WEBVTT 00:00:00.000 --> 00:00:02.259 Okay, so welcome to the deep dive everybody. 00:00:02.620 --> 00:00:06.040 Today we are going to be tackling. Sounds fun. 00:00:06.280 --> 00:00:09.740 A really interesting topic that I think a lot 00:00:09.740 --> 00:00:11.240 of people are probably familiar with to some 00:00:11.240 --> 00:00:13.720 degree. But hopefully we'll be able to cover 00:00:13.720 --> 00:00:15.679 it in a new and exciting way today. All right. 00:00:16.179 --> 00:00:21.039 So that is the topic. Of dark matter and dark 00:00:21.039 --> 00:00:24.160 energy. Okay. And for people who aren't already 00:00:24.160 --> 00:00:28.339 aware, this stuff basically makes up. 95 % of 00:00:28.339 --> 00:00:32.039 the universe. Wow. Which is kind of a crazy thing 00:00:32.039 --> 00:00:34.200 to think about. That's a lot we don't know, then. 00:00:34.259 --> 00:00:37.700 It is. So everything that we can see, the stars, 00:00:37.880 --> 00:00:39.899 the planets, the galaxies, all the stuff that 00:00:39.899 --> 00:00:43.759 we're made of, makes up a measly 5%. Really, 00:00:43.920 --> 00:00:47.140 just 5 %? Of the known universe. Wow. So, yeah. 00:00:47.479 --> 00:00:49.740 Kind of crazy. That's more than a little humbling, 00:00:49.840 --> 00:00:52.140 isn't it? It is it is to think everything we've 00:00:52.140 --> 00:00:54.960 ever observed is just a tiny fraction Yeah, and 00:00:54.960 --> 00:00:59.240 it really puts into perspective Just how much 00:00:59.240 --> 00:01:01.600 we still don't know exactly and how much there 00:01:01.600 --> 00:01:03.719 still is out there to learn So I guess you could 00:01:03.719 --> 00:01:05.980 say that we're kind of like explorers going on 00:01:05.980 --> 00:01:08.120 this journey together Okay, like that to make 00:01:08.120 --> 00:01:10.420 sense of these really mind -bending ideas. Yeah, 00:01:10.420 --> 00:01:14.040 let's dive in. We'll do our best to kind of Explain 00:01:14.040 --> 00:01:16.340 them in a way that Hopefully is approachable. 00:01:16.359 --> 00:01:19.159 Yeah and easy to understand but yeah, it's gonna 00:01:19.159 --> 00:01:21.480 be a wild ride I think it's incredible that 95 00:01:21.480 --> 00:01:24.640 % isn't just empty space. It's not like we're 00:01:24.640 --> 00:01:27.519 missing a few puzzle pieces here It's this huge 00:01:27.519 --> 00:01:30.180 unknown presence that's actually shaping the 00:01:30.180 --> 00:01:32.840 universe. Yeah. Yeah, that has a real effect 00:01:32.840 --> 00:01:35.480 Yeah on the things that we can see absolutely 00:01:35.480 --> 00:01:37.840 and that's what makes it so fascinating to study 00:01:37.840 --> 00:01:41.200 exactly So when we talk about dark matter You 00:01:41.200 --> 00:01:43.400 know, we're talking about this really elusive 00:01:43.400 --> 00:01:45.840 thing. You could almost think of it as like the 00:01:45.840 --> 00:01:51.519 universe's shy, introverted cousin. OK. You know, 00:01:51.579 --> 00:01:53.099 it barely interacts with anything. It's like 00:01:53.099 --> 00:01:55.700 the cosmic wallflower. Exactly. You know, it 00:01:55.700 --> 00:01:57.359 doesn't even interact with light. That's why 00:01:57.359 --> 00:01:59.480 it's so hard to even detect. Yeah, that's why 00:01:59.480 --> 00:02:02.180 we can't see it. So let's unpack how we even 00:02:02.180 --> 00:02:04.819 got to the point of knowing that this stuff exists. 00:02:04.879 --> 00:02:07.180 Yeah. How did we figure out something was there 00:02:07.180 --> 00:02:09.830 if we can't see it? Right. So the first clues 00:02:09.830 --> 00:02:13.349 for dark matter actually came from astronomers 00:02:13.349 --> 00:02:16.509 looking at how galaxies spin. Galaxy spin. Yeah. 00:02:16.909 --> 00:02:19.409 So think about our solar system. All right. So 00:02:19.409 --> 00:02:21.830 you have the sun at the center. Right. And then 00:02:21.830 --> 00:02:25.509 the planets orbit around it and the planets further 00:02:25.509 --> 00:02:29.210 away from the sun orbit much slower. That makes 00:02:29.210 --> 00:02:32.169 sense. The sun's gravity is weaker further out. 00:02:32.469 --> 00:02:35.210 Exactly. And that's because the force of gravity 00:02:35.210 --> 00:02:38.340 decreases. with distance. Right, so it's a most 00:02:38.340 --> 00:02:40.199 predictable system. Right, it's like a cosmic 00:02:40.199 --> 00:02:43.060 ballet. Exactly, and so scientists initially 00:02:43.060 --> 00:02:46.319 thought that the same would apply to stars within 00:02:46.319 --> 00:02:49.340 galaxies. Stars closer to the center would be 00:02:49.340 --> 00:02:51.159 zipping around much faster than those further 00:02:51.159 --> 00:02:53.300 out. That makes sense, yeah, because the center 00:02:53.300 --> 00:02:55.659 of the galaxy would have the strongest gravitational 00:02:55.659 --> 00:02:57.699 pull. Right, all that mass concentrated in the 00:02:57.699 --> 00:03:01.740 middle. Right, then they started looking at these... 00:03:02.060 --> 00:03:05.180 distant spiral galaxies. Okay. And they found 00:03:05.180 --> 00:03:08.080 something really weird. What was that? The stars 00:03:08.080 --> 00:03:10.759 at the edges of the galaxies were moving way 00:03:10.759 --> 00:03:13.500 faster than they should have been based on the 00:03:13.500 --> 00:03:17.180 amount of visible matter that was there. So like 00:03:17.180 --> 00:03:20.379 the visible stars and gas weren't enough to explain 00:03:20.379 --> 00:03:22.639 those speeds. Exactly. It was almost as if there 00:03:22.639 --> 00:03:25.819 was way more mass. Holding those galaxies together 00:03:25.819 --> 00:03:27.520 interesting. So like there was some invisible 00:03:27.520 --> 00:03:29.960 source of gravity right then what we could actually 00:03:29.960 --> 00:03:33.139 see that's wild and so this mismatch between 00:03:33.139 --> 00:03:38.319 The observed spees the stars and the amount of 00:03:38.319 --> 00:03:40.960 visible matter was a really strong indication 00:03:40.960 --> 00:03:43.879 That there had to be some kind of invisible matter 00:03:43.879 --> 00:03:46.719 that was providing this extra gravitational pull 00:03:46.719 --> 00:03:49.419 dark matter. Yeah dark matter It's like imagine 00:03:49.419 --> 00:03:53.150 a figure skater spinning really fast. Okay. You 00:03:53.150 --> 00:03:55.750 know, like so fast that their arms should just 00:03:55.750 --> 00:03:58.030 fly off. Yeah. But something invisible is keeping 00:03:58.030 --> 00:04:00.349 them glued on. I like that analogy. It really 00:04:00.349 --> 00:04:02.750 captures the idea. So that invisible force is 00:04:02.750 --> 00:04:04.870 like the dark matter. And it's not just preventing 00:04:04.870 --> 00:04:07.810 galaxies from flying apart. Right. It also played 00:04:07.810 --> 00:04:10.169 a key role in how galaxies formed in the first 00:04:10.169 --> 00:04:12.550 place. Okay, so we know it's there because of 00:04:12.550 --> 00:04:14.569 its effects. Yeah. But the big question is... 00:04:14.569 --> 00:04:16.930 What is it? What is this stuff? So scientists 00:04:16.930 --> 00:04:19.810 have come up with all sorts of different theories. 00:04:20.120 --> 00:04:22.779 about what dark matter could be. And one of the 00:04:22.779 --> 00:04:25.800 most popular ones for a long time has been the 00:04:25.800 --> 00:04:29.399 idea of... Weekly Interacting Massive Particles. 00:04:29.720 --> 00:04:31.579 Weekly Interacting Massive Particles. Which is 00:04:31.579 --> 00:04:35.259 a mouthful. It is, but luckily we have the acronym... 00:04:35.259 --> 00:04:39.639 Soap Knees. Wings. And the name kind of gives 00:04:39.639 --> 00:04:42.300 you some hints about what scientists thought 00:04:42.300 --> 00:04:44.199 these things might be. I break it down for me. 00:04:44.500 --> 00:04:47.019 So massive implies that they have a lot of mass. 00:04:47.259 --> 00:04:49.560 OK, that makes sense. Massive things have a lot 00:04:49.560 --> 00:04:52.639 of mass. Right. And in fact, the source material 00:04:52.639 --> 00:04:55.339 actually mentions that they could have masses 00:04:55.339 --> 00:04:57.300 of several to V. Several to V, that's pretty 00:04:57.300 --> 00:04:59.220 heavy. Yeah. So for anyone who's not a particle 00:04:59.220 --> 00:05:02.279 physicist, which is probably most of us, can 00:05:02.279 --> 00:05:04.439 you put that into perspective? Sure. So a TeV 00:05:04.439 --> 00:05:07.600 is a tera electron volt, which is a unit of energy. 00:05:07.779 --> 00:05:10.199 But in particle physics, we often use it to describe 00:05:10.199 --> 00:05:12.779 mass because of that famous equation, EMCO. Right. 00:05:12.899 --> 00:05:14.680 Energy and mass are basically two sides of the 00:05:14.680 --> 00:05:17.569 same coin. Exactly. So to put that into perspective, 00:05:18.230 --> 00:05:21.069 a proton, which is one of the particles that 00:05:21.069 --> 00:05:24.529 makes up an atom's nucleus, has a mass of about 00:05:24.529 --> 00:05:28.829 one GV or giga electron volt. Okay. So a Tevi 00:05:28.829 --> 00:05:31.110 is a thousand times larger than that. So we're 00:05:31.110 --> 00:05:33.670 talking about particles that are potentially... 00:05:33.529 --> 00:05:35.949 thousands of times more massive than a proton. 00:05:36.149 --> 00:05:38.629 That's right. These are heavy hitters in the 00:05:38.629 --> 00:05:41.170 particle world. Wow. And because they are so 00:05:41.170 --> 00:05:42.949 massive, it's thought that they wouldn't be zipping 00:05:42.949 --> 00:05:45.170 around at the speed of light. OK. Instead, they'd 00:05:45.170 --> 00:05:48.209 be moving much slower, what we call cold dark 00:05:48.209 --> 00:05:51.029 matter. Cold dark matter. Yeah. So their average 00:05:51.029 --> 00:05:53.230 speed would be well below the speed of light. 00:05:53.529 --> 00:05:55.209 And why is that important, the speed? I mean. 00:05:55.610 --> 00:05:57.790 Well, it turns out that the speed of dark matter 00:05:57.790 --> 00:06:00.910 particles would have had a major impact on how 00:06:00.910 --> 00:06:02.850 galaxies formed. Oh, really? Yeah. If they were 00:06:02.850 --> 00:06:04.949 moving too fast, they wouldn't clump together 00:06:04.949 --> 00:06:06.970 gravitationally. OK. And they wouldn't be able 00:06:06.970 --> 00:06:09.449 to provide the scaffolding for galaxies to form. 00:06:09.449 --> 00:06:11.649 Oh, I see. They need to be slow enough to kind 00:06:11.649 --> 00:06:14.310 of hang out in one place and exert their gravitational 00:06:14.310 --> 00:06:16.649 influence. Like the cosmic glue that holds everything 00:06:16.649 --> 00:06:19.550 together. Exactly. OK. So we've got massive. 00:06:19.930 --> 00:06:21.589 Let's talk about the weakly interacting part. 00:06:21.670 --> 00:06:23.709 Right. Because that's the other half of the name. 00:06:23.829 --> 00:06:26.310 Right. So this means that they're predicted to 00:06:26.310 --> 00:06:29.720 only interact with other matter. through the 00:06:29.720 --> 00:06:32.180 weak nuclear force. The weak nuclear force. Okay, 00:06:32.279 --> 00:06:34.540 so that's one of the four fundamental forces 00:06:34.540 --> 00:06:37.720 of nature. Right. But as the name suggests, it's 00:06:37.720 --> 00:06:40.759 very weak. And it only acts over extremely short 00:06:40.759 --> 00:06:43.560 distances. Yeah, it's kind of like the shy introvert 00:06:43.560 --> 00:06:46.480 of the force world. So because of this, the chances 00:06:46.480 --> 00:06:48.939 of a wimp actually interacting with a regular 00:06:48.939 --> 00:06:52.660 particle are incredibly low. Yeah, like astronomically 00:06:52.660 --> 00:06:55.420 low. Which makes them... extremely difficult 00:06:55.420 --> 00:06:57.319 to detect. Is it trying to find a needle in a 00:06:57.319 --> 00:07:00.339 cosmic haystack? Yeah, and WIMP, the source mentions 00:07:00.339 --> 00:07:03.519 that WIMPs are predicted to exist in theories 00:07:03.519 --> 00:07:05.899 like supersymmetry and extra dimensions. Right, 00:07:05.939 --> 00:07:07.800 so those are some pretty mind -bending concepts. 00:07:07.920 --> 00:07:10.360 Yeah, so for those of us who aren't theoretical 00:07:10.360 --> 00:07:13.560 physicists, can you kind of give us a simplified 00:07:13.560 --> 00:07:16.639 explanation of what those are? Sure, so supersymmetry 00:07:16.639 --> 00:07:19.670 is essentially a theory that proposes that. For 00:07:19.670 --> 00:07:22.089 every known particle in the universe, there's 00:07:22.089 --> 00:07:24.230 a corresponding partner particle. OK, like a 00:07:24.230 --> 00:07:27.129 mirror image. Exactly. And some of these hypothetical 00:07:27.129 --> 00:07:30.149 partner particles could have the properties of 00:07:30.149 --> 00:07:32.569 WIMPs. So it's like a whole new set of particles 00:07:32.569 --> 00:07:34.769 that we haven't discovered yet. Right, and extra 00:07:34.769 --> 00:07:37.569 dimensions is the idea that our universe might 00:07:37.569 --> 00:07:40.329 have more than the three spatial dimensions that 00:07:40.329 --> 00:07:42.569 we're familiar with. Right, length, width, and 00:07:42.569 --> 00:07:45.709 height. Exactly. So imagine like a fourth spatial 00:07:45.709 --> 00:07:49.139 dimension. that we can't perceive directly. And 00:07:49.139 --> 00:07:52.300 these extra dimensions could contain particles 00:07:52.300 --> 00:07:54.879 that behave like dark matter? OK, so it's like 00:07:54.879 --> 00:07:58.399 there's this whole hidden realm that we can't 00:07:58.399 --> 00:08:00.339 see. Kind of like a shadow universe. Where dark 00:08:00.339 --> 00:08:02.959 matter might be hanging out. Yeah, it's a fascinating 00:08:02.959 --> 00:08:05.360 idea. It is. So there have been a lot of experiments 00:08:05.360 --> 00:08:10.149 to try and detect WIMPs. One of those famous 00:08:10.149 --> 00:08:13.009 ones was the Large Underground Xenon Detector, 00:08:13.250 --> 00:08:15.730 or LUX. LUX, yeah, I've heard of that one. And 00:08:15.730 --> 00:08:19.550 this thing was built deep underground. Why underground? 00:08:19.629 --> 00:08:21.870 To shield it from all the other radiation. Oh, 00:08:21.870 --> 00:08:23.430 that makes sense. You don't want interference 00:08:23.430 --> 00:08:24.930 from other particles. Right, you want to make 00:08:24.930 --> 00:08:27.589 sure that if you detect something, it's actually 00:08:27.589 --> 00:08:30.509 a wimp, and not just some other random particle. 00:08:30.949 --> 00:08:32.870 Smart. So they ran this experiment for several 00:08:32.870 --> 00:08:38.269 years, but unfortunately, they didn't find... 00:08:38.379 --> 00:08:41.639 any conclusive evidence of wimps. Oh, that's 00:08:41.639 --> 00:08:44.860 a shame. It is. But the search continues with 00:08:44.860 --> 00:08:46.940 even more sensitive experiments being planned. 00:08:47.019 --> 00:08:48.919 That's the spirit of science. Never give up. 00:08:49.080 --> 00:08:52.360 Exactly. So wimps are still a possibility, but 00:08:52.360 --> 00:08:54.620 the jury's still out. Fair enough. So let's move 00:08:54.620 --> 00:08:57.580 on to another popular dark matter candidate. 00:08:57.600 --> 00:09:00.840 OK. And that is axions. Axions, all right. And 00:09:00.840 --> 00:09:03.220 these are very different from wimps. How so? 00:09:03.299 --> 00:09:05.980 Well, for starters, they're predicted to be incredibly 00:09:05.980 --> 00:09:08.370 light. Okay, so instead of massive, we have super 00:09:08.370 --> 00:09:10.629 light. Right. The source mentions that they could 00:09:10.629 --> 00:09:13.450 have masses on the order of microelectron volts. 00:09:14.110 --> 00:09:16.529 Microelectron volts? Wow, that's tiny! Yeah, 00:09:16.549 --> 00:09:18.649 so compared to wimps, they're like feathers. 00:09:18.970 --> 00:09:21.029 So what about their interactions? Are they also 00:09:21.029 --> 00:09:24.350 weakly interacting? Even more so. Axions are 00:09:24.350 --> 00:09:27.269 not predicted to interact through the electromagnetic 00:09:27.269 --> 00:09:30.389 force. Yeah. Or the strong nuclear force. So 00:09:30.389 --> 00:09:33.309 they're practically invisible to us, then. Yeah. 00:09:33.549 --> 00:09:35.710 And not affected by the force that holds atomic 00:09:35.710 --> 00:09:37.730 nuclei together. Right. So they're incredibly 00:09:37.730 --> 00:09:40.169 difficult to detect. So if they're so light and 00:09:40.169 --> 00:09:42.129 barely interact with anything, how could they 00:09:42.129 --> 00:09:44.230 possibly be dark matter? Well, that's a good 00:09:44.230 --> 00:09:48.169 question. And it turns out that they're very 00:09:48.169 --> 00:09:50.990 weak interactions. Yeah. And incredibly light 00:09:50.990 --> 00:09:54.269 mass actually make them very stable. Stable? 00:09:54.330 --> 00:09:56.700 Yeah. They could essentially exist. Yeah. for 00:09:56.700 --> 00:09:59.179 the entire age of the universe. OK, so they're 00:09:59.179 --> 00:10:01.840 like the cosmic Methuselahs of particles. Exactly. 00:10:02.000 --> 00:10:04.120 And that fits in really nicely with the fact 00:10:04.120 --> 00:10:08.759 that we know dark matter has been around for 00:10:08.759 --> 00:10:10.620 billions of years. That makes sense. And despite 00:10:10.620 --> 00:10:13.940 the challenges, there are actually experiments 00:10:13.940 --> 00:10:16.620 trying to detect axions. Oh, wow, really? Yeah, 00:10:16.620 --> 00:10:18.799 like the ADMX experiment. ADMX, what does that 00:10:18.799 --> 00:10:20.940 stand for? It stands for the Axion Dark Matter 00:10:20.940 --> 00:10:23.379 Experiment. Catchy. And there's also the Casper 00:10:23.379 --> 00:10:25.820 experiment. Casper? OK. I don't even want to 00:10:25.820 --> 00:10:27.620 know what that stands for. Yeah, it's a bit of 00:10:27.620 --> 00:10:29.919 a mouthful. But basically, these experiments 00:10:29.919 --> 00:10:33.480 are trying to observe the really subtle effects 00:10:33.480 --> 00:10:38.200 that axions might have on things like electromagnetism 00:10:38.200 --> 00:10:41.700 and the spins of atomic nuclei. So it's like 00:10:41.700 --> 00:10:43.960 they're looking for the faintest whisper against 00:10:43.960 --> 00:10:46.639 the roar of the universe. Exactly. It's a really 00:10:46.639 --> 00:10:49.460 difficult thing to do. I bet. OK, so we've talked 00:10:49.460 --> 00:10:52.539 about massive and weakly interacting particles. 00:10:52.679 --> 00:10:55.960 Limpies. We've talked about. super light, and 00:10:55.960 --> 00:10:59.240 even more weakly interacting particles. Axions. 00:10:59.440 --> 00:11:01.399 So what's next? Well, there's another interesting 00:11:01.399 --> 00:11:04.419 candidate, sterile neutrinos. Sterile neutrinos. 00:11:04.659 --> 00:11:07.200 OK, so we already know that neutrinos exist. 00:11:07.360 --> 00:11:09.299 Right, those tiny little ghosts of the particle 00:11:09.299 --> 00:11:12.120 world. Yeah, they're these incredibly tiny particles 00:11:12.120 --> 00:11:15.220 that interact through the weak nuclear force. 00:11:16.009 --> 00:11:18.370 and they're incredibly difficult to detect. But 00:11:18.370 --> 00:11:20.509 we have managed to detect them, right? We have, 00:11:20.610 --> 00:11:22.750 with these massive underground detectors. Yeah, 00:11:22.889 --> 00:11:24.850 those huge golden chamber detectors that look 00:11:24.850 --> 00:11:26.750 like something out of a sci -fi movie. Right, 00:11:26.929 --> 00:11:29.129 so how are sterile neutrinos different? Well, 00:11:29.210 --> 00:11:32.429 the key difference is that they're theorized 00:11:32.429 --> 00:11:35.389 to interact solely through gravity. Just gravity? 00:11:35.509 --> 00:11:38.210 Yeah, no weak force, no electromagnetic force, 00:11:38.389 --> 00:11:41.370 nothing else. Wow, so they're even more isolated 00:11:41.370 --> 00:11:44.750 than whips. True cosmic loners. Yeah, it's like 00:11:44.750 --> 00:11:47.230 they're practically ghosts. So how could we ever 00:11:47.230 --> 00:11:49.509 hope to find something that only interacts through 00:11:49.509 --> 00:11:51.750 gravity? Ah, that's a great question. And the 00:11:51.750 --> 00:11:54.389 truth is, it would be incredibly difficult. But 00:11:54.389 --> 00:11:56.809 the source does mention a couple of possible 00:11:56.809 --> 00:11:59.950 detection methods. One is by looking for x -ray 00:11:59.950 --> 00:12:03.350 emissions from galaxy clusters, which might be 00:12:03.350 --> 00:12:05.950 produced if sterile neutrinos decay. Interesting. 00:12:06.029 --> 00:12:08.370 And another possibility is to search for subtle 00:12:08.370 --> 00:12:11.220 imprints they might have left. on the cosmic 00:12:11.220 --> 00:12:14.379 microwave background radiation. The CMB. Exactly 00:12:14.379 --> 00:12:16.139 that afterglow of the Big Bang. So it's like, 00:12:16.440 --> 00:12:18.759 we're trying to look for the faintest shadows 00:12:18.759 --> 00:12:22.000 that they might have cast on the universe. Pretty 00:12:22.000 --> 00:12:24.740 much. But as with all these candidates, we haven't 00:12:24.740 --> 00:12:28.440 actually found any direct evidence of their existence. 00:12:28.539 --> 00:12:31.600 So for now, they remain hypothetical. OK, so 00:12:31.600 --> 00:12:33.519 moving on, we have another interesting idea, 00:12:33.639 --> 00:12:37.879 and that is dark photons. Dark photons. So. Are 00:12:37.879 --> 00:12:41.159 these like regular photons? The dark. That's 00:12:41.159 --> 00:12:44.059 a good way to think about it. Ah. But instead 00:12:44.059 --> 00:12:47.200 of carrying the electromagnetic force, they would 00:12:47.200 --> 00:12:50.679 carry a new fundamental force. OK. A dark force, 00:12:50.720 --> 00:12:53.059 if you will. A dark force. That would only interact 00:12:53.059 --> 00:12:55.980 between dark matter particles. OK. It's like 00:12:55.980 --> 00:12:59.570 dark matter has its own. separate communication 00:12:59.570 --> 00:13:01.789 network. You got it. That we're not privy to. 00:13:02.309 --> 00:13:05.429 Exactly. Interesting. So if dark matter is made 00:13:05.429 --> 00:13:07.909 up of particles that can talk to each other through 00:13:07.909 --> 00:13:10.850 this dark force, it could potentially explain 00:13:10.850 --> 00:13:13.710 some of the things we observe about its distribution 00:13:13.710 --> 00:13:16.350 in the universe. Like what? Well for example, 00:13:16.809 --> 00:13:19.389 the distribution of dark matter in dwarf galaxies 00:13:19.389 --> 00:13:22.669 doesn't quite match. what our simulations predict. 00:13:23.470 --> 00:13:25.470 If dark matter particles were only interacting 00:13:25.470 --> 00:13:28.169 through gravity, we'd expect to see a much denser 00:13:28.169 --> 00:13:30.309 concentration of dark matter in the centers of 00:13:30.309 --> 00:13:32.590 these galaxies. But that's not what we observe. 00:13:32.649 --> 00:13:35.250 So the dark force could smooth things out a bit. 00:13:35.789 --> 00:13:38.330 Exactly. If these particles can interact with 00:13:38.330 --> 00:13:40.549 each other through this dark force, it could 00:13:40.549 --> 00:13:43.289 lead to a more even distribution of dark matter. 00:13:43.590 --> 00:13:46.649 And the source mentions that. These dark photons 00:13:46.649 --> 00:13:50.629 might even be able to mix. with regular photons. 00:13:50.789 --> 00:13:54.129 Mix. Yeah. It's a theoretical possibility. And 00:13:54.129 --> 00:13:56.629 if it happens, it could lead to some really subtle 00:13:56.629 --> 00:13:59.549 signals that we might be able to detect. OK, 00:13:59.549 --> 00:14:01.289 so that would be a way of indirectly detecting 00:14:01.289 --> 00:14:04.690 them. Exactly. But so far, no such luck. OK, 00:14:04.690 --> 00:14:07.429 so no evidence yet. Not yet, but the search continues. 00:14:07.669 --> 00:14:10.529 So we've gone through this whole list of potential 00:14:10.529 --> 00:14:14.309 dark matter particles. But what if dark matter 00:14:14.309 --> 00:14:16.509 isn't a particle at all? Now you're talking. 00:14:16.590 --> 00:14:20.019 Yeah. And that brings us to the idea of emergent 00:14:20.019 --> 00:14:22.080 gravity. Emergent gravity, that sounds pretty 00:14:22.080 --> 00:14:24.659 radical. It is. It's a completely different way 00:14:24.659 --> 00:14:27.259 of thinking about dark matter. All right. I'm 00:14:27.259 --> 00:14:29.360 intrigued. Lay it on me. So instead of saying 00:14:29.360 --> 00:14:34.039 that there's this new type of particle that we 00:14:34.039 --> 00:14:37.100 haven't discovered yet, emergent gravity proposes. 00:14:37.940 --> 00:14:40.840 That gravity itself isn't what we think it is. 00:14:40.980 --> 00:14:43.279 Whoa, hold on. You're saying gravity isn't fundamental. 00:14:43.580 --> 00:14:46.100 Well. Not in the way we currently understand 00:14:46.100 --> 00:14:48.039 it. Emergent gravity suggests that. Gravity might 00:14:48.039 --> 00:14:50.620 actually emerge from a more fundamental phenomenon. 00:14:50.659 --> 00:14:53.440 And what phenomenon would that be? Quantum entanglement. 00:14:53.639 --> 00:14:55.779 Quantum entanglement, you mean that spoopy action 00:14:55.779 --> 00:14:58.120 at a distance? The one and only. So as you know, 00:14:58.620 --> 00:15:01.960 quantum entanglement is this really strange connection 00:15:01.960 --> 00:15:05.539 where two particles become linked no matter how 00:15:05.539 --> 00:15:07.179 far apart they are. Right, it's like they're 00:15:07.179 --> 00:15:10.059 communicating instantaneously even across vast 00:15:10.059 --> 00:15:13.860 distances. And so the idea with emergent gravity 00:15:14.220 --> 00:15:18.139 is that the effects that we attribute to dark 00:15:18.139 --> 00:15:20.799 matter, like the faster than expected rotation 00:15:20.799 --> 00:15:24.470 of galaxies, might actually be caused by... this 00:15:24.470 --> 00:15:28.490 intricate web of quantum entanglement that permeates 00:15:28.490 --> 00:15:30.750 the universe. So it's like gravity is an emergent 00:15:30.750 --> 00:15:33.590 property of this underlying quantum entanglement. 00:15:33.789 --> 00:15:36.450 Exactly. It's a really mind -blowing concept. 00:15:36.730 --> 00:15:38.629 It really challenges our understanding of what 00:15:38.629 --> 00:15:40.750 gravity is. Yeah, and it's important to point 00:15:40.750 --> 00:15:44.230 out that this is still a very theoretical idea. 00:15:44.509 --> 00:15:47.559 There's no direct experimental evidence. to support 00:15:47.559 --> 00:15:49.919 it yet but it's a fascinating possibility it 00:15:49.919 --> 00:15:51.980 is okay so we've covered a lot of ground we have 00:15:51.980 --> 00:15:54.039 on dark matter now let's shift gears okay talk 00:15:54.039 --> 00:15:57.139 about the other big mysterious component hmm 00:15:57.139 --> 00:16:01.200 of the universe dark energy dark energy the cosmic 00:16:01.200 --> 00:16:04.580 accelerator yeah so this is the force that's 00:16:04.580 --> 00:16:07.159 thought to be responsible for the accelerating 00:16:07.159 --> 00:16:09.740 expansion of the universe. Right. So for billions 00:16:09.740 --> 00:16:11.840 of years, the universe has been expanding. Since 00:16:11.840 --> 00:16:14.120 the Big Bang. Exactly. Right. And if gravity 00:16:14.120 --> 00:16:16.919 was the only force at play, we'd expect that 00:16:16.919 --> 00:16:19.440 expansion to slow down. Right. Because gravity 00:16:19.440 --> 00:16:23.379 pulls things together. But in the late 1990s, 00:16:23.799 --> 00:16:26.059 astronomers made a shocking discovery. What was 00:16:26.059 --> 00:16:28.279 that? The expansion of the universe wasn't slowing 00:16:28.279 --> 00:16:30.879 down. It was speeding up. Speeding up. Yeah, 00:16:30.879 --> 00:16:32.879 like someone had stepped on the cosmic gas pedal. 00:16:33.179 --> 00:16:35.960 OK, so what could possibly be causing that? Well, 00:16:35.960 --> 00:16:38.480 that's the million dollar question. And the leading 00:16:38.480 --> 00:16:41.620 explanation is dark energy. Dark energy. It's 00:16:41.620 --> 00:16:44.600 mysterious force that's pushing everything apart. 00:16:44.720 --> 00:16:46.820 And it's not just pushing things apart. It's 00:16:46.820 --> 00:16:49.659 pushing them apart at an accelerating rate. Right. 00:16:49.679 --> 00:16:51.700 So the expansion is getting faster and faster. 00:16:51.940 --> 00:16:55.659 OK. So what are some of the leading theories 00:16:55.659 --> 00:16:59.360 about what dark energy could be? All right. Well, 00:16:59.399 --> 00:17:02.779 one of the simplest explanations. is the cosmological 00:17:02.779 --> 00:17:06.519 constant. And this idea actually goes all the 00:17:06.519 --> 00:17:09.380 way back to Einstein. Oh, wow. Yeah, he originally 00:17:09.380 --> 00:17:12.240 introduced it as a way to keep the universe static. 00:17:12.480 --> 00:17:15.269 Static? Because back then, people thought the 00:17:15.269 --> 00:17:17.309 universe was eternal and unchanging. Right, the 00:17:17.309 --> 00:17:19.769 steady state model. But later when it was discovered 00:17:19.769 --> 00:17:22.190 that the universe was actually expanding, he 00:17:22.190 --> 00:17:24.670 abandoned the cosmological constant, calling 00:17:24.670 --> 00:17:26.930 it his biggest blunder. Oh, the irony. Right. 00:17:27.150 --> 00:17:29.309 But now the cosmological constant is back. And 00:17:29.309 --> 00:17:32.390 it's being used to explain the accelerating expansion. 00:17:32.549 --> 00:17:35.269 Exactly. So what is it exactly? It's basically 00:17:35.269 --> 00:17:39.369 the idea that empty space itself has energy. 00:17:39.480 --> 00:17:42.119 Empty space having energy, that's pretty mind 00:17:42.119 --> 00:17:45.220 -blowing. Yeah, and this energy is thought to 00:17:45.220 --> 00:17:47.140 have a negative pressure. Negative pressure. 00:17:47.140 --> 00:17:50.720 What does that mean? Well, it means that instead 00:17:50.720 --> 00:17:53.380 of pulling things together, like gravity does, 00:17:53.700 --> 00:17:56.799 it pushes them apart. Oh, I see. And so this 00:17:56.799 --> 00:18:00.019 energy of empty space is what's thought to be 00:18:00.019 --> 00:18:02.720 driving the accelerating expansion. OK, but where 00:18:02.720 --> 00:18:06.240 does this energy come from? Well, that's where 00:18:06.240 --> 00:18:07.940 things get a little tricky. One way to think 00:18:07.940 --> 00:18:10.599 about it. Is in terms of quantum fluctuations 00:18:10.599 --> 00:18:13.539 quantum fluctuations. So in the quantum world 00:18:13.539 --> 00:18:16.279 Empty space. It's actually empty. It's full of 00:18:16.279 --> 00:18:20.279 these Virtual particles that are constantly popping 00:18:20.279 --> 00:18:23.140 into and out of existence So like a constant 00:18:23.140 --> 00:18:25.759 sea of particles being created and destroyed 00:18:25.759 --> 00:18:28.640 exactly and these particles Have energy. Okay, 00:18:28.640 --> 00:18:30.299 so that's the source of the energy. Yeah, and 00:18:30.299 --> 00:18:32.859 so the energy associated with these quantum fluctuations 00:18:32.859 --> 00:18:37.609 Is what's thought to contribute? To the cosmological 00:18:37.609 --> 00:18:39.849 constant that makes sense, but there's a problem 00:18:39.849 --> 00:18:43.329 a problem. What is it a big one? Oh when physicists 00:18:43.329 --> 00:18:46.710 try to calculate The value of the cosmological 00:18:46.710 --> 00:18:49.569 constant based on quantum field theory they get 00:18:49.569 --> 00:18:52.009 an answer. That's way too big way too big How 00:18:52.009 --> 00:18:54.369 big are you talking? We're talking 120 orders 00:18:54.369 --> 00:18:57.109 of magnitude 120 orders magnitude. That's a one 00:18:57.109 --> 00:18:59.890 with 120 zeros after it Yeah, so it's like trying 00:18:59.890 --> 00:19:03.490 to measure the distance to the moon and being 00:19:03.490 --> 00:19:07.089 off by the size of the entire solar system. Wow, 00:19:07.130 --> 00:19:09.170 that's not just a little off that's astronomically 00:19:09.170 --> 00:19:11.829 off. It is. And so this is a huge problem for 00:19:11.829 --> 00:19:14.410 the cosmological constant idea. Yeah, it's a 00:19:14.410 --> 00:19:18.109 major discrepancy. Because it suggests that there's 00:19:18.109 --> 00:19:20.650 something fundamental that we're missing. About 00:19:20.650 --> 00:19:23.009 how quantum mechanics and gravity work together. 00:19:23.190 --> 00:19:26.289 Exactly. So while the cosmological constant is 00:19:26.289 --> 00:19:29.390 the simplest explanation for dark energy, it 00:19:29.390 --> 00:19:31.809 also comes with this huge baggage. Yeah, a lot 00:19:31.809 --> 00:19:33.130 of unanswered questions.