WEBVTT - What is the dark flow? 0:00:08.480 --> 0:00:11.800 Hey, Orge, do you think stuff in science is typically 0:00:11.920 --> 0:00:14.560 well named or sometimes misleading? 0:00:15.160 --> 0:00:17.599 Depends on which science. Do you mean physics? Then no? 0:00:20.079 --> 0:00:21.919 I mean sometimes we do a good job, right, like 0:00:21.960 --> 0:00:22.520 black hole? 0:00:22.800 --> 0:00:25.520 Yeah? Yeah, those are black and their holes in space. 0:00:25.600 --> 0:00:27.479 So I guess you did good on that one. 0:00:27.880 --> 0:00:30.200 But in other cases you're saying we didn't do as well. 0:00:30.400 --> 0:00:34.320 Well, let's see, how about quark flavors which don't have flavor, 0:00:34.320 --> 0:00:36.960 how about quantum colors which don't have colored? 0:00:37.600 --> 0:00:40.960 All right, that's fair, So let's do a little experiment. 0:00:41.080 --> 0:00:42.800 I'm going to give you a physics name, and you're 0:00:42.800 --> 0:00:44.919 gonna guess what it means. All right, go for it. 0:00:44.920 --> 0:00:46.400 It's called the dark flow. 0:00:46.640 --> 0:00:49.479 That sounds like maybe a plumbing product to get your 0:00:49.560 --> 0:00:50.440 toilet moving again? 0:00:52.280 --> 0:00:52.560 Nice? 0:00:52.720 --> 0:00:55.240 Did I get that right? Stay tuned and find out 0:00:55.440 --> 0:00:56.680 you have to go with the flow. Is that what 0:00:56.720 --> 0:00:57.120 you're saying? 0:01:12.240 --> 0:01:12.399 Hi? 0:01:12.400 --> 0:01:15.880 I'm Horia mccartoonist and the author of Oliver's Great Big Universe. 0:01:16.040 --> 0:01:18.760 Hi, I'm Daniel. I'm a particle physicist and a professor 0:01:18.760 --> 0:01:21.320 at U see erline, and I hate plumbing projects. 0:01:21.600 --> 0:01:23.800 Who loves plumbing projects? I guess plumbers? 0:01:24.280 --> 0:01:28.400 Yeah? Maybe, I mean I hope they love plumbing projects. 0:01:28.400 --> 0:01:29.440 Otherwise it's kind of sad. 0:01:29.520 --> 0:01:31.959 It's probably interesting to them. Sure, But how many plumbing 0:01:31.959 --> 0:01:33.319 projects have you worked on recently? 0:01:33.480 --> 0:01:36.320 Oh? Too many? You know, anytime you have a plumbing project, 0:01:36.319 --> 0:01:40.240 that means bad news. Something's leaking somewhere, something's broken somewhere. 0:01:40.440 --> 0:01:42.200 You know, the joys of home ownership. 0:01:42.319 --> 0:01:43.480 What did you call a plumber? 0:01:43.600 --> 0:01:45.800 Yeah, that's like paying somebody to deliver the bad news 0:01:45.800 --> 0:01:46.080 to you. 0:01:46.720 --> 0:01:49.520 Hey, yeah, and then then hopefully they fix it for you. 0:01:49.680 --> 0:01:51.000 What kind of plumbers are you calling? 0:01:52.320 --> 0:01:53.960 Yeah, they fix it. We had a leak in our 0:01:54.040 --> 0:01:57.160 upstairs bathroom recently and had to tear out and rehab 0:01:57.280 --> 0:01:59.080 all the walls in our garage. It was a big 0:01:59.120 --> 0:01:59.760 pain in the butt. 0:01:59.800 --> 0:02:02.960 Oh boy, what happened? The tub overfluid or something? 0:02:03.040 --> 0:02:04.080 It was a dark flow. 0:02:04.320 --> 0:02:06.640 You took too long of a bath there thinking about physics. 0:02:06.720 --> 0:02:08.760 I was dunking stuff in the bathtub hoping for a 0:02:08.840 --> 0:02:10.639 Eureka moment, and it just didn't happen. 0:02:10.840 --> 0:02:13.680 It just didn't happen. You didn't figure out how bullyiancy works. 0:02:15.240 --> 0:02:17.280 No, it's too complicated for a particle physicists. 0:02:18.080 --> 0:02:21.040 Too many particles in a tub of water. But anyways, 0:02:21.080 --> 0:02:23.600 welcome to our podcast Daniel and Jorge Explain the Universe, 0:02:23.639 --> 0:02:26.079 a production of iHeartRadio. 0:02:25.400 --> 0:02:27.800 In which we teach you to think like a physicist 0:02:27.840 --> 0:02:30.480 instead of a plumber. Don't dunk stuff in a tub 0:02:30.520 --> 0:02:33.560 of water, but immerse your brain in the mysteries of 0:02:33.680 --> 0:02:36.320 the universe and try to learn how the universe works. 0:02:36.320 --> 0:02:38.440 By Oz most is hanging out with us as we 0:02:38.480 --> 0:02:41.120 talk and joke about everything that's out there in the 0:02:41.200 --> 0:02:43.679 universe and try to explain all of it to you. 0:02:43.880 --> 0:02:45.640 That's right. We try to fill the tub in your 0:02:45.680 --> 0:02:49.440 mind of knowledge and wonder until it overflows and hopefully 0:02:50.200 --> 0:02:51.919 maybe leaks into your garage a little bit. 0:02:53.720 --> 0:02:55.800 You know, in that scenario, maybe a plumbing project is 0:02:55.840 --> 0:02:57.880 good news because you got to rip everything out and 0:02:57.919 --> 0:03:00.400 build up a new understanding of the universe. That's what 0:03:00.440 --> 0:03:02.079 I'm always saying I want to do. Wait. 0:03:02.080 --> 0:03:04.160 Wait, you're saying the universe is plumbing. 0:03:05.120 --> 0:03:07.959 I'm saying, our mental understanding of how the universe works 0:03:08.200 --> 0:03:10.399 has plumbing. When you got to tear all that out 0:03:10.440 --> 0:03:12.320 and rebuild it, that means you're coming up with a 0:03:12.400 --> 0:03:16.079 new idea about how the universe works, and that's a revolution. 0:03:15.800 --> 0:03:19.240 Oh wait, what's the plumbing for ideas? Also, what do 0:03:19.240 --> 0:03:20.880 you flush down the toilet of physics? 0:03:21.160 --> 0:03:23.720 Imagine the inside of a physicist mind and model it 0:03:23.760 --> 0:03:26.600 as a bunch of pipes with ideas flushing and flowing 0:03:26.639 --> 0:03:27.320 and swirling. 0:03:28.560 --> 0:03:32.000 I see. I always love physicists sticky pipes themselves in 0:03:32.080 --> 0:03:35.040 their brains. But aren't physicists sort of the plumbers of 0:03:35.040 --> 0:03:37.200 the universe? Kind of you're trying to figure out how 0:03:37.200 --> 0:03:38.760 the plumbing of the universe works. 0:03:39.000 --> 0:03:42.120 Yeah, everything in the universe is out there, sloshing around, 0:03:42.240 --> 0:03:45.440 banging it into itself. It's not like a stable situation. 0:03:45.600 --> 0:03:47.360 You look up at the night sky and you might think, Hi, 0:03:47.360 --> 0:03:49.400 everything's just sort of hanging it out. But that's just 0:03:49.440 --> 0:03:53.040 because we live for such brief moments. On cosmic scales, 0:03:53.200 --> 0:03:55.760 if you look at the universe over millions or billions 0:03:55.760 --> 0:03:58.640 of years, you would see things expanding and exploding and 0:03:58.680 --> 0:04:00.160 smashing and flowing. 0:04:00.320 --> 0:04:03.840 Well, usually pipes expanding is not a good sign. Like, 0:04:03.880 --> 0:04:06.360 if your pipe expands, it's probably going to burst. 0:04:06.480 --> 0:04:09.480 M Yes, that's true. Dark energy would be bad news 0:04:09.480 --> 0:04:10.120 for your plumbing. 0:04:10.240 --> 0:04:12.280 Yeah, better flush it down the toilet or call a 0:04:12.280 --> 0:04:13.560 real professional. 0:04:14.400 --> 0:04:16.520 Someone who has actually useful skills you mean. 0:04:16.520 --> 0:04:20.560 Yeah, someone who can fix a leak, someone who knows 0:04:20.560 --> 0:04:21.360 how to use a wrench. 0:04:22.160 --> 0:04:24.919 You mean not a physicist and not a cartoonist. 0:04:24.440 --> 0:04:26.719 No, but an engineer. Maybe I'm also an engineer. I 0:04:26.760 --> 0:04:27.360 have that skill. 0:04:27.720 --> 0:04:29.640 So you tackle the plumbing projects in your house to 0:04:29.680 --> 0:04:30.640 do you call it professional? 0:04:30.839 --> 0:04:33.280 Sometimes I do fix it. Yeah, I fix my washing 0:04:33.320 --> 0:04:36.280 machine the other day. I was pretty proud of myself. 0:04:36.200 --> 0:04:37.599 My kicking it and swearing at it. 0:04:38.200 --> 0:04:41.000 No, but looking it up on YouTube and looking at 0:04:41.000 --> 0:04:44.120 a video of how to replace the little valve there inside. 0:04:44.279 --> 0:04:45.520 We're all YouTube engineers. 0:04:45.560 --> 0:04:47.520 Did you type how to replace that little valve there 0:04:47.560 --> 0:04:49.160 into Google and just follow the instructions? 0:04:49.520 --> 0:04:49.599 No? 0:04:49.680 --> 0:04:51.800 I think I just typed the model of my dishwasher, 0:04:51.839 --> 0:04:54.320 and then, you know, helpfully, a lot of people out 0:04:54.320 --> 0:04:57.360 there have posted videos about it. I had to order apart, 0:04:57.720 --> 0:05:02.760 pulled it apart. Surprisingly, nothing exploded. 0:05:02.960 --> 0:05:03.400 So far. 0:05:03.640 --> 0:05:04.080 That's right. 0:05:04.120 --> 0:05:07.040 So far the Jorge has mastered his washing machine and 0:05:07.080 --> 0:05:09.400 the plumbing of his house. Physicists are still trying to 0:05:09.400 --> 0:05:11.480 figure out how the universe out there works. 0:05:11.720 --> 0:05:14.039 Yeah, because it is an amazing universe full of what 0:05:14.200 --> 0:05:18.240 seems like internal plumbing where forces and things like particles 0:05:18.279 --> 0:05:21.080 and quantum fields all flow around and flush together and 0:05:21.080 --> 0:05:23.240 get mixed up and flow from one side to the other. 0:05:23.400 --> 0:05:25.800 There's all sorts of stuff involved. There's normal matter, there's 0:05:25.880 --> 0:05:29.640 dark matter, there's dark energy. There's the expansion of the universe. 0:05:29.680 --> 0:05:32.080 There's the shape of the universe, in the size of 0:05:32.120 --> 0:05:34.760 the universe, all of which play a role in what's 0:05:34.800 --> 0:05:35.800 going on out there. 0:05:35.920 --> 0:05:37.760 Yeah, we've learned a lot about the universe, but there's 0:05:37.760 --> 0:05:41.200 still a lot we don't know, big giant concepts that 0:05:41.279 --> 0:05:43.800 are still a big mystery. So today on the podcast, 0:05:43.880 --> 0:05:53.440 we'll be tackling the question what is the dark flow? Now, 0:05:53.480 --> 0:05:55.400 this is not related to on flow, is it. 0:05:55.480 --> 0:05:58.080 Well, there are a lot of bodily functions that do flow, 0:05:58.279 --> 0:06:01.400 but we are strictly a physics podcast asked not biology. 0:06:01.960 --> 0:06:04.040 Although putting the word dark in front of it it 0:06:04.080 --> 0:06:06.520 does make me think of toilet functions. 0:06:07.640 --> 0:06:09.719 There are a lot of dark matters that get flushed 0:06:09.720 --> 0:06:10.640 down the toilet, that's true. 0:06:10.720 --> 0:06:12.440 Yeah, a lot of dark matter that requires a lot 0:06:12.440 --> 0:06:15.320 of dark energy. Especially if you haven't been eating fiber. 0:06:16.880 --> 0:06:18.720 When your plumber is telling you you need a new diet, 0:06:18.800 --> 0:06:20.320 then you definitely have a problem. 0:06:20.920 --> 0:06:24.640 Yeah, aren't doctors sort of like body plumbers? 0:06:24.680 --> 0:06:28.400 Hmmm, yeah, exactly. Yeah, the digestive system sometimes needs a 0:06:28.480 --> 0:06:29.080 new valves. 0:06:29.120 --> 0:06:31.800 But this is an intriguing name for a concept, that 0:06:31.960 --> 0:06:35.240 dark flow. And so as usual, we were wondering how 0:06:35.240 --> 0:06:38.360 many people out there had heard of this concept or 0:06:38.640 --> 0:06:40.560 have any idea what it could meet. 0:06:40.640 --> 0:06:42.920 Thanks to all the volunteers who answered this question. If 0:06:42.920 --> 0:06:45.560 it sounds like fun to you to get a dose 0:06:45.600 --> 0:06:48.440 of random physics questions in your inbox every week, right 0:06:48.520 --> 0:06:51.320 to me too, questions at Danielandhorghey dot com. 0:06:51.360 --> 0:06:52.960 So think about it for a second. What do you 0:06:53.000 --> 0:06:56.280 think is the dark flow? Here's what people have to say. 0:06:56.440 --> 0:06:59.600 That sounds like it has to do with dark energy 0:07:00.200 --> 0:07:05.520 and or dark matter and it's relative distribution through the 0:07:05.600 --> 0:07:07.080 cosmos over time. 0:07:07.520 --> 0:07:09.080 I want to say, has something to do with like 0:07:09.360 --> 0:07:14.360 dark matter flowing between stars or galaxies, or like how 0:07:14.360 --> 0:07:17.040 we have a solar wind, so like a dark matter wind. 0:07:17.320 --> 0:07:19.680 All right, I'm going to go with Batman as well. 0:07:20.840 --> 0:07:23.800 It's like when Batman's in a state where he's totally 0:07:23.840 --> 0:07:26.880 concentrated in his crime fighting, it's just you know, having 0:07:27.200 --> 0:07:29.120 the artistic flow of fighting crime. 0:07:29.200 --> 0:07:31.480 At least it would be something new in the Batman story. 0:07:31.520 --> 0:07:33.800 I feel like Batman is just the same story over 0:07:33.880 --> 0:07:34.600 and over again. 0:07:34.720 --> 0:07:37.040 Oh boy, are we're going to get into Batman in 0:07:37.080 --> 0:07:41.120 this episode? How long is this episode? It sounds like 0:07:41.160 --> 0:07:42.400 you've thought about Batman a lot. 0:07:42.480 --> 0:07:45.120 I just saw like Batman Year one rebooted. It's like, 0:07:45.160 --> 0:07:46.760 how many times can you go back and tell the 0:07:46.760 --> 0:07:49.840 same story about his parents getting shot and he's a 0:07:49.920 --> 0:07:53.280 dark soul? Dot dot dot, Like, come up with something new, people, Well, 0:07:53.240 --> 0:07:54.880 do you mean like change his origin story? 0:07:54.920 --> 0:07:55.840 Then he wouldn't be Batman. 0:07:56.360 --> 0:08:00.960 Tell us a story about somebody else. Don't just keep 0:08:01.000 --> 0:08:02.080 rehashing the same. 0:08:01.880 --> 0:08:05.400 Ipedi tell us about possum Man. 0:08:05.600 --> 0:08:09.760 Yeah, exactly, And squirrel Man squirrel girl is a thing, right. 0:08:09.840 --> 0:08:11.280 Ryan North writes that it's great. 0:08:11.480 --> 0:08:17.119 Yeah, yeah, But anyways, without getting too dark here about 0:08:17.160 --> 0:08:21.239 comic books, this is apparently a real physics concept, dark flow. 0:08:21.400 --> 0:08:24.440 It is a real physics concept, and it's kind of 0:08:24.480 --> 0:08:27.520 connected to dark matter and dark energy, but it's neither 0:08:27.720 --> 0:08:31.600 of those things. It's more related to the overall expansion 0:08:31.640 --> 0:08:33.760 and flow of galaxies in the universe. 0:08:34.600 --> 0:08:37.960 I wonder if maybe you've overused the word dark in 0:08:38.000 --> 0:08:40.560 physics that's a dark thought, yeah, because people think it's 0:08:40.600 --> 0:08:42.680 just all connected. If you use the same word for 0:08:42.760 --> 0:08:44.160 the set for different things. 0:08:44.040 --> 0:08:46.720 You're absolutely right. People confuse dark matter and dark energy 0:08:46.840 --> 0:08:49.320 all the time because they're both called dark. Really, what 0:08:49.360 --> 0:08:51.719 we mean by dark is we don't understand this. It's 0:08:51.840 --> 0:08:53.920 mysterious and unknown. 0:08:53.720 --> 0:08:56.360 And invisible, right that you can't see it in space. 0:08:56.440 --> 0:08:59.600 M hmm, yeah, exactly, all right, Well, Daniel step us 0:08:59.600 --> 0:09:03.320 through this. So what is the dark flow? What's exactly 0:09:03.480 --> 0:09:06.160 flowing in the universe and where is it flowing too 0:09:06.400 --> 0:09:07.000 or down too? 0:09:07.160 --> 0:09:09.320 That's part of the question. How is the universe flowing 0:09:09.360 --> 0:09:12.480 and where is it flowing to? So to understand the 0:09:12.559 --> 0:09:15.280 dark flow, we have to think about the expansion of 0:09:15.320 --> 0:09:18.440 the universe, like where all the galaxies are going, because 0:09:18.440 --> 0:09:20.079 the dark flow is a mystery that sort of sits 0:09:20.160 --> 0:09:23.040 on top of our current understanding of the expansion of 0:09:23.080 --> 0:09:25.880 the universe, how all the galaxies are moving away from 0:09:25.920 --> 0:09:26.320 each other. 0:09:26.440 --> 0:09:29.360 And like expansion, you mean, like the expansion of space itself, 0:09:29.440 --> 0:09:32.560 Like the space where all the galaxies sit in is expanding, 0:09:32.559 --> 0:09:33.720 It's getting bigger and bigger. 0:09:33.840 --> 0:09:38.760 Exactly, space is being created between galaxies, which effectively increases 0:09:38.880 --> 0:09:42.680 the distance between those galaxies, and that's the expansion of 0:09:42.720 --> 0:09:46.400 the universe, which is a really interesting and fascinating history. Right. 0:09:46.400 --> 0:09:49.600 We know that the universe expanded very quickly early. 0:09:49.360 --> 0:09:51.640 On, that's the Big Bang, right, and inflation. 0:09:52.000 --> 0:09:54.800 Yeah, loosely speaking, though we don't understand what caused that 0:09:54.840 --> 0:09:58.600 initial expansion. And then after that period things were still expanding, 0:09:58.880 --> 0:10:01.400 but the signs sort of flow. The universe took its 0:10:01.400 --> 0:10:04.040 foot off the gas and started hitting the brake. So 0:10:04.240 --> 0:10:07.280 still expanding, but now it was decelerating. There's all this 0:10:07.400 --> 0:10:09.720 mass in the universe that was starting to slow down 0:10:09.760 --> 0:10:13.200 the expansion. So universe still getting bigger, space still getting 0:10:13.200 --> 0:10:15.920 created between the galaxies, but at a lower. 0:10:15.720 --> 0:10:19.400 Rate now, meaning like the mass of the stuff in 0:10:19.440 --> 0:10:22.959 the universe was somehow slowing down the expansion. Why was that? 0:10:23.200 --> 0:10:26.320 Was it like the gravity of the stuff or just 0:10:26.440 --> 0:10:29.880 the having stuff in space makes it not expand. 0:10:29.679 --> 0:10:32.960 That's just the gravity, right. What determines whether space is 0:10:33.040 --> 0:10:37.360 expanding or contracting, and whether it's accelerating or decelerating is 0:10:37.400 --> 0:10:39.760 the amount of stuff in space, like the matter density, 0:10:39.840 --> 0:10:42.439 and that sends to pull stuff together. And then also 0:10:42.480 --> 0:10:45.040 the shape of the universe, is it flat, is it negative? 0:10:45.120 --> 0:10:48.120 Is it open? And the amount of dark energy which 0:10:48.160 --> 0:10:50.880 is pushing things out and accelerating things. Early on in 0:10:50.920 --> 0:10:53.400 the universe, there really wasn't a whole lot of dark energy, 0:10:53.440 --> 0:10:57.079 and so matter dominated and it was slowing down that expansion. 0:10:57.760 --> 0:11:01.480 But because it was still expanding, and as the universe expands, 0:11:01.520 --> 0:11:04.920 it makes more dark energy because dark energy doesn't get diluted, 0:11:05.000 --> 0:11:08.000 and then dark energy makes the universe expand more. That 0:11:08.040 --> 0:11:12.240 continuing expansion, even though it was decelerating, made more dark energy, 0:11:12.480 --> 0:11:16.520 which turned things around again back to acceleration. So we 0:11:16.559 --> 0:11:20.800 had constant expansion, but we had like initial acceleration, then deceleration, 0:11:21.200 --> 0:11:23.640 and now for the last five or six billion years 0:11:23.920 --> 0:11:27.000 we've had acceleration. Again that's dark energy. 0:11:27.559 --> 0:11:29.680 So it's been sort of a roller coaster ride for 0:11:29.720 --> 0:11:33.120 the universe, right, Like it stressed out really fast and 0:11:33.160 --> 0:11:35.880 then its slowed down and now it's picking up speed 0:11:35.880 --> 0:11:39.040 again and potentially it's going to keep picking up speed 0:11:39.080 --> 0:11:42.040 until it grows it and super duper fast rate. 0:11:42.280 --> 0:11:44.200 Yeah, we don't know what the future holds because we 0:11:44.200 --> 0:11:47.640 don't really understand dark energy like at all. If this 0:11:47.760 --> 0:11:50.760 simple model of dark energy is just like some energy 0:11:50.800 --> 0:11:54.320 that permeates space and causes the expansion of the universe 0:11:54.360 --> 0:11:57.320 to accelerate. Is valid, then yeah, I'll just keep going 0:11:57.360 --> 0:11:59.760 forever and things will get more and more distant. Space 0:11:59.800 --> 0:12:04.560 will continue to expand between galaxies, creating vaster and vaster 0:12:04.679 --> 0:12:09.000 distances between superclusters which will collapse into supermassive black holes. 0:12:09.400 --> 0:12:11.640 And that's sort of the far future of the universe. 0:12:11.760 --> 0:12:14.440 And we're talking about this today because this expansion of 0:12:14.480 --> 0:12:17.959 the universe, this creation of space between galaxies, is sometimes 0:12:18.000 --> 0:12:20.320 called by astronomers the Hubble flow. 0:12:20.720 --> 0:12:23.520 Now, first of all, I guess this expansion of space 0:12:23.600 --> 0:12:27.120 is happening everywhere, all at once, the same everywhere, or 0:12:27.160 --> 0:12:29.040 does it happen more in certain spots. 0:12:29.440 --> 0:12:31.839 We think it happens the same everywhere. 0:12:31.400 --> 0:12:33.520 Even the spots with lots of stuff in it. 0:12:33.640 --> 0:12:35.600 Yeah, that's the current idea, and it's sort of a 0:12:35.640 --> 0:12:38.439 theory and an observation. It's sort of the simplest idea 0:12:38.440 --> 0:12:40.160 you could have if you don't know what dark energy is. 0:12:40.559 --> 0:12:43.720 Possibly it's something like a cosmological constant, some energy in 0:12:43.800 --> 0:12:46.480 space that causes this to happen. The simplest thing to 0:12:46.480 --> 0:12:48.600 do is to say, oh, that's just a number it's 0:12:48.600 --> 0:12:51.200 the same everywhere. We don't see any evidence for it 0:12:51.240 --> 0:12:54.680 being different in different parts of the universe. But you know, 0:12:54.760 --> 0:12:57.040 that's a tricky because we can't see it in different 0:12:57.040 --> 0:12:59.160 parts of the universe. At the same time, we can 0:12:59.200 --> 0:13:02.320 see the expansion happening locally now ish we can see 0:13:02.320 --> 0:13:06.080 the expansion happening far away, like a billion years ago. 0:13:06.400 --> 0:13:08.080 It's hard to compare point to point. 0:13:08.480 --> 0:13:10.760 But didn't you say stuff slows down the expansion of 0:13:10.760 --> 0:13:13.360 the universe. So I wonder if in this bus, where 0:13:13.360 --> 0:13:16.000 there's a lot of stuff in it, maybe it's expanding 0:13:17.320 --> 0:13:17.880 less fast. 0:13:18.280 --> 0:13:20.840 Stuff does slow down the expansion of the universe. That 0:13:20.880 --> 0:13:23.960 sort of contributes overall, but everything is actually balanced so 0:13:24.000 --> 0:13:27.240 that overall the universe is flat. But yeah, we don't 0:13:27.360 --> 0:13:30.199 know how stuff and dark energy interact with each other. 0:13:30.320 --> 0:13:33.680 Dark energy is so dilute and so actually weak that 0:13:33.800 --> 0:13:36.360 anywhere there is stuff, you can basically just ignore the 0:13:36.480 --> 0:13:38.839 dark energy and just treat it as if there was 0:13:38.920 --> 0:13:40.920 gravity there, just like from its stuff. 0:13:41.720 --> 0:13:44.440 But I guess as we look around, we see the 0:13:44.480 --> 0:13:46.800 whole of the universe expanding evenly in all. 0:13:46.640 --> 0:13:49.199 Directions exactly, And the idea of the Hubble flow, I 0:13:49.240 --> 0:13:51.559 think It's kind of helpful visually for you to think 0:13:51.600 --> 0:13:53.800 about this because we tend to think about this in 0:13:53.920 --> 0:13:58.320 terms of velocities, Like those galaxies are moving away from 0:13:58.400 --> 0:14:01.319 us at some speed, and we measure those velocities in 0:14:01.400 --> 0:14:04.359 terms of red shift. How the light from those galaxies 0:14:04.440 --> 0:14:07.120 is getting stretched out because those galaxies are moving away 0:14:07.120 --> 0:14:10.160 from us. This is how Hubble sort of initially described it. 0:14:10.400 --> 0:14:13.160 You know, the velocity of these galaxies. It makes more sense, 0:14:13.200 --> 0:14:14.880 it's sort of more natural to think about it in 0:14:15.000 --> 0:14:19.200 terms of general relativity. Has expansion of space itself, That 0:14:19.280 --> 0:14:22.280 those galaxies aren't accelerating away from us, but space is 0:14:22.320 --> 0:14:25.280 being created between us, and so like, there is an 0:14:25.280 --> 0:14:28.040 increased distance between us and those galaxies, but you're not 0:14:28.080 --> 0:14:29.560 feeling any acceleration. 0:14:30.520 --> 0:14:33.200 And you said, this motion is called the Hubble flow. 0:14:33.440 --> 0:14:35.680 What do you mean motion? And why is it called 0:14:35.920 --> 0:14:36.320 a flow? 0:14:36.520 --> 0:14:39.280 So motion there is a bit misleading. The distance between 0:14:39.320 --> 0:14:43.920 the galaxies is increasing, and so for nearby galaxies, you 0:14:43.960 --> 0:14:46.840 can like measure their velocity and say that galaxy has 0:14:46.880 --> 0:14:50.040 a certain velocity in our reference frame. But across really 0:14:50.080 --> 0:14:52.560 really big distances, it doesn't make sense to talk about 0:14:52.800 --> 0:14:55.680 relative velocity for objects that are super far away from 0:14:55.680 --> 0:14:59.520 each other and have different non inertial frames. So astronomers 0:14:59.520 --> 0:15:02.520 sometimes use velocities as a sort of way to talk 0:15:02.560 --> 0:15:05.720 about those objects. But it doesn't actually make any sense 0:15:05.800 --> 0:15:08.080 in terms of general relativity. And why it's called the 0:15:08.120 --> 0:15:10.040 hubble flow. Yeah, it's a good question. I think it 0:15:10.120 --> 0:15:12.640 just helps you visualize how like space is being created 0:15:12.680 --> 0:15:16.000 everywhere and things are being carried along by that stream. 0:15:16.000 --> 0:15:18.600 Okay, so the hubble flow. And then I guess maybe 0:15:18.600 --> 0:15:21.040 it's sort of like the flow of space being created 0:15:21.400 --> 0:15:23.280 pushing everything apart in the universe. 0:15:23.560 --> 0:15:26.280 Yeah, exactly, And so you can think about those galaxies 0:15:26.280 --> 0:15:28.760 as sort of like sitting in that hubble flow. Space 0:15:28.840 --> 0:15:31.280 is being created, everything is moving further and further apart. 0:15:31.520 --> 0:15:33.880 But there's a wrinkle on top of that motion. Right, 0:15:33.880 --> 0:15:36.360 the hubble flow is not the only thing that's happening 0:15:36.440 --> 0:15:38.920 in the universe. Each of these galaxies is also moving 0:15:39.280 --> 0:15:42.560 relative to the hubble flow. Like people often write in 0:15:42.600 --> 0:15:45.680 and ask, hey, if galaxies are being pushed apart because 0:15:45.680 --> 0:15:47.920 of dark energy, why is the Milky Way going to 0:15:47.960 --> 0:15:50.720 slam into Andromeda? And the answer is that these things 0:15:50.760 --> 0:15:53.280 are moving because of gravity. Right, we talked earlier about 0:15:53.320 --> 0:15:56.479 how dark energy sort of wins over really large distances, 0:15:56.800 --> 0:15:59.760 but for short distances it's too weak. Well, gravity is 0:15:59.800 --> 0:16:03.120 the opposite. Gravity gets really weak at large distances, so 0:16:03.200 --> 0:16:06.600 like between superclusters you hardly feel anything. But short distances, 0:16:06.640 --> 0:16:09.160 like just between two galaxies or between the Sun and 0:16:09.200 --> 0:16:12.560 the Earth, gravity totally dominates dark energy right like. 0:16:12.600 --> 0:16:15.920 Right now, even where I'm sitting, space is expanding, right like, 0:16:15.920 --> 0:16:17.280 if I look at my hand in front of me, 0:16:17.560 --> 0:16:20.240 the space that my hand is in is technically expanding, 0:16:20.280 --> 0:16:22.280 just like the rest of the universe is just expanding 0:16:22.400 --> 0:16:24.800 so little that the force is keeping my hand together 0:16:25.440 --> 0:16:28.520 when over the expansion of the space. 0:16:28.720 --> 0:16:31.240 Yeah, that's exactly right. And in the case of your 0:16:31.240 --> 0:16:34.680 body and your hand, the forces are electromagnetic, those chemical 0:16:34.720 --> 0:16:37.600 bonds are tying everything together, So the hubble flow is 0:16:37.640 --> 0:16:40.280 like the gentless little current that your body can overpower. 0:16:40.400 --> 0:16:42.600 Even on the scale of the Solar System, gravity winds 0:16:42.880 --> 0:16:44.960 and the motion of the Earth around the Sun is 0:16:45.040 --> 0:16:47.760 totally dominated just by gravity. Like if it wasn't, we 0:16:47.800 --> 0:16:50.640 might have discovered dark energy much much earlier, because we 0:16:50.640 --> 0:16:53.080 wouldn't have understood the orbits of the planets without it, 0:16:53.120 --> 0:16:55.480 and we have very precise measurements of things happening in 0:16:55.520 --> 0:16:58.600 the Solar System. Any little deviation from gravity we would 0:16:58.640 --> 0:17:01.280 have noticed if it was at all measurable. But dark 0:17:01.400 --> 0:17:03.360 energy is not even measurable on the scale of our 0:17:03.400 --> 0:17:06.200 Solar System. You have to go out to between clusters 0:17:06.240 --> 0:17:08.159 of galaxy in order to see it. So on the 0:17:08.200 --> 0:17:11.360 local scale, dark energy is basically irrelevant, and it's gravity 0:17:11.400 --> 0:17:13.840 that winds over and so these galaxies are moving in 0:17:13.920 --> 0:17:16.440 random directions and feeling tugs from gravity and having all 0:17:16.440 --> 0:17:19.280 sorts of dynamics. We talked about how galaxies merge and 0:17:19.320 --> 0:17:23.240 form together and make superclusters. So this motion relative to 0:17:23.280 --> 0:17:26.639 the hubble flow, that's called the peculiar motion of a galaxy. 0:17:26.920 --> 0:17:29.720 It just means how it's moving relative to the flow 0:17:30.040 --> 0:17:30.440 I see. 0:17:30.520 --> 0:17:33.080 So like, for example, in our Solar System, the space 0:17:33.160 --> 0:17:35.600 is expanding, but our planet is tied to the Sun, 0:17:35.720 --> 0:17:37.560 just like we're tied to the Earth. So like me 0:17:37.680 --> 0:17:40.960 sitting here on Earth and not moving relative to the Earth, 0:17:41.000 --> 0:17:43.960 there is sort of almost like space flowing through me 0:17:44.040 --> 0:17:46.440 as it expands through. 0:17:46.200 --> 0:17:50.320 Me exactly, So your peculiar motion is towards the Earth, 0:17:50.320 --> 0:17:53.360 and the Earth's peculiar motion is towards you. And then 0:17:53.400 --> 0:17:56.320 on the scale of galaxies, like the Milky Way's peculiar 0:17:56.359 --> 0:17:59.639 motion is relative to Andromeda, and Andromeda's peculiar motion is 0:17:59.640 --> 0:18:02.560 relative to the Milky Way. If we only were following 0:18:02.560 --> 0:18:05.040 the Hubble flow and had no peculiar motion, then the 0:18:05.080 --> 0:18:07.760 space between us would be expanding, right, the distance would 0:18:07.760 --> 0:18:10.320 be growing. But it's not. Milky Way Andromeda are coming 0:18:10.359 --> 0:18:13.200 towards each other. And that's the case for lots of galaxies, 0:18:13.400 --> 0:18:15.320 even the ones that aren't going to smash into each other. 0:18:15.440 --> 0:18:18.040 They're all basically pointing in random directions. Very few are 0:18:18.080 --> 0:18:20.359 like at rest relative to the Hubble flow. 0:18:20.640 --> 0:18:23.960 So like our galaxy and Andromena galaxy are moving relative 0:18:24.000 --> 0:18:27.359 to each other. And there's two components to that. You're saying, right, like, 0:18:27.400 --> 0:18:31.800 there's how much we're moving because the space is expanding 0:18:31.840 --> 0:18:35.080 between us, and there's also how much we're moving if 0:18:35.080 --> 0:18:37.119 the space wasn't expanding between us. 0:18:37.240 --> 0:18:39.640 Yeah, that's exactly right. You can think about it either 0:18:39.760 --> 0:18:42.160