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 Astronomi 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 00:00:26.920 --> 00:00:29.039 Today we are doing something a little different. We're not 8 00:00:29.120 --> 00:00:33.920 just analyzing a document. We're trying to tune in to 9 00:00:34.320 --> 00:00:38.039 the core rhythm of the cosmos, the actual beat of 10 00:00:38.119 --> 00:00:39.439 space and time itself. 11 00:00:39.640 --> 00:00:41.439 It's a pretty profound idea to start with. 12 00:00:41.560 --> 00:00:44.600 It is, imagine the entire universe, I mean everything, all 13 00:00:44.640 --> 00:00:49.240 the galaxies, the stars, the empty voids, constantly you know, oscillating. 14 00:00:49.240 --> 00:00:51.880 We're talking about the gravitational wave background. 15 00:00:51.479 --> 00:00:55.640 The concept that for decades was pure theory. It sounds 16 00:00:55.640 --> 00:00:57.479 like something straight out of science fiction, but it's not. 17 00:00:57.719 --> 00:01:02.079 We now know it is absolute, polutely, demonstrably real. It 18 00:01:02.159 --> 00:01:03.960 is the ultimate cosmic hum. 19 00:01:03.880 --> 00:01:05.560 That's a good way to put it. If you could 20 00:01:05.560 --> 00:01:08.959 somehow turn up the volume on the universe, the gravitational 21 00:01:09.000 --> 00:01:13.120 wave background, or the GWB would be this fundamental low 22 00:01:13.159 --> 00:01:15.480 frequency roar you'd hear underneath everything else. 23 00:01:15.519 --> 00:01:17.439 And the one description that really stuck with me from 24 00:01:17.439 --> 00:01:20.439 our source material today, a fantastic study from the University 25 00:01:20.439 --> 00:01:23.680 of Colorado Boulder published in the Astrophysical Journal, is that 26 00:01:23.719 --> 00:01:26.439 these are ripples in space and time right, and that 27 00:01:26.480 --> 00:01:30.200 these ripples are moving constantly through the cosmos. And this 28 00:01:30.239 --> 00:01:32.879 is the great part. Jiggles is almost like jello. 29 00:01:33.280 --> 00:01:36.239 It's a fantastic analogy for trying to visualize it, isn't 30 00:01:36.280 --> 00:01:38.159 it It is? Though, you know, we should be clear 31 00:01:38.159 --> 00:01:42.599 that the jiggle is it's infinitesimly subtle, you'd never ever 32 00:01:42.640 --> 00:01:46.920 feel it. But it's detection that is anything but subtle. 33 00:01:47.200 --> 00:01:51.040 Monumental achievement, oh absolutely. It represents a huge triumph for 34 00:01:51.120 --> 00:01:54.840 physics and it's opened up this profound new way of 35 00:01:54.879 --> 00:01:59.159 investigating how the largest structures in the universe actually evolve 36 00:01:59.280 --> 00:02:00.480 and interact with each other. 37 00:02:00.599 --> 00:02:02.959 And that's really our mission for this deep dive. We 38 00:02:03.000 --> 00:02:06.879 want to unpack a massive cosmic mystery that really it 39 00:02:06.920 --> 00:02:10.719 arose the very instant this background was first detected. 40 00:02:10.800 --> 00:02:13.800 The celebration and the confusion arrived at almost the exact 41 00:02:13.840 --> 00:02:14.639 same moment. 42 00:02:14.520 --> 00:02:17.639 Exactly, and the CU Boulder research we're looking at offers 43 00:02:17.680 --> 00:02:20.599 what might be the most compelling solution to this puzzle yet, 44 00:02:20.919 --> 00:02:24.879 a puzzle that has stumped astrophysicists since that groundbreaking GWB 45 00:02:25.039 --> 00:02:27.439 detection was announced back in twenty twenty three. 46 00:02:27.439 --> 00:02:29.800 Right, And I think to really appreciate the solution, we 47 00:02:30.280 --> 00:02:33.639 first have to understand the main players in this enormous 48 00:02:33.719 --> 00:02:34.439 cosmic drama. 49 00:02:34.560 --> 00:02:35.400 Let's set the stage. 50 00:02:35.479 --> 00:02:40.400 Okay, So the GWB itself is fundamentally created by this constant, 51 00:02:40.439 --> 00:02:41.560 relentless merging of. 52 00:02:41.520 --> 00:02:45.800 Galaxies, a process that's been going on for billions of. 53 00:02:45.800 --> 00:02:50.479 Years, billions of years. It's central to how complex galaxies 54 00:02:50.960 --> 00:02:53.680 like our own Milky Way actually form and grow and 55 00:02:53.719 --> 00:02:55.639 mature over cosmic time. 56 00:02:55.759 --> 00:02:57.280 And at the heart of those mergers you have the 57 00:02:57.319 --> 00:02:59.759 real behemoths, the anchors of the whole thing. 58 00:03:00.039 --> 00:03:04.280 Exactly at the core of every significant galaxy you'll find 59 00:03:04.319 --> 00:03:08.599 a super massive black hole in SMBH. So when two 60 00:03:08.639 --> 00:03:12.560 galaxies are drawn together by gravity, their two central SMBHs 61 00:03:12.599 --> 00:03:18.319 are also drawn into this spiraling, inevitable and ultimately violent dance. 62 00:03:18.439 --> 00:03:20.560 And it's the end of that dance, the collision that 63 00:03:20.639 --> 00:03:22.240 sends out the signal we're talking about. 64 00:03:22.280 --> 00:03:25.039 That's the moment. The final collision of these supermassive black 65 00:03:25.039 --> 00:03:29.240 holes sends these incredibly powerful ripples and spacetime gravitational waves 66 00:03:29.280 --> 00:03:30.479 flooding out of the cosmos. 67 00:03:30.479 --> 00:03:32.879 So, if you just zoom out across all of cosmic time, 68 00:03:32.879 --> 00:03:36.159 you've got billions of years of these galaxies merging, billions 69 00:03:36.159 --> 00:03:38.360 of these black hole collisions, right, and all of them 70 00:03:38.360 --> 00:03:44.080 are contributing their own individual ripples to this subtle, omnipresent background. Hum. 71 00:03:44.120 --> 00:03:48.759 That's it, exactly. That cumulative overlapping signature is the gravitational 72 00:03:48.840 --> 00:03:52.840 wave background. It's like imagine turning on a trillion distant 73 00:03:52.960 --> 00:03:55.439 radios all at once and just listening to the collective 74 00:03:55.479 --> 00:03:56.400 static they generate. 75 00:03:56.520 --> 00:03:58.360 And this brings us right to the heart of the puzzle. 76 00:03:58.759 --> 00:04:02.039 The one data point that made this whole field of research. 77 00:04:01.800 --> 00:04:03.680 Just turn on its head, the big reveal. 78 00:04:03.919 --> 00:04:04.280 Yeah. 79 00:04:04.639 --> 00:04:08.319 In twenty twenty three, several big international collaborations, the most 80 00:04:08.319 --> 00:04:11.919 prominent being nanograph that's the North American Nanohurtz Observatory for 81 00:04:11.960 --> 00:04:15.000 gravitational waves, they announced they had done it for the 82 00:04:15.039 --> 00:04:18.920 first time. They had definitively detected the GWB, an. 83 00:04:18.759 --> 00:04:21.800 Earth shattering moment for gravitational wave astronomy, I mean, a 84 00:04:21.839 --> 00:04:24.319 Nobel prizeworthy discovery, without a doubt. 85 00:04:24.360 --> 00:04:27.120 But then they looked closer at the signal itself, and that's. 86 00:04:26.959 --> 00:04:30.480 Where the confusion started. They'd found the song, but it 87 00:04:30.519 --> 00:04:32.240 was being played far, far too loud. 88 00:04:32.360 --> 00:04:35.600 Exactly when they measured the waves, they were significantly, I 89 00:04:35.639 --> 00:04:38.879 mean shockingly larger than anyone had predicted. 90 00:04:39.040 --> 00:04:42.759 All of the best theoretical models, the most sophisticated simulations 91 00:04:42.759 --> 00:04:45.759 we had, they all pointed to a much quieter background. 92 00:04:45.959 --> 00:04:49.920 So you have this clear, massive discrepancy between what the 93 00:04:49.920 --> 00:04:53.759 physics models calculated should be there and what our observations 94 00:04:53.759 --> 00:04:54.839 were actually showing us. 95 00:04:55.040 --> 00:04:57.319 Right, And as Julie Kammerford, the lead author of the 96 00:04:57.319 --> 00:04:59.639 study were focusing on, said, when she saw the data, 97 00:05:00.279 --> 00:05:02.720 it was a surprise and a fun new puzzle to 98 00:05:02.720 --> 00:05:03.480 figure out. 99 00:05:03.560 --> 00:05:07.000 A fun puzzle, I guess if you're a brilliant astrophysicist. 100 00:05:07.079 --> 00:05:09.639 For everyone else, it was just a massive mystery. 101 00:05:09.800 --> 00:05:12.000 And that's exactly what her team set out to solve. 102 00:05:12.240 --> 00:05:16.000 Okay, so let's try to unpack this, this cosmic symphony. 103 00:05:16.439 --> 00:05:19.360 We need to really establish the fundamental concept of the 104 00:05:19.399 --> 00:05:22.920 GWB because you know, the name is familiar, but the 105 00:05:22.959 --> 00:05:23.959 physics behind it. 106 00:05:23.920 --> 00:05:26.000 Is just vast, it really is. 107 00:05:26.360 --> 00:05:29.160 Let's start with that analogy you like from the source material, 108 00:05:29.199 --> 00:05:31.360 the swimming pool, just to make it a bit more concrete. 109 00:05:31.680 --> 00:05:34.120 The swimming pool analogy is probably the best way to 110 00:05:34.199 --> 00:05:39.120 visualize what's happening. So picture a massive, cosmic sized swimming pool. 111 00:05:39.959 --> 00:05:42.160 This pool represents the entirety of space time. 112 00:05:42.240 --> 00:05:42.879 Okay, got it. 113 00:05:43.240 --> 00:05:46.040 Now, imagine there are lots and lots of people in 114 00:05:46.120 --> 00:05:51.360 this pool. These people are the galaxies, and more specifically, 115 00:05:51.600 --> 00:05:56.639 they're merging black holes, and they are all constantly moving around, kicking, splashing, and. 116 00:05:56.680 --> 00:05:59.959 Every one of those splashes, every kick, that's an individual 117 00:06:00.079 --> 00:06:03.279 gravitational wave being sent out by a black hole merger 118 00:06:03.519 --> 00:06:04.639 somewhere in the universe. 119 00:06:04.680 --> 00:06:07.360 That's it, exactly. And the gravitational wave background is just 120 00:06:07.399 --> 00:06:10.480 the accumulation of all of it, the overlap, the interference 121 00:06:10.560 --> 00:06:15.040 pattern of every single one of those individual waves happening 122 00:06:15.079 --> 00:06:18.399 all across the universe over billions and billions of years. 123 00:06:18.480 --> 00:06:21.600 So you don't feel one distinct splash, You just feel 124 00:06:21.639 --> 00:06:23.920 the constant, chaotic churn of the water. 125 00:06:24.120 --> 00:06:27.480 You end up with a constant wash of overlapping ripples 126 00:06:27.519 --> 00:06:30.480 across the entire surface of the pool. That's the background. 127 00:06:30.600 --> 00:06:32.920 Now, I think a really critical piece of context here 128 00:06:33.000 --> 00:06:35.839 is the scale. Because when we talk about gravitational waves, 129 00:06:36.120 --> 00:06:38.480 a lot of people probably think of the first ones 130 00:06:38.519 --> 00:06:39.720 that were detected by Lego. 131 00:06:39.959 --> 00:06:42.040 Right, that's a very important distinction to make. 132 00:06:42.199 --> 00:06:46.160 Those came from much smaller stellar mass black holes colliding, 133 00:06:46.240 --> 00:06:49.560 and those waves were well, they had very high frequencies, 134 00:06:49.639 --> 00:06:52.160 relatively speaking, they were like a quick, sharp chup. 135 00:06:52.399 --> 00:06:55.639 That's a crucial difference. The Lego waves are in what 136 00:06:55.639 --> 00:06:58.879 we call the acoustical range hundreds of hertz. They last 137 00:06:58.920 --> 00:07:01.519 for a fraction of a second. You can literally convert 138 00:07:01.560 --> 00:07:03.439 them into a sound, that famous chirp. 139 00:07:03.560 --> 00:07:06.160 But the GWB we're talking about now, the one detected 140 00:07:06.160 --> 00:07:08.920 by nanograph, that's in the nanohertz range nanohurts. 141 00:07:08.959 --> 00:07:11.680 I mean, it's in almost incomprehensibly low frequency. 142 00:07:11.759 --> 00:07:13.519 What does that actually mean nanohertz? 143 00:07:13.879 --> 00:07:16.480 It means one single cycle of one of these waves 144 00:07:16.800 --> 00:07:19.120 takes billions of seconds to complete. 145 00:07:18.839 --> 00:07:22.959 Billions of seconds, so years, many many years for one 146 00:07:22.959 --> 00:07:23.360 wave to. 147 00:07:23.360 --> 00:07:26.000 Pass by, decades even. And the reason the frequency is 148 00:07:26.000 --> 00:07:28.480 so incredibly low is because the source is just so vast. 149 00:07:29.240 --> 00:07:33.399 The nanohertz signal corresponds to the waves generated by those 150 00:07:33.399 --> 00:07:35.720 super massive black hole binaries we were talking about. 151 00:07:35.759 --> 00:07:37.439 These aren't two sun sized objects. 152 00:07:37.519 --> 00:07:41.040 These are bohemoth, gargantuan systems. You have two black holes 153 00:07:41.439 --> 00:07:44.079 each millions or billions of times the mass of our 154 00:07:44.120 --> 00:07:47.560 Sun orbiting each other. Their orbits are huge, sometimes taking 155 00:07:47.600 --> 00:07:50.680 several years or even decades to complete. So the waves 156 00:07:50.680 --> 00:07:55.040 they emit are themselves incredibly long, low frequency repels that 157 00:07:55.120 --> 00:07:56.879 stretch across light years of space. 158 00:07:57.079 --> 00:08:01.800 And it's the slow, steady, in enormous oscillation that makes 159 00:08:01.879 --> 00:08:02.720 up the GWB. 160 00:08:03.040 --> 00:08:05.439 That's the symphony. It's not a quick chirp. It's the 161 00:08:05.680 --> 00:08:07.079 deep base note of the universe. 162 00:08:07.279 --> 00:08:09.920 And yet, I mean, even though the sources are so massive, 163 00:08:10.079 --> 00:08:13.480 the actual effect on us is still basically nil. Right, 164 00:08:13.519 --> 00:08:16.120 We're being jiggled like jello, but we don't feel a thing. 165 00:08:16.240 --> 00:08:19.759 Correct. The ways have stretched out and weakened over cosmic distances. 166 00:08:19.920 --> 00:08:22.959 They're so incredibly subtle that we require instruments of just 167 00:08:23.160 --> 00:08:26.680 unimaginable sensitivity to even detect them. And this really brings 168 00:08:26.759 --> 00:08:29.040 us back to the source of those ripples, the mechanics 169 00:08:29.040 --> 00:08:30.199 of how galaxies evolve. 170 00:08:30.399 --> 00:08:33.720 So walk us through that mechanic. How do two huge 171 00:08:33.960 --> 00:08:37.559 galaxies miles apart end up with their central black holes 172 00:08:37.559 --> 00:08:39.279 in that final fatal dance. 173 00:08:39.559 --> 00:08:43.200 Well, the universe is constantly building bigger things from smaller things. 174 00:08:43.559 --> 00:08:47.399 It's a hierarchical process. Gravity is always at work pulling 175 00:08:47.440 --> 00:08:49.799 smaller galaxies into the orbits of larger ones. 176 00:08:50.120 --> 00:08:53.600 So you have countless galaxies constantly colliding and intertwining all 177 00:08:53.600 --> 00:08:54.399 over the cosmos. 178 00:08:54.519 --> 00:08:58.240 Yes, and as two large galaxies start to merge, they're 179 00:08:58.320 --> 00:09:01.679 two super massive black hole holes which were at their 180 00:09:01.720 --> 00:09:04.639 respective centers begin to lose orbital energy. 181 00:09:04.919 --> 00:09:06.159 How do they lose energy? 182 00:09:06.399 --> 00:09:10.080 Primarily through a process called dynamical friction. As they move 183 00:09:10.120 --> 00:09:13.480 through the now combined galaxy, their immense gravity pulls on 184 00:09:13.600 --> 00:09:16.519 all the stars and gas clouds around them, creating a 185 00:09:16.600 --> 00:09:18.000 sort of gravitational wake. 186 00:09:18.279 --> 00:09:20.840 Ah, So that wake pulls back on them, slowing them 187 00:09:20.840 --> 00:09:21.639 down exactly. 188 00:09:21.679 --> 00:09:24.519 It's like cosmic drag. This friction robs them of their 189 00:09:24.639 --> 00:09:27.879 orbital energy, causing them to spiral inward, closer and closer 190 00:09:27.919 --> 00:09:29.559 than the new center of the merged galaxy. 191 00:09:29.600 --> 00:09:31.879 And that's when they enter that final spiraling dance. 192 00:09:32.159 --> 00:09:35.919 Precisely, they form a binary system orbiting each other getting 193 00:09:35.919 --> 00:09:39.600 faster and closer and closer till they eventually they coalesce 194 00:09:39.600 --> 00:09:41.840 into a single even the larger black hole. 195 00:09:41.960 --> 00:09:46.000 And that final moment, that coalescence, that's the big splash, That's. 196 00:09:45.799 --> 00:09:48.840 The moment that releases a tremendous burst of energy in 197 00:09:48.919 --> 00:09:52.679 the form of these nanohertz gravitational ways, sending those ripples 198 00:09:52.679 --> 00:09:53.799 out through the cosmos. 199 00:09:53.879 --> 00:09:56.559 So, in terms of the broader context, why does charting 200 00:09:56.600 --> 00:09:59.639 this background, why does it matter so much to astrophysicists? 201 00:09:59.679 --> 00:10:00.879 What's the big prize here? 202 00:10:01.519 --> 00:10:04.679 The big prize is that the GWB is effectively a 203 00:10:04.720 --> 00:10:07.159 fossil record of galaxy. 204 00:10:06.679 --> 00:10:08.399 Formation, a fossil record. 205 00:10:08.519 --> 00:10:12.840 Yes, by studying the specific details, the frequency, of the amplitude, 206 00:10:12.879 --> 00:10:16.159 the whole signature of these waves, we can learn so much. 207 00:10:16.759 --> 00:10:20.200 It reveals new insights into the evolution of the universe itself. 208 00:10:20.440 --> 00:10:24.600 How so well, if we can correctly model the GWB, 209 00:10:25.000 --> 00:10:27.360 if we can understand why it has the strength it does, 210 00:10:27.960 --> 00:10:30.919 we can basically reconstruct how galaxies have been merging over 211 00:10:30.919 --> 00:10:35.159 the last say, ten billion years. We can understand how smaller, simpler, 212 00:10:35.159 --> 00:10:39.080 what we call primordial galaxies coalesced over time to create 213 00:10:39.080 --> 00:10:41.559 the large complex structures like the Milky Way that we 214 00:10:41.600 --> 00:10:42.720 see all around us today. 215 00:10:43.000 --> 00:10:46.000 It's a direct probe into the history of cosmic construction. 216 00:10:46.200 --> 00:10:48.279 It's one of the most direct probes we could ever 217 00:10:48.320 --> 00:10:48.720 hope for. 218 00:10:48.960 --> 00:10:51.879 Okay, let's talk about the size differences, because this is 219 00:10:51.919 --> 00:10:56.000 really where the assumptions that everyone was making fundamentally went wrong. 220 00:10:56.559 --> 00:11:00.440 When we say supermassive black holes, it sounds like one category, 221 00:11:00.440 --> 00:11:01.600 but it's not not at all. 222 00:11:01.639 --> 00:11:05.200 It's a huge category that spans an enormous range of masses. 223 00:11:05.360 --> 00:11:07.039 So on the upper end, what are we talking about. 224 00:11:07.200 --> 00:11:09.759 On the upper end, you have the true giants. These 225 00:11:09.759 --> 00:11:13.559 are SMBHs with a mass equal to billions of times 226 00:11:13.559 --> 00:11:16.840 the mass of our Sun. These are the absolute powerhouses, 227 00:11:17.159 --> 00:11:21.440 capable of generating incredibly strong individual waves when they collide. 228 00:11:21.519 --> 00:11:23.039 The heavyweights of the universe. 229 00:11:23.120 --> 00:11:26.519 Absolutely. But then you have the secondary category, which. 230 00:11:26.320 --> 00:11:28.639 Are still huge by any normal standard. 231 00:11:28.759 --> 00:11:30.440 Oh, of course, you have the ones that are still 232 00:11:30.440 --> 00:11:33.759 super massive, but as the study says, slightly less so, 233 00:11:34.399 --> 00:11:37.440 their masses are only millions of times larger than our Sun. 234 00:11:37.720 --> 00:11:40.399 Only millions, right, So, in the context of a galaxy merger, 235 00:11:40.440 --> 00:11:43.240 where you have say a billion Sun smbh meeting a 236 00:11:43.240 --> 00:11:47.440 million sun smbh, that million sun object is considered the 237 00:11:47.519 --> 00:11:51.279 secondary or the smaller player in that particular dance. 238 00:11:51.639 --> 00:11:53.600 And this is where we get to the crucial piece 239 00:11:53.639 --> 00:11:57.159 of conventional wisdom that this new study completely challenged. Why 240 00:11:57.240 --> 00:12:01.200 were those smaller black holes, the million million sun mass ones, 241 00:12:01.559 --> 00:12:04.799 Why were they basically ignored in the models for the GWB. 242 00:12:05.200 --> 00:12:07.279 It really came down to the basic physics of how 243 00:12:07.279 --> 00:12:10.519 gravitational waves are generated. The amplitude, or the strength of 244 00:12:10.519 --> 00:12:13.480 the wave, scales very significantly with the mass of the 245 00:12:13.519 --> 00:12:14.639 objects that are colliding. 246 00:12:14.720 --> 00:12:17.240 The bigger the splasher is the bigger the wave exactly. 247 00:12:17.600 --> 00:12:20.840 And because the waves produced by a merger between two 248 00:12:20.960 --> 00:12:26.120 billion sun SMBHs are so overwhelmingly powerful, the prevailing theoretical 249 00:12:26.159 --> 00:12:30.080 models basically assumed that the cumuative contribution from all those 250 00:12:30.200 --> 00:12:33.200 smaller million sun black holes would just be negligible. 251 00:12:33.480 --> 00:12:36.440 It would be lost in the noise from the bigger events. 252 00:12:36.159 --> 00:12:39.200 Correct or that their contribution would be too low frequency 253 00:12:39.240 --> 00:12:42.039 to be relevant to the bulk of what nanograph was 254 00:12:42.039 --> 00:12:42.759 actually measuring. 255 00:12:43.279 --> 00:12:45.919 So, to be clear, the models that existed before twenty 256 00:12:45.960 --> 00:12:49.320 twenty three, they focused almost entirely on the biggest, most 257 00:12:49.399 --> 00:12:52.879 massive sort of equal mass mergers, the big on big collisions. 258 00:12:53.159 --> 00:12:56.000 That's right. They were trying to calculate the expected strength 259 00:12:56.000 --> 00:13:00.399 of the GWB by focusing on those heavyweight fights. Assumed 260 00:13:00.480 --> 00:13:03.240 the contribution from all the big on small mergers was 261 00:13:03.360 --> 00:13:05.039 just too quiet to really matter. 262 00:13:05.399 --> 00:13:08.399 They minimized the role of the medium sized players in 263 00:13:08.480 --> 00:13:09.600 this cosmic symphony. 264 00:13:09.679 --> 00:13:12.639 They did, they assumed a much more predictable pathway for 265 00:13:12.720 --> 00:13:16.399 how these things contributed. And it was that minimized expectation 266 00:13:16.600 --> 00:13:20.480 that the NANOGRAVI anomaly just completely shattered. So let's really 267 00:13:20.480 --> 00:13:23.000 focus in on that specific data point, the one that 268 00:13:23.039 --> 00:13:27.840 made all this new research necessary. When NANOGrav finally successfully 269 00:13:27.840 --> 00:13:30.759 detected the GWB, they didn't just say we found it. 270 00:13:30.840 --> 00:13:33.440 They provided a measurement of its amplitude. 271 00:13:32.919 --> 00:13:35.519 Its overall strength or loudness exactly. 272 00:13:35.639 --> 00:13:40.279 And that amplitude measurement immediately, I mean immediately defied all 273 00:13:40.320 --> 00:13:41.000 the expectation. 274 00:13:41.120 --> 00:13:43.360 And again we're not talking about it being just slightly off. 275 00:13:43.360 --> 00:13:45.879 This wasn't a minor calibration issue. 276 00:13:45.559 --> 00:13:49.440 No, No, this was a significant statistical outlier. The measured 277 00:13:49.440 --> 00:13:51.559 waves were substantially stronger than predicted. 278 00:13:51.639 --> 00:13:53.440 How much stronger are we talking the. 279 00:13:53.519 --> 00:13:56.559 Est of its vary a bit, but perhaps as much 280 00:13:56.600 --> 00:14:00.559 as fifty percent larger than what the most sophisticated current 281 00:14:00.600 --> 00:14:02.039 theoretical models had predicted. 282 00:14:02.120 --> 00:14:03.240 Fifty percent. Wow. 283 00:14:03.360 --> 00:14:07.120 Yeah, it's a huge number, and it forced astrophysicists into 284 00:14:07.120 --> 00:14:11.039 a really difficult position. If the real universe is generating 285 00:14:11.120 --> 00:14:15.639 ways that are this much stronger than we predict, then 286 00:14:15.679 --> 00:14:18.679 our model is fundamentally flawed. Something is missing. 287 00:14:19.120 --> 00:14:21.960 So if the actual waves are bigger than the model predicted, 288 00:14:22.360 --> 00:14:26.000 you're missing a source of amplification. And logically it seems 289 00:14:26.000 --> 00:14:28.320 like you have two main paths to investigate that goo 290 00:14:28.399 --> 00:14:31.279 One well, either there are just way more mergers happening 291 00:14:31.320 --> 00:14:33.480 across the cosmos than we thought there were, a higher 292 00:14:33.480 --> 00:14:36.519 merger rate R or the black holes that are involved 293 00:14:36.519 --> 00:14:39.879 in the mergers we do count are somehow effectively larger 294 00:14:39.919 --> 00:14:41.879 than we assume them to be when they collide. 295 00:14:41.960 --> 00:14:44.799 That's a perfect summary of the two main possibilities. And 296 00:14:44.840 --> 00:14:47.159 this is where a critical point about the data comes in. 297 00:14:47.399 --> 00:14:51.000 If the discrepancy was simply because there were more mergers overall, 298 00:14:51.440 --> 00:14:54.120 that higher merger rate, that would typically show up in 299 00:14:54.120 --> 00:14:57.200 a different way in the GWB signal. It might change 300 00:14:57.240 --> 00:15:00.360 the slope of the background spectrum across different frequencies. 301 00:15:00.000 --> 00:15:02.000 We would change the character of the sound, not just 302 00:15:02.039 --> 00:15:03.000 the volume. 303 00:15:02.679 --> 00:15:05.159 A good way to put it, huh, but the specific 304 00:15:05.240 --> 00:15:10.000 shape and the magnitude of what NANOGrav detected. It strongly, 305 00:15:10.120 --> 00:15:14.279 strongly favored the second idea that the individual events themselves 306 00:15:14.480 --> 00:15:16.200 were just more energetic. 307 00:15:16.120 --> 00:15:19.399 So the data was pointing toward the mass involved in 308 00:15:19.480 --> 00:15:22.000 the collisions being larger, not just the sheer number of 309 00:15:22.000 --> 00:15:23.039 collisions being higher. 310 00:15:23.200 --> 00:15:26.639 Exactly. The conclusion was that the current models of how 311 00:15:26.639 --> 00:15:30.480 these SMBH mergers work must be missing a key factor, 312 00:15:30.919 --> 00:15:33.600 a factor that amplifies the wave strength right at that 313 00:15:33.679 --> 00:15:34.720 moment of coalescence. 314 00:15:34.919 --> 00:15:37.679 And if the solution wasn't something you know within the 315 00:15:37.720 --> 00:15:41.360 known physics of how galaxies evolve, what was the alternative. 316 00:15:41.720 --> 00:15:44.159 Well, the alternative was what people started referring to as 317 00:15:44.360 --> 00:15:46.919 requiring new exotic physics, which. 318 00:15:46.759 --> 00:15:49.240 Is always exciting but also a little scary for physicists. 319 00:15:49.519 --> 00:15:53.159 It is it means maybe there are entirely new populations 320 00:15:53.159 --> 00:15:56.159 of black holes we don't know about, or some cosmological 321 00:15:56.159 --> 00:15:59.240 phenomena we hadn't even conceived of. And this is why 322 00:15:59.279 --> 00:16:02.639 that fifty percent discrepancy was so shocking. It opened the 323 00:16:02.679 --> 00:16:04.320 door to some pretty wild ideas. 324 00:16:04.440 --> 00:16:07.679 So let's go back to our swimming pool analogy. If 325 00:16:07.720 --> 00:16:10.559 the waves in the cosmic pool are way bigger than 326 00:16:10.559 --> 00:16:13.639 you expect, right, it means either we have a lot 327 00:16:13.639 --> 00:16:16.759 more swimmers in the pool than we counted, or the 328 00:16:16.799 --> 00:16:20.120 swimmers we did count are suddenly doing much much bigger 329 00:16:20.159 --> 00:16:22.759 cannon balls than we estimated they could, and. 330 00:16:22.759 --> 00:16:25.080 The data was just screaming that the cannon balls must 331 00:16:25.120 --> 00:16:25.519 be bigger. 332 00:16:25.759 --> 00:16:28.799 So why does a bigger cannon ball make a bigger wave? 333 00:16:29.000 --> 00:16:30.320 Let's get into the physics of that. 334 00:16:30.759 --> 00:16:33.679 It goes right back to Einstein's theory of general relativity. 335 00:16:34.159 --> 00:16:37.759 When these two immense masses spiral together and crash, the 336 00:16:37.799 --> 00:16:40.720 amount of energy that gets converted and released is Gravitational 337 00:16:40.799 --> 00:16:44.360 radiation scales nonlinearly. 338 00:16:43.799 --> 00:16:46.879 Nonlinearly, meaning a small increase in mass gives you a 339 00:16:46.919 --> 00:16:48.279 big increase in wave. 340 00:16:48.080 --> 00:16:52.879 Strength, a disproportionately large increase in the gravitational wave amplitude. 341 00:16:53.279 --> 00:16:55.879 It depends on the total mass of the system, but 342 00:16:55.960 --> 00:16:59.559 also critically on the mass ratio between the two black holes. 343 00:17:00.039 --> 00:17:03.399 So the logical conclusion here seems pretty inescapable. If the 344 00:17:03.440 --> 00:17:06.440 measured waves were substantially larger than predicted. 345 00:17:06.839 --> 00:17:10.000 Then the supermassive black holes involved in those mergers must 346 00:17:10.039 --> 00:17:13.279 have had a larger effective mass right before they collided 347 00:17:13.640 --> 00:17:15.279 than our models assumed they had. 348 00:17:15.559 --> 00:17:17.559 They were heavier at the finish line than they were 349 00:17:17.559 --> 00:17:18.400 at the starting line. 350 00:17:18.440 --> 00:17:21.359 In a manner of speaking, yes, and this is precisely 351 00:17:21.400 --> 00:17:24.839 where the brilliant hypothesis developed by Commerford and Simon comes 352 00:17:24.839 --> 00:17:28.079 into play. They decided to go back and revisit that 353 00:17:28.160 --> 00:17:29.440 original assumption. 354 00:17:29.359 --> 00:17:31.960 The one that said the smaller million sun black holes 355 00:17:32.200 --> 00:17:33.559 didn't really matter exactly. 356 00:17:33.839 --> 00:17:36.559 They started to ask a very simple question, what if 357 00:17:36.599 --> 00:17:39.839 those black holes aren't that size right at the moment 358 00:17:39.880 --> 00:17:43.480 the waves are created. What if? What if they grow 359 00:17:43.599 --> 00:17:46.200 significantly during the merger process itself. 360 00:17:46.480 --> 00:17:49.160 That's the intellectual pivot right there. That's what unlocks the 361 00:17:49.160 --> 00:17:50.480 whole mystery it is. 362 00:17:50.599 --> 00:17:52.680 It means you have to stop looking just at the 363 00:17:52.680 --> 00:17:55.519 starting mass that we measure from galaxy surveys, and you 364 00:17:55.559 --> 00:17:58.119 have to start looking at the dynamics during that several 365 00:17:58.119 --> 00:18:01.279 million year period when the two black holes are spiraling 366 00:18:01.319 --> 00:18:02.119 in toward each other. 367 00:18:02.720 --> 00:18:04.160 What happens in the dance itself? 368 00:18:04.720 --> 00:18:08.559 Correct The researchers hypothesized, and this is based on some 369 00:18:08.640 --> 00:18:12.440 hints from earlier simulations that when a smaller SMBH starts 370 00:18:12.440 --> 00:18:15.279 to merge with a much larger one, something strange happens. 371 00:18:15.960 --> 00:18:20.279 The smaller black hole seems to undergo this incredible, unexpected 372 00:18:20.319 --> 00:18:20.960 growth spurt. 373 00:18:21.200 --> 00:18:23.799 It gains a lot of mass very quickly. 374 00:18:23.559 --> 00:18:25.880 A lot of mass, pushing it up into a mass 375 00:18:26.000 --> 00:18:30.119 range where its eventual collision generates a wave that's powerful 376 00:18:30.200 --> 00:18:34.640 enough to dramatically influence the GWB measurement. It transforms that 377 00:18:34.720 --> 00:18:38.200 black hole from a negligible ripple maker into a significant 378 00:18:38.200 --> 00:18:39.000 cannonball maker. 379 00:18:39.160 --> 00:18:41.839 So the player we had dismissed from the team as 380 00:18:41.920 --> 00:18:45.000 just a minor league contributor suddenly became a major force 381 00:18:45.039 --> 00:18:45.559 on the field. 382 00:18:45.680 --> 00:18:49.119 And that unexpected extra energy from all these newly promoted 383 00:18:49.119 --> 00:18:53.359 players provided the missing fifty percent amplification. It was exactly 384 00:18:53.400 --> 00:18:55.359 what was needed to match the nanograph data. 385 00:18:55.359 --> 00:18:58.200 And this mechanism, this differential growth, that's the key. 386 00:18:58.559 --> 00:19:02.119 It's the key to resolving the whole anomaly. It explains 387 00:19:02.160 --> 00:19:05.359 where all that extra wave amplitude comes from without needing 388 00:19:05.440 --> 00:19:08.920 to invent any new or unknown forces. It just means 389 00:19:08.960 --> 00:19:11.599 our calculation of the black hole's mass at the moment 390 00:19:11.599 --> 00:19:15.480 of collision was wrong because we weren't correctly accounting for 391 00:19:15.519 --> 00:19:16.839 how much they eat along the way. 392 00:19:16.960 --> 00:19:18.920 So this is where the discussion really gets into the 393 00:19:19.000 --> 00:19:22.440 nitty gritty physics of how this growth spurt actually happens. 394 00:19:22.480 --> 00:19:24.960 And what's so great about this explanation is that it's 395 00:19:25.039 --> 00:19:30.079 not some mysterious new phenomenon. It's a simple, almost elegant 396 00:19:30.119 --> 00:19:33.480 quirk of geometry and the environment during the merger. 397 00:19:33.359 --> 00:19:36.119 Yes, what the scientists termed preferential accretion. 398 00:19:36.400 --> 00:19:37.640 Preferential accretion. 399 00:19:37.759 --> 00:19:39.799 Let's break that down to understand it. We first have 400 00:19:39.880 --> 00:19:43.400 to really visualize the environment. We need to remember that 401 00:19:43.480 --> 00:19:46.920