universe, the matter, the things, 0:04:05.520 --> 0:04:08.520 the bits and pieces that move around in our universe, 0:04:08.720 --> 0:04:11.120 are not the things that make up me and you 0:04:11.520 --> 0:04:15.800 and weird beakers of bubbling green goo in chemistry laboratories, 0:04:16.040 --> 0:04:19.719 it's something else, something different, something we do not yet understand, 0:04:20.040 --> 0:04:21.920 but we have a cool name for it. That's right, 0:04:21.920 --> 0:04:23.719 we have a cool name for it. It's dark matter. 0:04:23.839 --> 0:04:26.320 And that is an interesting analogy, Daniel. I guess the 0:04:26.400 --> 0:04:29.240 universe is like a giant buffet, and five percent of 0:04:29.240 --> 0:04:33.360 it is like regular food, right, chicken and bread and 0:04:33.560 --> 0:04:37.200 pasta slad, And about twenty seven percent of that buffet 0:04:37.520 --> 0:04:41.920 is a big giant mystery, some kind of weird dark stuff. 0:04:41.960 --> 0:04:44.719 And I guess you would be right in there tiling 0:04:44.760 --> 0:04:46.280 it on on your plate. Yeah, I don't know if 0:04:46.279 --> 0:04:47.800 I'll go back for seconds. You know, I got to 0:04:47.839 --> 0:04:50.240 try the first serving, but yes, serve me up some 0:04:50.360 --> 0:04:52.400 dark matter. I want to know what it is. Does 0:04:52.440 --> 0:04:55.360 it in the end just taste like chicken? Would it taste? 0:04:55.360 --> 0:04:57.680 I guess it would just go through your tongue, wouldn't it. 0:04:57.839 --> 0:05:01.640 That's right. Dark matter sounds like something black and heavy, 0:05:02.080 --> 0:05:06.800 but actually it's invisible and intangible. Dark matter would pass 0:05:07.040 --> 0:05:10.120 right through you like a cloud of neutrinos, because we 0:05:10.160 --> 0:05:12.640 don't think that it interacts with normal matter in any 0:05:12.680 --> 0:05:15.920 way other than with gravity, and gravity is a very 0:05:16.040 --> 0:05:18.760 very weak force, so you couldn't even like pick up 0:05:18.800 --> 0:05:21.360 a spoon of dark matter. You had like a blob 0:05:21.400 --> 0:05:23.679 of dark matter, and you dropped it, it would fall 0:05:23.760 --> 0:05:26.159 to the center of the Earth, right, And we don't 0:05:26.200 --> 0:05:28.120 even know if it is matter. We don't even know 0:05:28.160 --> 0:05:30.400 if it is stuff. All we know about it is 0:05:30.440 --> 0:05:33.120 its effect on the rest of the universe and the 0:05:33.120 --> 0:05:35.240 rest of the stars and galaxies out there that we 0:05:35.240 --> 0:05:38.120 can see. That's right. We have never confirmed the particle 0:05:38.200 --> 0:05:40.640 nature of dark matter. We don't even really know a 0:05:40.720 --> 0:05:42.960 hundred percent that it's stuff. And we've talked about it 0:05:43.000 --> 0:05:44.840 on the podcast a lot of times and we've said 0:05:44.839 --> 0:05:47.520 that we are pretty sure it's matter, but there are 0:05:47.560 --> 0:05:49.800 some questions that remain. There are some things that we 0:05:49.880 --> 0:05:52.159 see out there in the universe that the idea that 0:05:52.240 --> 0:05:55.240 dark matter is stuff is some kind of invisible new 0:05:55.360 --> 0:05:59.320 matter in the universe doesn't quite explain. And there are 0:05:59.360 --> 0:06:02.159 a few other our ideas, different hypotheses to try to 0:06:02.200 --> 0:06:03.839 explain it. And I know that it's one of the 0:06:03.880 --> 0:06:06.560 favorite pastime of our listeners because they're always writing the 0:06:06.600 --> 0:06:09.520 emails about maybe dark matter is actually this other thing, 0:06:09.640 --> 0:06:11.560 or what if dark matter it could be something else 0:06:11.600 --> 0:06:14.560 totally different. It is one of the biggest most accessible 0:06:14.640 --> 0:06:17.960 mysteries in the universe. That's right. Maybe it is a 0:06:18.000 --> 0:06:21.560 giant tree of some kind of cosmic buffet for some 0:06:21.600 --> 0:06:25.719 giant beings. Perhaps. Yeah, maybe it's just like weird pasta 0:06:25.839 --> 0:06:28.360 instead of squidding, because they put something else into it 0:06:28.440 --> 0:06:31.400 to make it like really dark or invisible. But I 0:06:31.440 --> 0:06:33.360 guess what you mean, right, Yeah, if there's some kind 0:06:33.360 --> 0:06:36.119 of animal, let's bray something that makes it invisible, maybe 0:06:36.120 --> 0:06:38.560 they've just inserted that into the dark matter pasta. Well, 0:06:38.600 --> 0:06:40.760 hopefully it's more like the dessert of the universe, because 0:06:40.760 --> 0:06:42.720 that would be pretty neat, right. A third of the 0:06:42.800 --> 0:06:46.280 universe is just desserts. Does that tell us about your diet? 0:06:46.400 --> 0:06:50.120 Or Hey, are you like desserts? Sure? Why not? I 0:06:50.160 --> 0:06:55.920 guess it all depends on your perspective. Vegetables can't be dessert, right, sure, yeah, 0:06:55.960 --> 0:06:58.640 I guess so I like a nice zucchini bread. Yeah, which, 0:06:58.720 --> 0:07:01.040 it's that's technically what whread you eat at the end? Right, 0:07:01.320 --> 0:07:03.840 sounds good. Well, I'm hoping that we can gobble of 0:07:03.960 --> 0:07:06.440 the mystery of dark matter one day, but until then, 0:07:06.800 --> 0:07:09.000 we have to think carefully about what we have seen, 0:07:09.279 --> 0:07:11.800 what we know, what we can explain, what we can't 0:07:11.840 --> 0:07:15.000 explain and what new ideas we might need to tell 0:07:15.000 --> 0:07:18.160 a complete and true story about everything that's happening out 0:07:18.160 --> 0:07:20.800 there in the universe. Yeah, it is an ongoing debate 0:07:20.840 --> 0:07:23.760 about what dark matter is, and it's an ongoing exploration 0:07:23.800 --> 0:07:25.800 of what it could be. So today we'll be talking 0:07:25.840 --> 0:07:29.080 about one possible idea for dark matter. So today on 0:07:29.120 --> 0:07:36.760 the program, we'll be tackling the question could dark matter 0:07:37.160 --> 0:07:41.280 be a super fluid? And if so, is it's sparkling? 0:07:41.640 --> 0:07:43.680 And will it kill you? I guess if you drink 0:07:43.720 --> 0:07:46.120 it question number two? Right, First, I want to know 0:07:46.160 --> 0:07:48.960 if it's carbonated, and then question number two is can 0:07:49.000 --> 0:07:51.720 I survive drinking it? Well, it dissolve all the carbon 0:07:51.800 --> 0:07:54.520 in your body is the second question. I guess you 0:07:54.560 --> 0:07:57.160 would want to know before you drink anything, unless you're 0:07:57.240 --> 0:07:59.800 Daniel whites It. Yeah, and it's a really interesting question. 0:08:00.000 --> 0:08:02.200 Other dark matter is a super fluid, and you might 0:08:02.240 --> 0:08:04.200 wonder like, why do we care? Why do we think 0:08:04.280 --> 0:08:06.600 it might be a super fluid. And for all the 0:08:06.680 --> 0:08:10.119 successes that dark matter has had in explaining the large 0:08:10.120 --> 0:08:13.440 scale structure of the universe and gravitational lensing and the 0:08:13.480 --> 0:08:16.880 wiggles we see in the cosmic microwave background radiation. We'll 0:08:16.880 --> 0:08:18.320 talk about it in a minute, But there are some 0:08:18.400 --> 0:08:21.400 things that dark matter as a theory of some weird 0:08:21.440 --> 0:08:25.240 invisible particle really struggles to explain in our universe. It 0:08:25.280 --> 0:08:27.120 needs a little bit of help. Yeah, and I feel 0:08:27.120 --> 0:08:28.960 like we maye skipped the question here, Like I feel 0:08:28.960 --> 0:08:31.240 like we never even tackled the question could dark matter 0:08:31.280 --> 0:08:33.840 be a fluid? Like do we even know if it 0:08:33.840 --> 0:08:36.440 could be a fluid or a solid or gas? You will, Actually, 0:08:36.480 --> 0:08:40.160 dark matter already we think is kind of a collisionless fluid, 0:08:40.480 --> 0:08:42.440 sort of like an ideal gas. You know, we think 0:08:42.480 --> 0:08:44.680 about it as these particles flying around in the universe, 0:08:44.920 --> 0:08:48.000 not interacting with each other at all, because again, the 0:08:48.000 --> 0:08:51.240 only interactions we think it has is gravity, and gravity 0:08:51.240 --> 0:08:54.240 between particles is basically zero. So already we think of 0:08:54.320 --> 0:08:56.840 dark matter sort of like a collisionless fluid or an 0:08:56.880 --> 0:08:59.240 ideal gas. So here we're talking about it having like 0:08:59.320 --> 0:09:03.480 special produced from being a super fluid like a superhero. 0:09:03.960 --> 0:09:06.959 But shouldn't be like a super gas then? Yeah, exactly, 0:09:07.000 --> 0:09:09.280 Maybe it gets bitten by a radioactive spider and then 0:09:09.320 --> 0:09:12.200 turns into a super fluid or super gas. Yea radioactive 0:09:12.280 --> 0:09:14.520 fluid spider, I guess it would have to be the 0:09:14.559 --> 0:09:17.040 marvel theory of the universe. Well, as usually, we were 0:09:17.080 --> 0:09:19.960 wondering how many people out there had considered this question 0:09:19.960 --> 0:09:23.320 whether dark matter could be a super fluid. So Daniel 0:09:23.320 --> 0:09:25.760 went out there into the wilds of the internet, or 0:09:25.800 --> 0:09:28.640 maybe the campus of UC Irvine, which one this time? Daniel, 0:09:28.760 --> 0:09:32.080 these are Internet answers, So thanks very much to everybody 0:09:32.080 --> 0:09:35.040 who participates in these and waits patiently for us to 0:09:35.120 --> 0:09:37.520 get to the episode. If you'd like to participate for 0:09:37.679 --> 0:09:40.320 future episodes, please don't be shy right to us. Two 0:09:40.440 --> 0:09:43.480 questions at Daniel and Jorge dot com. So think about 0:09:43.480 --> 0:09:46.280 it for a second. Do you think dark matter could 0:09:46.360 --> 0:09:50.040 be a super fluid? Here's what people had to say. Well, 0:09:50.080 --> 0:09:52.000 I wish I knew what a super fluid was, because 0:09:52.240 --> 0:09:54.319 that sounds like a really awesome thing to get to know. 0:09:54.840 --> 0:09:57.240 The dark matter could be a lot of different things 0:09:57.400 --> 0:10:00.599 still at this point, um, so sure it could be 0:10:00.600 --> 0:10:02.560 a super fluid. It could be a great soft drink 0:10:03.240 --> 0:10:06.320 that sadly just passes right through your body. I don't 0:10:06.360 --> 0:10:10.160 really think that's true because the other superfluids that we've 0:10:10.240 --> 0:10:12.800 created are all made out of hawks, they're just in 0:10:12.800 --> 0:10:15.800 a different arrangement, and I don't think quarks can, you know, 0:10:16.040 --> 0:10:19.600 display the behaviors that dark matter does. So no, I 0:10:19.600 --> 0:10:21.920 don't think that dark matter is a super fluid, but 0:10:22.520 --> 0:10:25.959 who knows, well, what's a super fluid. I believe that's 0:10:25.960 --> 0:10:29.240 material that doesn't lose energy when it's moving, So it 0:10:29.320 --> 0:10:31.800 sounds like document that would really hit that point because 0:10:31.800 --> 0:10:34.319 it doesn't even inter fear with itself. On the other hand, 0:10:34.360 --> 0:10:36.880 we know that doc meto is impacted by gravity, so 0:10:36.960 --> 0:10:39.120 that would speak against it. So I'm not really sure. 0:10:39.280 --> 0:10:40.679 But if I would have to make a bed, I 0:10:40.679 --> 0:10:43.520 would say, yeah, it's a super fluid. I don't see 0:10:43.559 --> 0:10:47.720 how can anything out in space be considered a fluid 0:10:48.280 --> 0:10:51.640 given how low density it is, and if it isn't 0:10:51.640 --> 0:10:54.480 a fluid, it could be a super fluid. No idea. 0:10:55.080 --> 0:10:58.400 That's fascinating. I would not have thought dark matter was 0:10:58.440 --> 0:11:00.319 dense enough to be a fluid in the sense that 0:11:00.360 --> 0:11:03.760 we understand it. Um my understanding that dart matter is 0:11:03.760 --> 0:11:07.840 going to be as diffuse as regular matter and its 0:11:07.840 --> 0:11:12.000 distribution through a nearly infinite universe, so that it's going 0:11:12.040 --> 0:11:14.360 to behave like a guess. I would think if I 0:11:14.360 --> 0:11:17.800 remember correctly, a super fluid doesn't as a fluid fluid 0:11:17.840 --> 0:11:22.240 without viscosity. I think it was viscosity. And well, we 0:11:22.360 --> 0:11:25.840 know dark matter as supposed to interact with matter only 0:11:25.880 --> 0:11:29.400 through gravity. But I suppose if it interacts any through gravity, 0:11:29.400 --> 0:11:32.240 that implies some form of attraction, which means, if we 0:11:32.320 --> 0:11:35.120 regarded as a fluid, it must have some sort of viscosity, which, 0:11:35.400 --> 0:11:38.719 if I remember the definition of a superfluid currictly means 0:11:38.760 --> 0:11:41.240 that can't be a super fluid. Sure, I guess it 0:11:41.280 --> 0:11:44.680 could be anything. We really don't know much about dark 0:11:44.720 --> 0:11:49.680 matter other than it exists and it has gravity. Um, 0:11:49.720 --> 0:11:53.120 so why not? Could be, could be super fluid, could be, 0:11:53.840 --> 0:11:58.600 could be anything? Really m M interesting answers some skepticism. 0:11:58.760 --> 0:12:00.480 Some people were like, I don't know, don't think so 0:12:01.800 --> 0:12:04.400 this one sounds weird to people. I think the idea 0:12:04.440 --> 0:12:08.240 of having like a fluid out in space sounds weird. 0:12:08.320 --> 0:12:12.640 People think of spaces like cold and mostly empty, maybe 0:12:12.640 --> 0:12:15.320 filled with tiny little crystals or particles flying around, But 0:12:15.360 --> 0:12:17.120 like a fluid is a weird thing to think about 0:12:17.120 --> 0:12:19.800 having in space, right, And it seems like a lot 0:12:19.800 --> 0:12:21.320 of people are like, maybe it could be a fluid, 0:12:21.360 --> 0:12:23.240 but a super fluid, I don't know if I would 0:12:23.320 --> 0:12:26.360 give it, you know, supernatural powers. Yeah, well, at least 0:12:26.400 --> 0:12:27.920 one guy is ready to taste it, though, you know 0:12:27.960 --> 0:12:31.880 he's thinking about super soft drinks. Hmm, interesting, super soda, 0:12:32.080 --> 0:12:34.960 super dark soda. Exactly. It's very massive. It's like Coca 0:12:35.040 --> 0:12:39.400 cola dark coke. Give us a call. We got ideas, 0:12:39.559 --> 0:12:43.040 special dark recipe. But a lot of people didn't seem 0:12:43.080 --> 0:12:46.040 to know what even a super fluid is. I guess 0:12:46.040 --> 0:12:49.000 that's not a common word even I'm not sure what 0:12:49.040 --> 0:12:51.920 it quite means. Yeah, super fluids are not the kind 0:12:51.960 --> 0:12:54.640 of thing you have experience with that you don't see 0:12:54.679 --> 0:12:56.800 them in your everyday life. The river that runs through 0:12:56.800 --> 0:12:58.640 the park in the middle of your town doesn't ever 0:12:58.720 --> 0:13:01.520 become a super fluid. You don't make super fluids in 0:13:01.559 --> 0:13:05.240 your kitchen. They're like a weird quantum state of matter 0:13:05.360 --> 0:13:09.560 that we only recently even we're sure could exist. And 0:13:09.640 --> 0:13:12.080 so it's a sort of a new theoretical idea, and 0:13:12.080 --> 0:13:15.040 whenever that happens, people have fun applying into like, oh, 0:13:15.120 --> 0:13:17.920 this is new and cool. Maybe this works also over 0:13:17.960 --> 0:13:20.760 here in this different part of physics where there's something 0:13:20.800 --> 0:13:23.080 we don't understand we have a new hammer. So let's 0:13:23.080 --> 0:13:25.120 see what else could be a nail. Yeah, it seems 0:13:25.120 --> 0:13:27.480 like maybe dark matter being a super fluid could maybe 0:13:27.480 --> 0:13:29.839 explain some of the things we can't quite figure out 0:13:29.960 --> 0:13:31.920 about it. And so Dania, let's step people through this. 0:13:32.000 --> 0:13:34.280 First of all, I guess let's recap what dark matter 0:13:34.520 --> 0:13:36.840 is and what is it that we don't understand about 0:13:36.880 --> 0:13:39.360 it that is making us consider this idea. So we 0:13:39.520 --> 0:13:43.120 think that the universe has a bunch of invisible matter 0:13:43.240 --> 0:13:46.040 in it because we see a lot of gravity out 0:13:46.080 --> 0:13:49.640 there in the universe that we can't explain from visible matter. 0:13:49.800 --> 0:13:52.120 You know, we know that stars are huge balls of 0:13:52.200 --> 0:13:54.160 hydrogen and they have a lot of mass, and so 0:13:54.200 --> 0:13:56.480 they have a lot of gravity, and the Earth spins 0:13:56.520 --> 0:13:59.320 around the Sun, for example, because of the mass of 0:13:59.360 --> 0:14:01.760 all that grab But if you look at a galaxy 0:14:01.840 --> 0:14:03.560 and you add up all of the mass from all 0:14:03.600 --> 0:14:06.439 the stars that you can see, you can't explain all 0:14:06.480 --> 0:14:09.920 the gravity that's happening in the galaxy. Like the galaxy 0:14:10.000 --> 0:14:12.760 is spinning like all galaxies do, and that spinning would 0:14:12.800 --> 0:14:15.680 tend to toss stars into outer space the way like 0:14:15.760 --> 0:14:17.760 ping pong balls on a Merry Go Round. If you spin, 0:14:17.840 --> 0:14:20.320 it would toss those ping pong balls out past the 0:14:20.360 --> 0:14:23.160 Merry Go Round, But the gravity of the galaxy keeps 0:14:23.160 --> 0:14:26.000 those stars in place. That's why this Milky Way is 0:14:26.000 --> 0:14:28.360 not just like throwing all of our stars away. But 0:14:28.440 --> 0:14:31.720 the galaxies are spinning really really fast, and in order 0:14:31.720 --> 0:14:34.360 to hold them in place, they need more gravity than 0:14:34.440 --> 0:14:36.840 we can account for. The gravity from the stars we 0:14:36.880 --> 0:14:39.480 can see doesn't give us enough gravity to hold the 0:14:39.480 --> 0:14:42.920 galaxy together. And on a bigger scale, we also need 0:14:43.000 --> 0:14:47.120 to hold galaxy clusters together. Galaxy clusters are big groups 0:14:47.160 --> 0:14:50.400 of galaxies, and we don't think those galaxies seem to 0:14:50.440 --> 0:14:53.920 have enough mass to hold themselves together. Galaxy clusters are 0:14:53.920 --> 0:14:57.040 also spinning. But if you add a bunch of invisible 0:14:57.120 --> 0:15:01.000 mass to the galaxies, then it all works because it's invisible, 0:15:01.120 --> 0:15:04.800 explains why you can't see it, and it's new matter, 0:15:04.960 --> 0:15:08.240 so it adds more gravity, and so it solves those 0:15:08.280 --> 0:15:11.320 problems if you add this weird new invisible stuff to 0:15:11.400 --> 0:15:14.000 the universe. The weirdest thing about it, though, is that 0:15:14.040 --> 0:15:17.040 you need much more dark matter than visible stuff. It's 0:15:17.080 --> 0:15:19.240 not like you just add a little sprinkling of dark matter. 0:15:19.520 --> 0:15:22.240 You need to take every star and add five stars 0:15:22.280 --> 0:15:25.880 worth of dark matter to explain all the missing gravity. Yeah, 0:15:25.880 --> 0:15:28.200 it's it's almost like dark matter is kind of like 0:15:28.240 --> 0:15:31.600 the missing piece. And what we see of the universe, right, 0:15:31.680 --> 0:15:34.160 and the way the galaxies stick together and the galaxy 0:15:34.160 --> 0:15:37.280 clusters stick together, they stick together more than they should 0:15:37.360 --> 0:15:40.640 given what we can see in them. And so one 0:15:40.720 --> 0:15:43.680 solution is that maybe there's invisible stuff out there that's 0:15:43.680 --> 0:15:45.640 holding it together. Yeah, and this is the kind of 0:15:45.640 --> 0:15:48.800 thing we're always trying to do, is reconcile everything we see. 0:15:49.000 --> 0:15:51.600 We think we understand how gravity works. Let's check it. 0:15:51.680 --> 0:15:55.000 Let's make sure that our explanation makes sense, that it 0:15:55.040 --> 0:15:57.160 works for this scenario. And that's how we discovered oh 0:15:57.160 --> 0:15:59.360 my gosh, it doesn't. And that was the clue that 0:15:59.480 --> 0:16:02.280 maybe it was something else going on, or there was 0:16:02.360 --> 0:16:05.880 something else new out there. But before you believe that, 0:16:06.000 --> 0:16:08.800 just like crazy new idea that the universe is filled 0:16:08.800 --> 0:16:11.240 with a huge amount of invisible stuff you just happened 0:16:11.240 --> 0:16:14.200 and never noticed before, you want other pieces of evidence. 0:16:14.200 --> 0:16:17.040 And so we have other clues that dark matter might 0:16:17.040 --> 0:16:19.640 be real. We see it affecting the way light moves 0:16:19.720 --> 0:16:22.760 through the universe because if it has mass, it changes 0:16:22.840 --> 0:16:25.800 the curvature of space, and so it can lends light. 0:16:25.960 --> 0:16:28.480 We see that it existed in the very early universe 0:16:28.480 --> 0:16:32.720 because it affected the wiggles in the cosmic microwave background radiation, 0:16:33.160 --> 0:16:36.440 these photons from the plasma that filled the universe very 0:16:36.520 --> 0:16:38.960 very early on. And we also know that affected the 0:16:38.960 --> 0:16:42.200 whole way that the universe form, the large scale structure 0:16:42.280 --> 0:16:45.320 of the universe. Galaxies we think wouldn't even exist if 0:16:45.400 --> 0:16:49.040 dark matter hadn't created little gravitational wells to pull stars 0:16:49.080 --> 0:16:52.840 into mostly all hangs together into a very nice story, right, 0:16:52.840 --> 0:16:54.560 But it's kind of interesting. I guess that it's a 0:16:54.600 --> 0:16:57.400 story we made up assuming that what we know of 0:16:57.440 --> 0:16:59.960 the rest of the universe is true. Right, Like, if 0:17:00.040 --> 0:17:02.400 we assume that the laws of fixed work the way 0:17:02.400 --> 0:17:04.480 we think they do, then you sort of need this 0:17:04.560 --> 0:17:07.399 invisible matter to just make what we see make sense. 0:17:07.440 --> 0:17:09.560 But that's only assuming that we're right about the laws 0:17:09.560 --> 0:17:13.080 of physics. Yeah, that's saying gravity works a certain way, 0:17:13.200 --> 0:17:15.399 and so in order to explain this missing gravity, we 0:17:15.440 --> 0:17:18.320 need more mass. But you're right, there are other ideas. 0:17:18.400 --> 0:17:21.480 People have also thought, well, maybe gravity doesn't work the 0:17:21.520 --> 0:17:24.120 way that we thought maybe there isn't any missing gravity. 0:17:24.240 --> 0:17:27.560 It's just signs that our theory of gravity is wrong. 0:17:28.160 --> 0:17:31.160 And people have tried to modify the theory of gravity 0:17:31.200 --> 0:17:33.760 to explain what we see, and this is called Mond 0:17:34.320 --> 0:17:38.160 modified Newtonian dynamics. The theory says that instead of gravity 0:17:38.200 --> 0:17:41.920 going like one over distance square the way Newton said, 0:17:42.160 --> 0:17:45.959 there's another factor there that when things have very low accelerations, 0:17:45.960 --> 0:17:48.560 gravity is a little bit different. It gets a little stronger. 0:17:48.640 --> 0:17:51.080 So if you tweak gravity in just this way, you 0:17:51.080 --> 0:17:54.760 can also explain how galaxies rotate without using dark matter. 0:17:55.359 --> 0:17:57.920 That's really interesting. So it could be that dark matter 0:17:58.320 --> 0:18:00.960 is not dark matter at all, Like maybe we just 0:18:01.119 --> 0:18:03.480 have the laws of physics a little bit wrong. But 0:18:03.520 --> 0:18:06.600 does that explain everything about the way the galaxies stick 0:18:06.600 --> 0:18:09.560 together and even the gravitational lensing. No, So Mond, this 0:18:09.680 --> 0:18:13.200 alternative theory gravity does in fact a better job at 0:18:13.240 --> 0:18:16.879 explaining how galaxies rotate than dark matter does. There's some 0:18:16.920 --> 0:18:19.840 things about galaxy rotations that dark matter just can't seem 0:18:19.880 --> 0:18:22.600 to get right, but it doesn't explain everything else that 0:18:22.720 --> 0:18:25.240 dark matter does. Like Mond does not do a good 0:18:25.320 --> 0:18:29.359 job of describing how galaxy clusters rotate and spin. Around themselves, 0:18:29.440 --> 0:18:31.760 and it's much more difficult for it to explain, like 0:18:31.800 --> 0:18:35.520 the cosmic microwave background and lensing and all sorts of 0:18:35.600 --> 0:18:38.600 other very strong evidence for dark matter. So mond is 0:18:38.600 --> 0:18:40.560 sort of a nice idea you can explain one thing 0:18:40.640 --> 0:18:43.480 actually better than dark matter can. But dark matter is 0:18:43.520 --> 0:18:46.399 sort of like a stronger idea across the board. But 0:18:46.480 --> 0:18:48.760 neither of them, I guess, is perfect. And let's maybe 0:18:48.760 --> 0:18:50.520 dig into that a little bit, like what is it 0:18:50.560 --> 0:18:53.280 about the idea that dark matter is invisible stuff that 0:18:53.480 --> 0:18:55.600 doesn't explain what we see out there? So by now 0:18:55.640 --> 0:18:58.200 we've seen a lot of galaxies, and what we try 0:18:58.240 --> 0:19:00.600 to do is understand like how am my dark matter 0:19:00.720 --> 0:19:03.160 is in a galaxy versus how much normal matter? And 0:19:03.400 --> 0:19:05.960 is that common across galaxies? Like do all galaxies have 0:19:06.000 --> 0:19:08.440 the same amount of dark matter and the same amount 0:19:08.440 --> 0:19:11.040 of normal matter? And we also try to understand how 0:19:11.080 --> 0:19:13.520 that could have happened. You know, if dark matter is 0:19:13.560 --> 0:19:16.600 this weird particle, this new heavy, invisible thing, then it 0:19:16.600 --> 0:19:18.879 would have clustered together and we can make models for 0:19:19.080 --> 0:19:21.600 how that would have happened informed galaxies. We look out 0:19:21.600 --> 0:19:24.200 into the universe, the galaxies we see don't really line 0:19:24.280 --> 0:19:26.840 up with what we expect for dark matter. So one 0:19:26.880 --> 0:19:29.760 thing specifically we look at is a relationship between how 0:19:29.800 --> 0:19:33.000 bright the galaxies are and how fast they are spinning. 0:19:33.440 --> 0:19:35.600 How bright they are is really interesting because it tells 0:19:35.640 --> 0:19:37.920 us like how much normal matter is there, how many stars, 0:19:38.400 --> 0:19:41.160 how fast they're spinning should tell us something about how 0:19:41.240 --> 0:19:43.600 much dark matter there is in the galaxy. And if 0:19:43.640 --> 0:19:45.840 you do a bunch of simulations, then you expect like 0:19:46.200 --> 0:19:49.439 a loose relationship there. You expect, like some galaxies to 0:19:49.480 --> 0:19:50.879 have a lot of dark matter and something to have 0:19:50.920 --> 0:19:53.080 a little bit, but there to be a lot of variation. 0:19:53.480 --> 0:19:55.560 What we see when we look at these galaxies, though, 0:19:55.600 --> 0:19:58.280