WEBVTT 1 00:00:03.399 --> 00:00:07.719 Welcome to Bedtime Astronomy. Explore the wonders of the cosmos 2 00:00:07.759 --> 00:00:12.279 with our soothing Bedtime Astronomie podcast. Each episode offers a 3 00:00:12.359 --> 00:00:16.320 gentle journey through the stars, planets, and beyond, perfect for 4 00:00:16.399 --> 00:00:20.239 unwinding after a long day. Let's travel through the mysteries 5 00:00:20.239 --> 00:00:22.440 of the universe as you drift off into a peaceful 6 00:00:22.480 --> 00:00:23.800 slumber under the night sky. 7 00:00:26.879 --> 00:00:29.800 Hello everyone, and welcome back to the show. We have 8 00:00:30.679 --> 00:00:33.679 a truly massive topic on the table today. And I 9 00:00:33.679 --> 00:00:36.280 don't mean massive in the you know, the casual sense. 10 00:00:36.320 --> 00:00:39.479 I mean we are literally talking about the heaviest, densest 11 00:00:39.560 --> 00:00:41.600 objects in the entire universe. 12 00:00:41.679 --> 00:00:42.280 We really are. 13 00:00:42.359 --> 00:00:45.280 We're looking at a paper that just dropped yesterday February twelve, 14 00:00:45.359 --> 00:00:47.960 twenty twenty six, in Physical Review Letters. 15 00:00:48.159 --> 00:00:50.159 Yeah, it's about as hot off the press as it 16 00:00:50.200 --> 00:00:51.079 gets in this field. 17 00:00:51.240 --> 00:00:53.679 It really is. And I have to say, reading through 18 00:00:53.679 --> 00:00:55.600 it this morning, it just felt like one of those 19 00:00:55.640 --> 00:00:58.439 moments where you realize we might have been looking at 20 00:00:58.439 --> 00:01:01.159 the sky in well in the wrong way for the 21 00:01:01.240 --> 00:01:02.200 last fifty years. 22 00:01:02.359 --> 00:01:05.480 That's honestly not much of an exaggeration. This paper. It's 23 00:01:05.480 --> 00:01:07.840 by Henksy Wang and a team from the Max Planck 24 00:01:07.920 --> 00:01:11.719 Institute in Oxford. It takes a problem that has frustrated 25 00:01:11.760 --> 00:01:15.920 astrophysicists for decades and just completely flips it on its head. 26 00:01:15.959 --> 00:01:18.200 It really does. It turns a limitation into a tool. 27 00:01:18.439 --> 00:01:18.920 Exactly. 28 00:01:19.079 --> 00:01:22.719 We are talking about the universe as hidden strobes. More specifically, 29 00:01:22.719 --> 00:01:27.760 we're talking about supermassive black hole binaries, two absolute monsters 30 00:01:27.799 --> 00:01:30.200 locked in a death spiral, hiding in the dark. 31 00:01:30.480 --> 00:01:33.400 And until yesterday, hiding was definitely the right word. 32 00:01:33.719 --> 00:01:36.599 Right, So let's set the stage here for everyone, because 33 00:01:36.640 --> 00:01:40.239 I think most people have this image of a black 34 00:01:40.280 --> 00:01:45.400 hole as this lonely cosmic vacuum cleaner floating out there by. 35 00:01:45.239 --> 00:01:47.239 Itself, a solitary monster. 36 00:01:47.599 --> 00:01:49.640 Yeah, but that's not really the whole story. 37 00:01:49.359 --> 00:01:51.840 Is it not at all? I mean, sure, solitary black 38 00:01:51.840 --> 00:01:54.200 holes exist, of course, But if you look at the 39 00:01:54.239 --> 00:01:58.680 grand structure of the cosmos, the whole thing is hierarchical. 40 00:01:59.079 --> 00:02:01.680 Galaxies grow by eating other galaxies. 41 00:02:01.760 --> 00:02:03.840 It's galactic cannibalism. 42 00:02:03.480 --> 00:02:07.200 It is. It's a violent, messy history. Our own Milky 43 00:02:07.239 --> 00:02:10.159 Way has eaten smaller dwarf galaxies in the past, and 44 00:02:10.199 --> 00:02:13.439 we're famously on a collision course with Andromeda. 45 00:02:12.919 --> 00:02:16.240 Which is by the way still terrifying, but that's a 46 00:02:16.280 --> 00:02:18.759 problem for a few billion years from now, I guess, right. 47 00:02:18.840 --> 00:02:20.680 But just think about the architecture for a second. We 48 00:02:20.719 --> 00:02:23.240 know that pretty much every massive galaxy, including our own, 49 00:02:23.400 --> 00:02:26.479 has a superassive black hole in SMBH at its heart. 50 00:02:26.639 --> 00:02:31.280 Ours is SAGITTARIUSA, SAGITTARIUSA exactly, Andromeda has one that's much bigger. 51 00:02:31.560 --> 00:02:34.800 So when Galaxy A smashes into Galaxy B, the. 52 00:02:34.800 --> 00:02:36.520 Two big black holes have to go somewhere. 53 00:02:36.599 --> 00:02:39.719 They have to. They don't just vanish. As the two 54 00:02:39.759 --> 00:02:43.080 galaxies merge and all the stars and gas settle down 55 00:02:43.080 --> 00:02:47.280 into a new larger structure. Those two central black holes 56 00:02:47.599 --> 00:02:49.319 they have to interact. They sink. 57 00:02:49.639 --> 00:02:52.280 They sink towards the middle of the new galaxy. 58 00:02:51.960 --> 00:02:54.719 Right through a process we call dynamical friction. They sink 59 00:02:54.759 --> 00:02:58.719 toward the new center of gravity, and so logic dictates 60 00:02:59.000 --> 00:03:00.479 they must eventually find each. 61 00:03:00.360 --> 00:03:03.840 Other and form a pair a binary. 62 00:03:03.479 --> 00:03:06.759 A binary two super massive black holes orbiting each other. 63 00:03:07.199 --> 00:03:09.560 And this is the invisible dance we're talking about. The 64 00:03:09.599 --> 00:03:12.960 physics says they have to be there. All our models 65 00:03:13.000 --> 00:03:15.879 of how galaxies evolve say the universe should be well 66 00:03:16.080 --> 00:03:17.080 teeming with these. 67 00:03:16.919 --> 00:03:18.800 Things, it should be a common phenomenon. 68 00:03:18.960 --> 00:03:21.960 But when we point our best telescopes at those centers 69 00:03:22.000 --> 00:03:24.800 of these big merged galaxies, we see nothing. 70 00:03:24.639 --> 00:03:27.240 Well, not quite nothing. We see the galaxy the bright core. 71 00:03:27.800 --> 00:03:30.919 But trying to resolve two distinct black holes that are 72 00:03:31.039 --> 00:03:35.840 orbiting closely. It's arguably one of the single hardest observational 73 00:03:35.919 --> 00:03:37.400 challenges in all of astronomy. 74 00:03:37.680 --> 00:03:40.960 Okay, but why I mean we have the event horizon telescope. 75 00:03:40.960 --> 00:03:43.719 We literally took a picture of a black hole shadow 76 00:03:43.759 --> 00:03:46.280 a few years back. Why can't we just zoom in 77 00:03:46.319 --> 00:03:47.080 and see two of them? 78 00:03:47.319 --> 00:03:51.199 It's all about scale and resolution. The event Horizon telescope 79 00:03:51.400 --> 00:03:54.800 was an incredible feat, but it essentially turned the entire 80 00:03:54.879 --> 00:03:58.439 planet Earth into one giant radio dish to look at 81 00:03:58.439 --> 00:04:03.120 two very specific tars targets our own Sagittarius A and 82 00:04:03.159 --> 00:04:04.319 the one in M eighty seven. 83 00:04:04.719 --> 00:04:06.840 So it's not something we can just point anywhere. 84 00:04:07.039 --> 00:04:10.120 No, not at all. And for these binarias, we're talking 85 00:04:10.159 --> 00:04:13.479 about objects that could be millions or even billions of 86 00:04:13.560 --> 00:04:16.800 light years away. Now, when they're still very far apart 87 00:04:16.800 --> 00:04:20.199 from each other, say a few thousand light years, we 88 00:04:20.279 --> 00:04:24.360 can sometimes see them. We have confirmed cases of dual 89 00:04:24.480 --> 00:04:25.959 active galactic nuclei. 90 00:04:26.279 --> 00:04:29.759 So two bright spots and a messy colliding galaxy exactly. 91 00:04:29.800 --> 00:04:32.240 You can resolve them as two distinct points of light. 92 00:04:32.399 --> 00:04:34.040 Okay, so we can see the ones that have just 93 00:04:34.040 --> 00:04:35.079 started dating. 94 00:04:34.920 --> 00:04:36.399 You could put it that way. They're just waving at 95 00:04:36.439 --> 00:04:38.639 each other from across the room. But the ones we 96 00:04:38.720 --> 00:04:41.319 really care about, the ones that are gravitationally bound and 97 00:04:41.360 --> 00:04:47.040 are spiraling in towards an inevitable cataclysmic merger, those are 98 00:04:47.040 --> 00:04:48.120 effectively invisible. 99 00:04:48.279 --> 00:04:50.639 Why because they're too close, way too close. 100 00:04:50.959 --> 00:04:53.639 At that distance, they're separated by less than a light year, 101 00:04:53.720 --> 00:04:56.600 maybe much less. To our telescopes, they just blur together 102 00:04:56.639 --> 00:04:58.040 into a single blob of light. 103 00:04:58.480 --> 00:05:02.079 So they're either hiding in the glare of the galaxy's core. 104 00:05:02.079 --> 00:05:04.720 Or hiding in complete darkness if there isn't a lot 105 00:05:04.759 --> 00:05:07.000 of gas around them for them to eat and light up. 106 00:05:07.279 --> 00:05:11.600 And this has created this huge frustrating gap in our understanding. 107 00:05:11.120 --> 00:05:12.560 A cosmic blind spot. 108 00:05:12.800 --> 00:05:15.079 It is. We see them when they're far apart, and 109 00:05:15.160 --> 00:05:17.959 thanks to detectors like Lego, we can hear them in 110 00:05:18.000 --> 00:05:21.279 the final fraction of a second when they actually crash 111 00:05:21.319 --> 00:05:22.199 together and merge. 112 00:05:22.360 --> 00:05:25.199 But the whole middle part, the millions of years they 113 00:05:25.199 --> 00:05:27.319 spend spiraling closer and closer. 114 00:05:27.519 --> 00:05:30.439 It's a complete black box. We have almost no confirmed 115 00:05:30.439 --> 00:05:34.519 observations of that inspiral phase for supermassive black hole. 116 00:05:34.439 --> 00:05:36.319 Which is where this new paper comes in because the 117 00:05:36.360 --> 00:05:39.879 team from Max Plank in Oxford is basically saying, Okay, 118 00:05:40.000 --> 00:05:41.240 stop looking for the black holes. 119 00:05:41.560 --> 00:05:46.720 Exactly. They're saying, stop trying to resolve the dark objects themselves. Instead, 120 00:05:47.079 --> 00:05:49.560 look at the background, look at the wallpaper, look at 121 00:05:49.600 --> 00:05:53.439 the stars sitting behind them. They are proposing we use 122 00:05:53.519 --> 00:05:57.160 these binary black holes not as targets, but as lenses, 123 00:05:57.639 --> 00:05:59.000 as natural telescopes. 124 00:05:59.040 --> 00:06:00.800 Okay, this is where we need to go into the details. 125 00:06:00.839 --> 00:06:04.000 Because gravitational lensing is a term that gets tossed around 126 00:06:04.040 --> 00:06:06.160 a lot. You always hear the same analogy, you know, 127 00:06:06.199 --> 00:06:08.279 a bowling ball on a trampoline. 128 00:06:07.720 --> 00:06:08.920 The trampoline analogy. 129 00:06:09.040 --> 00:06:10.720 Yeah, that's not going to cut it today. We need 130 00:06:10.759 --> 00:06:11.839 to go deeper, we really do. 131 00:06:11.920 --> 00:06:14.879 The trampoline analogy is well, it's fine for getting the 132 00:06:14.879 --> 00:06:19.360 basic idea across, but it completely fails to explain why 133 00:06:19.439 --> 00:06:21.639 what this paper's proposing is so groundbreaking. 134 00:06:21.759 --> 00:06:24.000 So walk us through the actual mechanism. Let's start with 135 00:06:24.079 --> 00:06:26.920 lensing one oh one, a single black hole, okay. 136 00:06:27.519 --> 00:06:31.720 Einstein's general relativity mass warps the fabric of space time, 137 00:06:32.160 --> 00:06:34.519 and light, which we normally think of as traveling in 138 00:06:34.560 --> 00:06:37.759 a straight line, has to follow the curves in space time. 139 00:06:38.040 --> 00:06:40.600 So it's not that gravity is pulling on the light, 140 00:06:41.040 --> 00:06:43.800 it's that the path itself is bent precisely. 141 00:06:44.519 --> 00:06:48.199 Light is just taking the straightest possible path through a 142 00:06:48.240 --> 00:06:52.920 curved space. So if you have a massive object, the lens, 143 00:06:53.279 --> 00:06:56.360 and a light source directly behind it, the light from 144 00:06:56.399 --> 00:06:58.959 that source has to travel around the lens to get. 145 00:06:58.759 --> 00:07:01.600 To us, flowing around a rock in a stream. 146 00:07:01.879 --> 00:07:05.240 That's a decent analogy. Now, if you have a single 147 00:07:05.519 --> 00:07:09.959 perfectly spherical mass, like a single non spinning black hole, 148 00:07:10.519 --> 00:07:13.920 that lensing effect is very, very symmetric, okay. And if 149 00:07:13.959 --> 00:07:16.560 the alignment is perfect, the background, star, the black hole, 150 00:07:16.600 --> 00:07:18.800 and you, the observer are all in a perfect line. 151 00:07:19.279 --> 00:07:22.199 That symmetry focuses the light into what we call an 152 00:07:22.240 --> 00:07:23.240 Einstein ring. 153 00:07:23.399 --> 00:07:25.519 A perfect circle of light, a perfect circle. 154 00:07:25.560 --> 00:07:28.560 We've seen these. We use them to weigh distant galaxies 155 00:07:28.600 --> 00:07:32.079 and find exoplanets. It's a standard tool in the astronomical toolkit. 156 00:07:32.199 --> 00:07:33.040 But what's the catch. 157 00:07:33.240 --> 00:07:35.959 The catch is that word perfect. For a single spherical 158 00:07:36.040 --> 00:07:38.399 lens that has a single focal point, you have to 159 00:07:38.399 --> 00:07:40.120 be exactly on that line of sight to get that 160 00:07:40.199 --> 00:07:42.560 extreme magnification to see the ring. It's like a laser beam. 161 00:07:42.800 --> 00:07:44.879 So if a background star is just a tiny bit 162 00:07:44.959 --> 00:07:46.600 off to the side of the black hole, we don't 163 00:07:46.639 --> 00:07:47.120 see the ring. 164 00:07:47.319 --> 00:07:51.040 You'll see some distortion. The star might appear as two separate, 165 00:07:51.279 --> 00:07:55.319 slightly brighter images, but you don't get that massive, massive 166 00:07:55.360 --> 00:07:59.639 spike and brightness unless the alignment is just so. It's rare, 167 00:08:00.360 --> 00:08:00.879 very rare. 168 00:08:00.959 --> 00:08:04.759 Okay, that's lensing one oh one. Now enter the second 169 00:08:04.839 --> 00:08:06.920 black hole lensing two to two. 170 00:08:07.399 --> 00:08:10.160 This is where everything changes. You aren't just adding a 171 00:08:10.199 --> 00:08:13.079 second lens next to the first one. You are creating 172 00:08:13.120 --> 00:08:17.480 a complex interference pattern between two deep gravitational wells. 173 00:08:17.560 --> 00:08:20.480 The paper uses this term costic curves, and I know 174 00:08:20.560 --> 00:08:22.800 costic usually means something that burns. 175 00:08:22.519 --> 00:08:25.240 It does, but in optics and in gravitational lensing, it 176 00:08:25.279 --> 00:08:27.800 means something else. Left you back to that swimming pool 177 00:08:27.800 --> 00:08:29.000 analogy I mentioned earlier. 178 00:08:29.040 --> 00:08:30.759 The squiggly lines on the bottom. 179 00:08:30.519 --> 00:08:33.320 Exactly Think about why those lines are there. The surface 180 00:08:33.360 --> 00:08:35.759 of the water isn't flat. It's covered in little waves 181 00:08:35.759 --> 00:08:39.840 and ripples. Right, each ripple acts like a tiny, imperfect lens. 182 00:08:40.039 --> 00:08:40.799 Okay, I'm with you. 183 00:08:41.039 --> 00:08:43.799 So these ripples aren't focusing the sunlight into lots of 184 00:08:43.840 --> 00:08:47.879 little single points. They're focusing the light into lines, into curves. 185 00:08:48.279 --> 00:08:51.000 And those bright, dancing webs of light on the bottom 186 00:08:51.039 --> 00:08:53.360 of the pool, those are the caustics. It's where the 187 00:08:53.440 --> 00:08:56.000 light rays pile up, where the intensity is highest. 188 00:08:56.080 --> 00:08:58.440 Got it. So the water ripples are the imperfect lens, 189 00:08:58.679 --> 00:09:01.159 and the bright lines are the You've got it. 190 00:09:01.519 --> 00:09:04.440 Now take that concept and scale it up to the cosmos. 191 00:09:04.840 --> 00:09:08.799 Instead of ripples on water, you have two supermassive black 192 00:09:08.840 --> 00:09:12.639 holes orbiting each other. Their combined gravity creates a very 193 00:09:12.679 --> 00:09:16.440 complex surface in space time. It's not a smooth bowl anymore. 194 00:09:16.519 --> 00:09:18.559 It has ridges and valleys exactly. 195 00:09:18.679 --> 00:09:22.200 And those gravitational ridges focus the light from background stars, 196 00:09:22.200 --> 00:09:24.720 not into a point, but into a network of high 197 00:09:24.799 --> 00:09:27.639 magnification lines. Those are the caustic curves. 198 00:09:27.879 --> 00:09:28.840 And what do they look like? 199 00:09:29.080 --> 00:09:32.480 For a binary system? This network creates a very specific shape. 200 00:09:32.720 --> 00:09:36.840 The paper describes it as a diamond shaped structure. In mathematics, 201 00:09:36.840 --> 00:09:37.559 it's called an. 202 00:09:37.399 --> 00:09:40.240 Astroid, an asteroid like video game. 203 00:09:40.480 --> 00:09:42.519 Spelled differently, but yeah, it looks a bit like that. 204 00:09:42.600 --> 00:09:45.519 It's a star shape, a diamond with four points, but 205 00:09:45.600 --> 00:09:47.639 the sides are curved inwards. They're concave. 206 00:09:47.919 --> 00:09:52.120 So let me picture this floating in empty space behind 207 00:09:52.159 --> 00:09:55.759 these two invisible black holes. Is this other invisible thing, 208 00:09:55.879 --> 00:09:58.799 this geometric diamond of pure magnification? 209 00:09:59.240 --> 00:10:01.480 That is exactly what it is. And here is the 210 00:10:01.759 --> 00:10:05.480 absolute critical difference between a single black hole and a binary. 211 00:10:06.159 --> 00:10:08.320 For a single black hole, the region of very high 212 00:10:08.360 --> 00:10:11.639 magnification is a tiny point. For a binary, the region 213 00:10:11.679 --> 00:10:16.279 of very high magnification is this entire much larger diamond 214 00:10:16.320 --> 00:10:17.120 shaped perimeter. 215 00:10:17.240 --> 00:10:18.840 So the target is just bigger. 216 00:10:18.759 --> 00:10:22.240 Massively bigger. Ben's Coxus, one of the authors, makes this 217 00:10:22.320 --> 00:10:25.360 exact point in the press release. The chances of a 218 00:10:25.399 --> 00:10:30.159 background star's light being hugely amplified increase enormously for a 219 00:10:30.200 --> 00:10:32.279 binary compared to a single black hole. 220 00:10:32.519 --> 00:10:34.639 It's the difference between trying to hit the bull's eye 221 00:10:34.639 --> 00:10:36.840 with a single dart and just trying to hit any 222 00:10:36.840 --> 00:10:37.519 part of the dark. 223 00:10:37.360 --> 00:10:39.759 Board, a much much bigger darkboard. You've gone from trying 224 00:10:39.759 --> 00:10:41.600 to thread a needle to trying to hit a barn door, 225 00:10:41.639 --> 00:10:43.440 the probability just skyrockets. 226 00:10:43.799 --> 00:10:46.360 Okay, so the net is bigger. We have a much 227 00:10:46.360 --> 00:10:48.639 better chance of a background star lining up with this 228 00:10:48.759 --> 00:10:53.200 diamond shape. But the black holes aren't just sitting there. 229 00:10:53.480 --> 00:10:56.279 No, they're not. And this is where the strobe concept 230 00:10:56.360 --> 00:10:59.360 finally clicks into place. If the two black holes were 231 00:10:59.399 --> 00:11:03.480 just frozen in space, the diamond caustic would be frozen too. 232 00:11:03.600 --> 00:11:05.679 A star that falls on it would just look like 233 00:11:05.720 --> 00:11:08.039 a permanently distorted, brightened image. 234 00:11:08.080 --> 00:11:09.919 But they're not frozen. They're orbiting each other. 235 00:11:09.960 --> 00:11:12.039 They're orbiting. They're in a constant dance. 236 00:11:12.120 --> 00:11:13.039 So the diamond spins. 237 00:11:13.120 --> 00:11:16.360 The diamond spins, it rotates right along with the binary. 238 00:11:17.000 --> 00:11:19.960 Imagine a lighthouse and the beam is rotating. But in 239 00:11:19.960 --> 00:11:22.320 this case, the beam isn't a simple ray of light. 240 00:11:22.720 --> 00:11:27.320 It's this entire complex caustic structure. It's a diamond shaped 241 00:11:27.360 --> 00:11:28.519 beam of magnification. 242 00:11:28.879 --> 00:11:31.679 And that beam is sweeping across the background sky. 243 00:11:31.559 --> 00:11:34.759 Constantly sweeping. And the background sky, even in a seemingly 244 00:11:34.840 --> 00:11:39.039 empty patch, is just filled with faint, distant stars. 245 00:11:39.240 --> 00:11:41.360 So it's only a matter of time before that sweeping 246 00:11:41.399 --> 00:11:43.799 line of magnification runs over one of those stars. 247 00:11:43.960 --> 00:11:47.639 It's inevitable, purely by chance, the caustic curve will cross 248 00:11:47.639 --> 00:11:48.799 a background star. 249 00:11:48.720 --> 00:11:50.799 And when it hits, yeah, what do we see? 250 00:11:50.919 --> 00:11:54.200 We see a flash. The mathematics of the caustic means 251 00:11:54.240 --> 00:11:58.039 the magnification just shoots up. Hanksy Wang, the lead author, 252 00:11:58.080 --> 00:12:02.519 describes it as an extraor ordinarily bright flash for a 253 00:12:02.559 --> 00:12:05.159 brief period of time. It could be hours, could be days. 254 00:12:05.480 --> 00:12:08.759 It depends on the geometry and the speed. That background 255 00:12:08.840 --> 00:12:11.600 star will suddenly become hundreds or even thousands of times 256 00:12:11.639 --> 00:12:12.639 brighter than it normally. 257 00:12:12.679 --> 00:12:14.759 Is like someone switched on a cosmic light. 258 00:12:14.720 --> 00:12:17.840 Bulb, a very powerful one. And then as the caustic 259 00:12:17.879 --> 00:12:19.720 line passes over the star and moves on. 260 00:12:20.039 --> 00:12:21.279 The star goes back to normal. 261 00:12:21.360 --> 00:12:25.519 It fades back to its regular faint baseline brightness, as 262 00:12:25.559 --> 00:12:26.519 if nothing happened. 263 00:12:26.720 --> 00:12:28.279 But the binary is still orbiting. 264 00:12:28.399 --> 00:12:30.799 The binary keeps spinning, so the diamond cost it comes 265 00:12:30.840 --> 00:12:31.679 around again. 266 00:12:31.519 --> 00:12:33.879 On the next orbit, and if that star is still 267 00:12:33.919 --> 00:12:35.080 in the path, it hits. 268 00:12:34.919 --> 00:12:35.480 The star again. 269 00:12:35.600 --> 00:12:38.639 Flash flash. So we get a repeating signal a. 270 00:12:38.600 --> 00:12:42.360 Blank Hey, you get a strobe light, a repeating predictable 271 00:12:42.440 --> 00:12:46.080 burst of starlight, all caused by two invisible objects dancing 272 00:12:46.120 --> 00:12:46.679 in front of it. 273 00:12:46.759 --> 00:12:49.320 Okay, okay, I have to play Devil's advocate here. You know, 274 00:12:49.399 --> 00:12:51.879 I love this idea. But the universe is a really 275 00:12:51.919 --> 00:12:54.840 noisy place. It's full of things that blink and flash. 276 00:12:55.120 --> 00:12:59.879 We have variable stars, sefeed's, flarre stars, cataclysmic variables, super no. 277 00:13:00.200 --> 00:13:02.679 I mean, the list goes on. How can we possibly 278 00:13:02.720 --> 00:13:05.120 be sure that a blink we see is from this 279 00:13:05.240 --> 00:13:08.879 weird lensing effect and not just you know, a star 280 00:13:09.000 --> 00:13:09.720 having a hiccup. 281 00:13:10.320 --> 00:13:13.000 That is probably the most important question in this entire 282 00:13:13.039 --> 00:13:17.039 field of time domain astronomy. Is this transient, this flash unique? 283 00:13:17.879 --> 00:13:19.960 And the authors of this paper argue that, yes, this 284 00:13:20.120 --> 00:13:23.759 signal is remarkably distinct. It has a fingerprint distinction. What way, 285 00:13:24.000 --> 00:13:27.240 the timing, the shape, It all comes down to the 286 00:13:27.320 --> 00:13:29.960 light curve. That's what astronomers call a plot of an 287 00:13:30.000 --> 00:13:32.840 object's brightness over time. Okay, so if you look at 288 00:13:32.840 --> 00:13:35.919 the light curve of a typical variable star like a sephid, 289 00:13:36.320 --> 00:13:38.919 it often looks like a smooth wave. The brightness swells 290 00:13:39.000 --> 00:13:41.279 up and then it shrinks back down. It's often quite symmetric. 291 00:13:41.679 --> 00:13:44.559 That's because the physics is internal to the star. It's 292 00:13:44.639 --> 00:13:47.720 literally breathing, expanding and contracting. 293 00:13:47.399 --> 00:13:49.240 And a costic crossing. What does that look like on 294 00:13:49.279 --> 00:13:49.639 the graph? 295 00:13:49.960 --> 00:13:53.919 It looks nothing like that. Acostic crossing is purely geometric. 296 00:13:54.360 --> 00:13:57.360 It's not about stellar physics. It's about the geometry of 297 00:13:57.399 --> 00:14:00.559 crossing a fold in space time. The light curve is sharp, 298 00:14:00.679 --> 00:14:04.159 it's asymmetric. Asymmetric, you get a very very sharp rise 299 00:14:04.200 --> 00:14:07.000 in brightness and then a slightly slower fall. The peak 300 00:14:07.159 --> 00:14:10.120 is less of a gentle curve and more of a well, 301 00:14:10.159 --> 00:14:14.480 a spike. The math predicts a very specific U shaped 302 00:14:14.519 --> 00:14:17.320 profile right at the peak. It's a fingerprint that just 303 00:14:17.320 --> 00:14:19.480 doesn't look like anything a star does on its own. 304 00:14:19.679 --> 00:14:23.720 So if we see that specific, jagged spike shape, we 305 00:14:23.799 --> 00:14:26.519 know we're looking at gravity, not chemistry or fusion. 306 00:14:26.600 --> 00:14:29.159 That's the idea, and there's another layer to it. It's 307 00:14:29.159 --> 00:14:32.399 not just periodic. The paper calls it quasi period. 308 00:14:32.120 --> 00:14:34.039 Quasi periodic, so almost repeating. 309 00:14:34.120 --> 00:14:36.879 It repeats, but not perfectly. And the reason it's not 310 00:14:36.960 --> 00:14:39.679 perfect is the most exciting part. It's the very thing 311 00:14:39.720 --> 00:14:41.000 we've been trying to measure for. 312 00:14:41.080 --> 00:14:42.480 Decades, the inspiral. 313 00:14:42.600 --> 00:14:45.360 The inspiral. We should probably pause here and really dig 314 00:14:45.399 --> 00:14:47.879 into why these black holes are merging in the first place. 315 00:14:48.440 --> 00:14:51.960 We mentioned dynamical fresh earlier how they sink to the center, 316 00:14:52.399 --> 00:14:56.440 but that process isn't perfect. It leads to a famous problem. 317 00:14:56.279 --> 00:14:59.240 The final parsec problem. I remember this one. It's the 318 00:14:59.279 --> 00:15:02.000 idea that they get really close and then they just 319 00:15:02.000 --> 00:15:03.480 get stuck exactly. 320 00:15:03.720 --> 00:15:07.120 Dynamical friction works great when the black holes are far apart, 321 00:15:07.399 --> 00:15:10.559 moving through a dense sea of stars. They transfer their 322 00:15:10.639 --> 00:15:13.559 orbital energy to the stars and sink. But once they 323 00:15:13.600 --> 00:15:16.960 get close enough to form a tight binary, they well, 324 00:15:17.399 --> 00:15:18.440 they clear out their neighborhood. 325 00:15:18.480 --> 00:15:22.000 They kick all the nearby stars away with gravitational slingshots, right. 326 00:15:22.120 --> 00:15:24.639 And once they've kicked all the nearby stars away, there's 327 00:15:24.720 --> 00:15:27.559 nothing left to create that friction. There's nothing left for 328 00:15:27.600 --> 00:15:30.919 them to transfer their energy to. So in theory, they 329 00:15:30.919 --> 00:15:33.440 should just stall. They should orbit each other forever at 330 00:15:33.480 --> 00:15:35.759 a distance of about a parsec or a few light years. 331 00:15:35.879 --> 00:15:37.679 They should never get close enough to merge. 332 00:15:38.360 --> 00:15:41.960 But we know they do merge. We hear the gravitational 333 00:15:41.960 --> 00:15:45.600 waves from stellar mass black holes merging with lego all the. 334 00:15:45.559 --> 00:15:49.519 Time, exactly, so something has to bridge that final parsec gap. 335 00:15:49.840 --> 00:15:51.519 For a long time, we weren't sure what it was. 336 00:15:51.759 --> 00:15:55.120 Maybe friction from a big gas disc, maybe interactions with 337 00:15:55.159 --> 00:15:58.840 a third black hole. But for these really massive binaries, 338 00:15:59.120 --> 00:16:01.759 the dominant four that brings them together in the end 339 00:16:02.200 --> 00:16:04.679 is the emission of gravitational waves. 340 00:16:04.600 --> 00:16:06.840 And this is the key to decoding the strobes. 341 00:16:06.960 --> 00:16:10.559 This is the Rosetta stone. As these two huge masses 342 00:16:10.559 --> 00:16:14.039 whip around each other, they are constantly churning space time, 343 00:16:14.559 --> 00:16:17.440 radiating enormous amounts of energy away in the form of 344 00:16:17.480 --> 00:16:18.679 these gravitational waves. 345 00:16:18.759 --> 00:16:20.320 And if they're losing energy. 346 00:16:20.080 --> 00:16:23.600 Their orbit has to shrink. As the orbit shrinks, conservation 347 00:16:23.679 --> 00:16:26.120 of angular momentum means they have to speed up. 348 00:16:26.159 --> 00:16:29.600 And if they orbit faster, the strobe light blinks faster. 349 00:16:29.799 --> 00:16:32.440 Precisely, this is the decoding part of the whole thing. 350 00:16:32.639 --> 00:16:34.759 If we can find one of these systems and watch 351 00:16:34.799 --> 00:16:37.600 the flashes over time, we aren't just seeing a blink. 352 00:16:37.879 --> 00:16:40.399 We are seeing a cosmic clock that is ticking faster 353 00:16:40.639 --> 00:16:42.200 and faster and faster. 354 00:16:42.440 --> 00:16:44.840 So we can literally measure the rate of orbital decay 355 00:16:45.080 --> 00:16:47.360 just by timing the flashes from a background star. 356 00:16:47.840 --> 00:16:51.120 Yes, we can watch general relativity happen in real time. 357 00:16:51.679 --> 00:16:54.399 But the paper points out something even more profound, even 358 00:16:54.440 --> 00:16:57.759 more subtle. What's that The emission of gravitational waves doesn't 359 00:16:57.799 --> 00:17:01.360 just change the orbital period, it actually changes the shape 360 00:17:01.360 --> 00:17:02.759 of the caustic diamond itself. 361 00:17:02.840 --> 00:17:06.359 Wait, what how the waves themselves are distorting the lens 362 00:17:06.599 --> 00:17:07.119 in a way. 363 00:17:07.240 --> 00:17:10.640 Yes, the loss of energy and the shrinking separation between 364 00:17:10.640 --> 00:17:14.400 the two black holes alters the overall gravitational potential of 365 00:17:14.440 --> 00:17:18.519 the system. This, in turn subtly alters the geometrre of 366 00:17:18.559 --> 00:17:21.480 the caustic curve. The paper states that it imprints a 367 00:17:21.559 --> 00:17:23.279 characteristic modulation on. 368 00:17:23.240 --> 00:17:25.759 The flashes, So the lens itself is warping as the 369 00:17:25.839 --> 00:17:27.119 orbit decays. 370 00:17:26.759 --> 00:17:30.000 In cosmic real time. Yes, so maybe the peak brightness 371 00:17:30.039 --> 00:17:32.599 of the flash gets a tiny bit higher with each pass, 372 00:17:33.039 --> 00:17:34.720 or the U shape of the light curve gets a 373 00:17:34.759 --> 00:17:37.599 little bit wider. The timing between the flashes will shift 374 00:17:37.599 --> 00:17:40.559 in a very specific, predictable way. That is a direct 375 00:17:40.599 --> 00:17:42.799 signature of gravitational wave emission. 376 00:17:43.200 --> 00:17:45.319 Let me see if we get this straight. We find 377 00:17:45.319 --> 00:17:47.960 a star that's flashing, We check the light curve and 378 00:17:48.000 --> 00:17:51.319 confirm it has that jagged asymmetric shape of a costa crossing. 379 00:17:51.480 --> 00:17:54.240 We watch it repeat from the time between the flashes, 380 00:17:54.559 --> 00:17:57.839 we get the orbital period. From the brightness and shape 381 00:17:57.839 --> 00:18:00.440 of the flash, we can work out the total mass 382 00:18:00.440 --> 00:18:01.519 of the binary. 383 00:18:01.200 --> 00:18:02.119 In their mass ratio. 384 00:18:02.240 --> 00:18:05.359 Yes, and then from the tiny change in the timing 385 00:18:05.440 --> 00:18:08.079 and the shape of the flashes over years, we can 386 00:18:08.119 --> 00:18:09.799 measure how fast they're spiraling. 387 00:18:09.839 --> 00:18:09.880 It. 388 00:18:10.039 --> 00:18:12.599 You've got it. It's a complete telemetry package for an 389 00:18:12.640 --> 00:18:16.680 invisible system. We can extract the masses of the black holes, 390 00:18:16.720 --> 00:18:19.000 their distance from each other, their orbital speed, and the 391 00:18:19.079 --> 00:18:21.440 exact rate at which they are falling towards too. 392 00:18:22.279 --> 00:18:25.400 That's just that's mind blowing. It's like finding the black 393 00:18:25.440 --> 00:18:28.640 box of a plane crash months before the plane actually crashes. 394 00:18:28.839 --> 00:18:32.839 That's a grim but surprisingly accurate analogy. We are watching 395 00:18:33.000 --> 00:18:34.319 the slow motion collision. 396 00:18:34.480 --> 00:18:36.759 But we have to talk about time scales here because 397 00:18:36.799 --> 00:18:39.599 in astronomy, slow motion usually means check back in a 398 00:18:39.640 --> 00:18:42.680 million years. If a big binary has an orbited period 399 00:18:42.680 --> 00:18:44.960 of say five years, do we have to wait a 400 00:18:45.000 --> 00:18:46.839 whole career to see the orbit change? 401 00:18:46.920 --> 00:18:48.119