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 --> 00:00:22.920 of the universe as you drift off into a peaceful slumber. 6 00:00:23.000 --> 00:00:26.760 Under the night sky. 7 00:00:26.879 --> 00:00:28.839 I want you to step outside with me for a second, 8 00:00:29.000 --> 00:00:31.640 just in your mind. Okay, there, picture the night sky. 9 00:00:31.800 --> 00:00:34.159 It's you know, it's clear, the air is cold, it's 10 00:00:34.200 --> 00:00:37.119 incredibly dark, right, and you are looking up at the 11 00:00:37.159 --> 00:00:41.159 constellation of Ursa Major, which most of us know as 12 00:00:41.200 --> 00:00:41.799 the Great Bear. 13 00:00:42.039 --> 00:00:44.039 Yeah, classic, exactly. 14 00:00:43.840 --> 00:00:46.200 Now, if you know exactly where to look. Sitting deep 15 00:00:46.240 --> 00:00:49.479 within that patch of sky is Messia one oh one, 16 00:00:49.960 --> 00:00:51.159 the pinwheel galaxy. 17 00:00:51.359 --> 00:00:52.640 Oh that is a beautiful one. 18 00:00:52.719 --> 00:00:55.119 It really is. And when we observe it using our 19 00:00:55.159 --> 00:00:58.960 most powerful orbital tools like Hubbles advanced camera for surveys, 20 00:00:59.359 --> 00:01:00.960 we're doing something pretty remarkable. 21 00:01:01.039 --> 00:01:02.479 We're literally looking back in time. 22 00:01:02.679 --> 00:01:05.959 Yeah, we are pulling in photons that have been traveling 23 00:01:05.959 --> 00:01:10.359 across the vacuum of space for like twenty one million years. 24 00:01:10.519 --> 00:01:12.680 We're just staggering to think about it is. 25 00:01:12.719 --> 00:01:16.159 And we route that light through these highly specialized green 26 00:01:16.200 --> 00:01:20.480 and infra red filters right hitting these incredibly sensitive charge 27 00:01:20.519 --> 00:01:24.799 coupled device sensors, and what we capture is this breath taking, 28 00:01:25.120 --> 00:01:28.959 hyper detailed image across a three point three by three 29 00:01:28.959 --> 00:01:31.200 point three arc minute field of view. 30 00:01:31.439 --> 00:01:32.640 Right you see the whole disc. 31 00:01:33.000 --> 00:01:38.239 Yeah, It's a vast, swirling disc of hundreds of billions 32 00:01:38.239 --> 00:01:42.640 of stars, glowing neon gas, dense dust lanes, And it 33 00:01:42.719 --> 00:01:44.200 feels so complete. 34 00:01:44.280 --> 00:01:46.359 It feels like you're seeing the whole universe right. 35 00:01:46.280 --> 00:01:49.280 There, exactly. It feels like we are seeing everything there 36 00:01:49.359 --> 00:01:52.920 is to see in that majestic spiral. But then you 37 00:01:53.000 --> 00:01:56.840 have to pivot sharply and ask yourself a really unsettling question, 38 00:01:57.079 --> 00:01:59.280 which is, what if the most important part of that 39 00:01:59.480 --> 00:02:02.200 entire image just the stuff we absolutely cannot see? 40 00:02:02.319 --> 00:02:03.000 Oh Man point? 41 00:02:03.040 --> 00:02:06.359 What if the real story, like the fundamental architecture holding 42 00:02:06.400 --> 00:02:10.680 that entire spinning pinwheel together, is hidden in the dark, 43 00:02:10.840 --> 00:02:14.280 completely invisible, completely invisible. No matter what filters we use, 44 00:02:14.319 --> 00:02:16.639 and no matter how long we leave the camera shutter. 45 00:02:16.439 --> 00:02:19.439 Open, it forces a profound shift in perspective, doesn't it. 46 00:02:19.439 --> 00:02:25.080 It really does because we are so conditioned biologically by 47 00:02:25.120 --> 00:02:28.439 our evolutionary history and technologically by the instruments we build 48 00:02:28.599 --> 00:02:29.759 to just focus on the light. 49 00:02:30.039 --> 00:02:31.960 Right, we're visual creatures, exactly. 50 00:02:32.039 --> 00:02:36.360 We design these billion dollar orbital observatories specifically to capture 51 00:02:36.400 --> 00:02:41.680 the electromagnetic spectrum photons. But the visible universe, those swirling galaxies, 52 00:02:41.719 --> 00:02:45.879 the brilliant stars, the eliminated nebulas, that's just the frosting. 53 00:02:45.560 --> 00:02:48.120 The frosting on a very big invisible cake. 54 00:02:48.400 --> 00:02:52.879 Exactly. The vast majority of the universe's mass doesn't interact 55 00:02:53.360 --> 00:02:57.039 with electromagnetism at all. It just ignores it, totally ignores it. 56 00:02:57.039 --> 00:02:59.240 It doesn't emit light, it doesn't reflect it, and it 57 00:02:59.240 --> 00:03:03.639 doesn't absorb it. Wow, it is completely invisible to our 58 00:03:03.800 --> 00:03:06.199 primary way of experiencing reality. 59 00:03:06.879 --> 00:03:11.159 Okay, let's unpack this, because understanding that invisible architecture is 60 00:03:11.199 --> 00:03:13.400 the core mission of our conversation today. 61 00:03:13.479 --> 00:03:13.960 Let's do it. 62 00:03:14.080 --> 00:03:17.960 We know this invisible stuff, dark matter, we know it exists, 63 00:03:18.080 --> 00:03:22.560 we do, and we know it's incredibly abundant, outnumbering regular 64 00:03:22.639 --> 00:03:25.360 matter by roughly five to one. Right, and we know 65 00:03:25.439 --> 00:03:28.759 it's there because of its gravitational pull on the visible matter. 66 00:03:28.800 --> 00:03:29.520 We just talked about. 67 00:03:29.639 --> 00:03:32.000 Yeah, Vera. Ruben showed us this beautifully back in the 68 00:03:32.080 --> 00:03:32.800 nineteen seventies. 69 00:03:32.960 --> 00:03:35.719 Oh yeah, the galactic rotation curves exactly. 70 00:03:35.800 --> 00:03:37.919 You look at a galaxy spinning and the stars on 71 00:03:37.960 --> 00:03:40.280 the outer edges are moving way too fast. 72 00:03:39.960 --> 00:03:42.039 Like they should just fly off into space, right. 73 00:03:42.159 --> 00:03:45.520 They absolutely should, based on the visible mass, those stars 74 00:03:45.560 --> 00:03:47.439 should be flung out into deep space. 75 00:03:47.560 --> 00:03:48.719 That they don't, right. 76 00:03:48.919 --> 00:03:51.199 The fact that they stay locked in orbit means there's 77 00:03:51.199 --> 00:03:54.560 an enormous amount of invisible mass holding the galaxy together. 78 00:03:54.680 --> 00:03:57.199 It's kind of like seeing the branches of trees whipping 79 00:03:57.280 --> 00:03:59.719 back and forth. You can't see the wind, but the 80 00:03:59.759 --> 00:04:03.199 cane energy of the branches tells you the wind is there. 81 00:04:03.479 --> 00:04:04.639 That's a great analogy. 82 00:04:04.759 --> 00:04:09.080 But yet, despite decades of searching, despite building the most 83 00:04:09.120 --> 00:04:14.360 sensitive detectors humanity has ever conceived, we have never observed 84 00:04:14.639 --> 00:04:16.680 a dark matter particle directly. 85 00:04:16.839 --> 00:04:17.720 Never, not once. 86 00:04:17.920 --> 00:04:22.360 It has never given us a clear, undeniable, reproducible signal. 87 00:04:22.639 --> 00:04:24.279 No, it's been incredibly frustrating. 88 00:04:24.720 --> 00:04:28.279 So today we are exploring a radical new idea in 89 00:04:28.360 --> 00:04:32.120 particle astrophysics. What if the universe's failure to give us 90 00:04:32.160 --> 00:04:34.759 a clear, uniform signal isn't a dead end? 91 00:04:35.040 --> 00:04:36.600 Oh, I like where this is going? 92 00:04:36.759 --> 00:04:38.959 What if the fact that the signal keeps disappearing is 93 00:04:39.000 --> 00:04:41.600 actually the crucial clue we've been waiting for all along. 94 00:04:41.839 --> 00:04:43.680 That is the crux of the current crisis in the 95 00:04:43.680 --> 00:04:47.199 field right there. Yeah. Yeah. We have spent decades operating 96 00:04:47.279 --> 00:04:51.199 under a very specific, mathematically elegant set of assumptions about 97 00:04:51.240 --> 00:04:52.360 what dark matter should. 98 00:04:52.079 --> 00:04:55.079 Look like, how it should behave fundamentally. 99 00:04:54.600 --> 00:04:58.560 Exactly, and when the observational data repeatedly fails to match 100 00:04:58.600 --> 00:05:02.519 those mathematical assumptions, the initial reaction within the community is 101 00:05:03.079 --> 00:05:05.199 understandably intense frustration. 102 00:05:05.480 --> 00:05:06.959 I mean, I'd be pulling my hair out. 103 00:05:07.360 --> 00:05:11.319 Plenty of physicists are. But in physics, a null result 104 00:05:12.040 --> 00:05:14.279 an absence of a signal in a place where your 105 00:05:14.319 --> 00:05:17.959 best theories strongly predict there absolutely should be one that 106 00:05:18.120 --> 00:05:19.519 is incredibly valuable data. 107 00:05:19.639 --> 00:05:21.800 It's not a failure, it's a clue exactly. 108 00:05:22.160 --> 00:05:25.040 It forces you to rethink the fundamental rules of the 109 00:05:25.040 --> 00:05:27.079 standard models you've built your whole career. 110 00:05:26.839 --> 00:05:30.120 On, right, and to understand this new set of rules. 111 00:05:30.360 --> 00:05:33.079 We are going to explore a really fascinating mystery. 112 00:05:32.800 --> 00:05:34.600 That's playing out right now where do we start. 113 00:05:34.879 --> 00:05:39.160 It starts with a strange, highly energetic, totally unexplained glow 114 00:05:39.560 --> 00:05:42.120 right at the center of our very own Milky Way galaxy, 115 00:05:42.199 --> 00:05:46.240 the galactic center. Yes, then to figure out what that 116 00:05:46.319 --> 00:05:50.160 glow is, will travel out to the quietest, most pristine 117 00:05:50.240 --> 00:05:54.519 laboratories in the universe, these tiny, faint systems known as 118 00:05:54.600 --> 00:05:56.920 dwarf spheroidal galaxies. 119 00:05:56.399 --> 00:05:58.480 The clean rooms of the cosmos exactly. 120 00:05:58.959 --> 00:06:01.920 And finally we'll look at a groundbreaking theory from Fermilab 121 00:06:02.120 --> 00:06:05.560 which suggests that we've been thinking about the fundamental nature 122 00:06:05.600 --> 00:06:08.600 of dark matter all wrong. Okay, that maybe it's not 123 00:06:08.680 --> 00:06:12.759 just one single monolithic particle, but a complex system of two. 124 00:06:13.079 --> 00:06:15.920 That distinction, by the way between dark matter being a 125 00:06:16.079 --> 00:06:20.879 single uniform substance versus a dynamic system of multiple interacting 126 00:06:20.959 --> 00:06:24.279 parts that is not just a minor tweak to the equations. 127 00:06:24.319 --> 00:06:24.959 It's a big deal. 128 00:06:25.079 --> 00:06:28.439 It changes absolutely everything about how we map the kinematic 129 00:06:28.519 --> 00:06:29.319 history of the universe. 130 00:06:29.439 --> 00:06:31.040 So let's start right at the center of the action. 131 00:06:31.279 --> 00:06:35.240 The galactic center is glowing mystery. For decades, cosmologists have 132 00:06:35.279 --> 00:06:38.920 proposed that dark matter isn't just some abstract mathematical modifier 133 00:06:38.959 --> 00:06:42.600 to gravity it's made of actual physical. 134 00:06:42.079 --> 00:06:45.199 Particles, right, tangible stuff, even if we can't see. 135 00:06:45.000 --> 00:06:48.800 It exactly, and within the standard paradigm, there is this 136 00:06:48.959 --> 00:06:52.839 leading hypothesis that when these dark matter particles meet under 137 00:06:52.879 --> 00:06:56.480 the right conditions, they can annihilate each other ellilation, yes, 138 00:06:56.519 --> 00:06:59.319 And when they do that, they produce high energy radiation, 139 00:06:59.480 --> 00:07:03.759 specific gamma ray photons. And right now, as we speak, 140 00:07:04.000 --> 00:07:07.040 the Fermi Gamma ray space telescope is locked onto the 141 00:07:07.079 --> 00:07:08.160 center of our Milky Way. 142 00:07:08.199 --> 00:07:09.079 And what is it seeing. 143 00:07:09.439 --> 00:07:13.120 It is seeing a massive, unexplained excess of these gamma 144 00:07:13.160 --> 00:07:17.879 ray photons. It's coming from this roughly spherical region enveloping 145 00:07:17.920 --> 00:07:20.600 the dense inner bulge of our galaxy, right. 146 00:07:20.439 --> 00:07:25.399 The GV excess. To really grasp what the Fermi telescope 147 00:07:25.439 --> 00:07:28.399 is capturing, we need to dive into what annihilation actually 148 00:07:28.439 --> 00:07:31.519 means when we talk about particle astrophysics. 149 00:07:30.839 --> 00:07:32.519 Because it's not just things bumping together. 150 00:07:32.639 --> 00:07:35.040 No, no, this isn't just things bumping into each other and 151 00:07:35.079 --> 00:07:40.879 breaking apart into smaller pieces. It's a fundamental conversion of reality. WHOA, yeah, yeah, 152 00:07:41.000 --> 00:07:44.279 imagine two dark matter particles. Let's assume they're what we 153 00:07:44.360 --> 00:07:48.319 call weakly interacting massive particles. Or whimps okay, wandering through 154 00:07:48.439 --> 00:07:53.040 the incredibly dense gravitational environment of the galactic center. If 155 00:07:53.079 --> 00:07:56.639 the cross sections align and they collide, they destroy each 156 00:07:56.639 --> 00:08:00.160 other entirely, like completely gone completely. All of their mass 157 00:08:00.240 --> 00:08:05.279 is instantaneously converted into pure kinetic energy and standard model particles. 158 00:08:05.319 --> 00:08:06.319 Well it sounds violent. 159 00:08:06.480 --> 00:08:09.439 It is incredibly energetic. Depending on the mass of the 160 00:08:09.519 --> 00:08:14.160 dark matter particles. This process eventually cascades into the production 161 00:08:14.600 --> 00:08:17.759 of gamma rays. Okay, we are talking about photons with 162 00:08:17.920 --> 00:08:21.160 energies in the gig little electron volt or JEV range. 163 00:08:21.720 --> 00:08:23.920 And just for scale, how energetic is that? 164 00:08:23.920 --> 00:08:27.240 That is billions of times more energetic than the visible 165 00:08:27.279 --> 00:08:28.000 light hitting. 166 00:08:27.759 --> 00:08:31.560 Your eye right now, billions wow. And the Fermi telescope 167 00:08:31.639 --> 00:08:35.519 is specifically designed to catch these incredibly energetic photons, right 168 00:08:35.679 --> 00:08:37.840 it is. It's an amazing piece of engineering. I mean, 169 00:08:37.840 --> 00:08:40.919 it doesn't use mirrors or lenses like a regular telescope 170 00:08:40.919 --> 00:08:44.360 because gamma rays would just blast right through a mirror. 171 00:08:44.039 --> 00:08:47.080 They'd shatter it or just pass right through without interacting exactly. 172 00:08:47.120 --> 00:08:50.399 So instead it uses alternating layers of tungsten foils and 173 00:08:50.480 --> 00:08:54.600 silicon strip detectors. When a gamma ray hits the dense tungsten, 174 00:08:54.960 --> 00:08:59.639 the photon actually undergoes pair production. It spontaneously transforms into 175 00:08:59.639 --> 00:09:03.399 an a lee ttron and a positron from energy. Yeah. Those 176 00:09:03.440 --> 00:09:07.200 newly created particles then streak through the silicon detectors, leaving 177 00:09:07.200 --> 00:09:10.279 a tiny electrical wake that the computers can track backward 178 00:09:10.320 --> 00:09:13.039 to figure out exactly where the original gamma ray came 179 00:09:13.080 --> 00:09:13.759 from in the sky. 180 00:09:13.960 --> 00:09:17.559 It is an absolute triumph of particle physics applied to astronomy, 181 00:09:17.600 --> 00:09:20.240 it really is. And when we use Fermi to look 182 00:09:20.279 --> 00:09:22.840 at the center of the Milky Way, it maps this 183 00:09:23.480 --> 00:09:27.320 spherical excess of JEV gamma rays with astonishing precision. 184 00:09:27.679 --> 00:09:27.960 Right. 185 00:09:28.200 --> 00:09:33.360 The data reveals a morphology, a shape, and a spatial 186 00:09:33.360 --> 00:09:36.639 distribution that is a nearly flawless match for what we 187 00:09:36.720 --> 00:09:39.519 call a Navaro Frank White or NFW profile. 188 00:09:39.639 --> 00:09:41.600 And what's an NFW profile that. 189 00:09:41.639 --> 00:09:45.399 Is the exact mathematical density profile we expect a dark 190 00:09:45.440 --> 00:09:48.639 matter halo to have. Oh wow, Yeah, it's intensely concentrated 191 00:09:48.639 --> 00:09:51.320 at the galactic core where gravity pulls everything together. Yeah, 192 00:09:51.320 --> 00:09:54.240 and the density smoothly falls off inversely with the radius 193 00:09:54.240 --> 00:09:55.200 as you move further out. 194 00:09:55.440 --> 00:09:57.799 So we have this massive halo of dark matter sitting 195 00:09:57.799 --> 00:10:00.639 around the Milky Way in the very center. It's packed 196 00:10:00.720 --> 00:10:04.960 so densely that the particles are constantly interacting, annihilating and 197 00:10:05.039 --> 00:10:08.440 creating this glowing sphere of givy gamma rays. 198 00:10:08.759 --> 00:10:09.399 That's a theory. 199 00:10:09.519 --> 00:10:13.240 It paints this incredibly vivid picture in your mind. You 200 00:10:13.279 --> 00:10:16.840 have the flat, swirling, relatively thin disc of the Milky 201 00:10:16.879 --> 00:10:21.360 Way filled with visible stars and gas, and then enveloping 202 00:10:21.399 --> 00:10:25.440 the center, ballooning out above and below the galactic plane. 203 00:10:25.879 --> 00:10:30.080 Is this glowing orb of pure high energy radiation. 204 00:10:30.279 --> 00:10:32.639 It sounds like a smoking gun for dark matter, doesn't it? 205 00:10:32.639 --> 00:10:33.480 It really does. 206 00:10:33.600 --> 00:10:36.240 It certainly felt like one. When the gvxcess was first 207 00:10:36.320 --> 00:10:39.600 isolated from the background noise, right the energy spectrum peaked 208 00:10:39.639 --> 00:10:43.120 exactly where theories predicted it should if dark matter particles 209 00:10:43.159 --> 00:10:46.840 weighing around thirty to fifty gv were annihilating into bottom. 210 00:10:46.600 --> 00:10:49.000 Quarks, which then decayed into gammers exactly. 211 00:10:49.080 --> 00:10:53.159 It was almost too perfect. But but astrophysics is rarely 212 00:10:53.200 --> 00:10:53.960 that accommodating. 213 00:10:54.120 --> 00:10:56.000 Yeah, I have to push back on the smoking gun 214 00:10:56.039 --> 00:10:59.519 idea because the center of the Milky Way is decidedly 215 00:10:59.559 --> 00:11:02.960 not quiet controlled environment, not even a little bit. It 216 00:11:03.039 --> 00:11:08.480 is arguably the most chaotic, noisy, crowded neighborhood in our 217 00:11:08.639 --> 00:11:10.159 entire cosmic zip code. 218 00:11:10.240 --> 00:11:11.120 It's a mess in there. 219 00:11:11.559 --> 00:11:15.639 We've got Sagittarius, a star, a four million solar mass 220 00:11:15.720 --> 00:11:18.039 supermassive black hole sitting right there. 221 00:11:18.200 --> 00:11:18.360 Yep. 222 00:11:18.759 --> 00:11:22.480 We've got massive stars living fast and dying young in 223 00:11:22.519 --> 00:11:27.080 spectacular supernobe. We've got shock waves heating up unbelievable amounts 224 00:11:27.120 --> 00:11:28.639 of interstellar gas. 225 00:11:28.360 --> 00:11:30.639 So much background radiation exactly. 226 00:11:30.639 --> 00:11:34.360 So, could this gamma ray glow just be regular violent 227 00:11:34.440 --> 00:11:38.919 space phenomena? Like, specifically, what about ordinary astrophysical sources acting 228 00:11:39.000 --> 00:11:40.039 as impostors? 229 00:11:40.159 --> 00:11:42.159 Ah, the impostors, Yeah. 230 00:11:41.759 --> 00:11:45.159 Like a dense, unresolved cluster of millisecond pulsars. 231 00:11:45.240 --> 00:11:47.360 See. This raises an important question, and it is the 232 00:11:47.399 --> 00:11:50.080 exact center of gravity for the debate that has dominated 233 00:11:50.080 --> 00:11:52.799 this field for the last decade because of the mess. Yes, 234 00:11:53.080 --> 00:11:55.720 you are highlighting the fundamental problem with the galactic center. 235 00:11:56.399 --> 00:12:00.399 It is incredibly messy. Let's look closely at your pulsar hypothesis. Okay, 236 00:12:00.600 --> 00:12:04.240 A pulsars are rapidly spinning incredibly dense neutron star. It's 237 00:12:04.240 --> 00:12:07.799 the crushed remnant of a massive star that went supernova. Now, 238 00:12:07.960 --> 00:12:11.480 a millisecond pulsar is an older neutron star that has 239 00:12:11.519 --> 00:12:15.120 been spun up to insane speeds, rotating hundreds of times 240 00:12:15.120 --> 00:12:18.360 a second because it cannibalized matter from a companion star. 241 00:12:18.600 --> 00:12:21.919 Right, and they have these terrifyingly strong magnetic fields they 242 00:12:22.000 --> 00:12:26.440 whip around, acting like naturally occurring particle accelerator. 243 00:12:26.519 --> 00:12:30.399 Precisely, they accelerate electrons and positrons to near the speed 244 00:12:30.399 --> 00:12:32.480 of light along their magnetic poles. 245 00:12:32.559 --> 00:12:35.039 So they're just shooting beams of particles out into space. 246 00:12:35.360 --> 00:12:39.559 Yes, and these relativistic electrons then smash into ambient low 247 00:12:39.639 --> 00:12:44.559 energy photons like starlight or cosmic microwave background radiation. Okay, 248 00:12:45.039 --> 00:12:49.120 through a process called inverse Compton scattering, the electrons transfer 249 00:12:49.159 --> 00:12:51.720 a massive amount of their kinetic energy to the photons, 250 00:12:52.080 --> 00:12:54.080 kicking them all the way up into the GV gamma 251 00:12:54.159 --> 00:12:54.879 ray spectrum. 252 00:12:54.919 --> 00:12:57.799 Oh wow, so they just power up the regular light exactly. 253 00:12:58.039 --> 00:13:00.000 And here's the brutal part for dark matter. Huh. 254 00:13:00.279 --> 00:13:00.639 What's that? 255 00:13:01.120 --> 00:13:05.120 The combined spectrum of thousands of these unresolved millisecond pulsars 256 00:13:05.360 --> 00:13:09.159 peaking in the Gevy range looks almost mathematically identical to 257 00:13:09.200 --> 00:13:13.320 the expected signal of a forty gvy dark matter particle annihilating. 258 00:13:13.480 --> 00:13:13.840 No way. 259 00:13:13.919 --> 00:13:17.320 Yes, they're the ultimate astrophysical imposters. 260 00:13:17.559 --> 00:13:20.879 So we have a massive degeneracy problem, a huge one. 261 00:13:20.960 --> 00:13:23.840 We have a glowing sphere of gamma rays, and we 262 00:13:23.960 --> 00:13:27.840 have two completely different mechanisms that could be creating it exactly. 263 00:13:27.919 --> 00:13:31.559 It could be the groundbreaking discovery of dark matter annihilating, 264 00:13:32.159 --> 00:13:34.399 or it could just be a swarm of thousands of 265 00:13:34.440 --> 00:13:37.840 spinning dead stars clustered around the galactic core. 266 00:13:38.080 --> 00:13:40.840 And because the galactic center is so thick with dust 267 00:13:40.879 --> 00:13:43.759 and other radiation, we can't just zoom in with an 268 00:13:43.759 --> 00:13:46.480 optical telescope and count the pulsars to rule them out. 269 00:13:46.559 --> 00:13:48.399 We just can't see them clearly enough. 270 00:13:48.600 --> 00:13:51.879 We simply cannot separate the signals based on that data alone. 271 00:13:52.759 --> 00:13:55.480 The background modeling of the galactic center is fraught with 272 00:13:55.600 --> 00:13:59.879 systematic uncertainties. If you tweak your model of how interstellar 273 00:14:00.080 --> 00:14:02.960 gas is distributed by just a few percent, the dark 274 00:14:03.000 --> 00:14:07.399 matter signal can either vanish entirely or double in statistical significance. 275 00:14:07.600 --> 00:14:09.519 That's a huge margin of error. 276 00:14:09.600 --> 00:14:12.279 That is not a robust foundation for claiming a Nobel 277 00:14:12.360 --> 00:14:16.159 prizeworthy discovery. No, definitely not to prove that this glow 278 00:14:16.279 --> 00:14:19.600 is actually dark matter and not just a population of 279 00:14:19.600 --> 00:14:23.240 exotic neutron stars, you have to find the exact same 280 00:14:23.440 --> 00:14:25.600 dark matter signature somewhere. 281 00:14:25.240 --> 00:14:26.440 Else, some more quieter. 282 00:14:26.720 --> 00:14:30.879 Exactly. If the fundamental laws of physics are universal, which 283 00:14:30.919 --> 00:14:34.799 is the bedrock of our scientific understanding, then dark matter 284 00:14:34.840 --> 00:14:37.840 should behave identically everywhere in the cosmos. 285 00:14:38.200 --> 00:14:41.039 Okay, I see where this is going. We can't definitively 286 00:14:41.159 --> 00:14:44.159 prove what's happening in the noisy, chaotic center of the 287 00:14:44.159 --> 00:14:47.200 Milky Way because there's simply too much background noise, far 288 00:14:47.240 --> 00:14:48.799 too much. I like to think of it this way. 289 00:14:49.360 --> 00:14:53.200 It's like trying to isolate and record the sound of 290 00:14:53.240 --> 00:14:56.000 a single acoustic guitar while you are standing in the 291 00:14:56.000 --> 00:14:59.240 front row of a heavy metal concert. Oh I love that, right, 292 00:14:59.360 --> 00:15:03.279 there's pyrote technic's going off. There's a wall of amplifiers blasting, 293 00:15:03.720 --> 00:15:05.039 the crowd is screaming. 294 00:15:05.080 --> 00:15:05.840 They're chaos. 295 00:15:05.960 --> 00:15:08.240 Yeah. You might look at your audio waveform and think 296 00:15:08.279 --> 00:15:11.519 you see the acoustic guitar's frequency, but it's completely washed 297 00:15:11.559 --> 00:15:14.120 out by the distortion of the bass guitars and the symbols. 298 00:15:14.279 --> 00:15:16.919 You'd never be able to trust the recording exactly. 299 00:15:16.960 --> 00:15:19.240 You can't be sure, so you have to take the 300 00:15:19.279 --> 00:15:21.720 acoustic guitar out of the stadium. You have to find 301 00:15:21.759 --> 00:15:24.240 a quieter laboratory to see if you can hear it clearly. 302 00:15:24.919 --> 00:15:28.480 And for astrophysicists, that leads us directly to the clean 303 00:15:28.559 --> 00:15:29.679 rooms of the cosmos. 304 00:15:29.960 --> 00:15:31.840 That analogy hits the nail on the head. If the 305 00:15:31.840 --> 00:15:35.279 galactic center is our heavy metal concert, we desperately need 306 00:15:35.320 --> 00:15:38.399 to find the cosmic equivalent of an antichoic chamber. 307 00:15:38.240 --> 00:15:39.840 A completely sound proof room. 308 00:15:40.000 --> 00:15:43.960 Yes, and for high energy astrophysics, those sound proof chambers 309 00:15:44.000 --> 00:15:48.440 are a specific class of satellites orbiting our galaxy called 310 00:15:48.559 --> 00:15:52.279 dwarf spheroidal galaxies or dfs for short. 311 00:15:52.519 --> 00:15:55.480 Let's really dive into these dwarf speroidal galaxies because they 312 00:15:55.480 --> 00:15:58.879 are fascinating, deeply weird little objects there really are. We 313 00:15:58.879 --> 00:16:02.480 are talking about systems like Reticulum two or Takana to 314 00:16:02.559 --> 00:16:05.799 second or Segue one. They are very small and they 315 00:16:05.799 --> 00:16:08.519 are incredibly faint. You cannot see them with the naked. 316 00:16:08.240 --> 00:16:09.080 Eye, not a chance. 317 00:16:09.159 --> 00:16:11.960 In fact, many of them were only discovered recently through 318 00:16:12.000 --> 00:16:15.720 massive automated sky surveys because they are so dim. But 319 00:16:15.759 --> 00:16:18.919 what makes them special is that they are overwhelmingly dominated 320 00:16:18.960 --> 00:16:19.799 by dark matter. 321 00:16:20.080 --> 00:16:24.720 Their mass to light ratios are absolutely staggering. How so, well, 322 00:16:24.799 --> 00:16:27.440 in the Milky Way, the ratio of dark matter to 323 00:16:27.519 --> 00:16:31.039 regular matter is roughly five to one. Okay, In some 324 00:16:31.080 --> 00:16:35.240 of these ultra faint dwarf cerroidal galaxies the ratio can 325 00:16:35.279 --> 00:16:36.799 be a thousand. 326 00:16:36.480 --> 00:16:37.720 To one, one thousand to one. 327 00:16:37.879 --> 00:16:43.519 Yes, they're essentially massive, dense lumps of dark matter holding 328 00:16:43.519 --> 00:16:48.440 on to just a tiny microscopic sprinkling of ancient stars. Wow. 329 00:16:48.879 --> 00:16:51.799 And the critical factor for our search is what they lack. 330 00:16:52.159 --> 00:16:55.000 They have absolutely no interstellar gas and no dust. 331 00:16:55.120 --> 00:16:57.679 And without gas and dust, you can't have stellar nurseries. 332 00:16:57.720 --> 00:16:59.399 You can't form new star. 333 00:16:59.399 --> 00:17:02.240 Exactly the point point because they aren't forming new stars. 334 00:17:02.639 --> 00:17:04.759 They haven't had any recent supernovae. 335 00:17:04.839 --> 00:17:05.480 Ah. 336 00:17:05.559 --> 00:17:08.519 Therefore, they do not possess massive populations of pulsars. 337 00:17:08.599 --> 00:17:09.440 That's huge. 338 00:17:09.759 --> 00:17:12.240 It is the stars that are there are billions of 339 00:17:12.319 --> 00:17:16.079 years old, small and stable. Furthermore, we understand structurally why 340 00:17:16.119 --> 00:17:18.720 they are so empty. Why is that as these dwarf 341 00:17:18.759 --> 00:17:21.680 galaxies orbit the Milky Way, they pass through our galaxies 342 00:17:21.720 --> 00:17:24.599 diffuse outer halo. The friction of that passage creates a 343 00:17:24.599 --> 00:17:26.519 phenomenon called ram pressure stripping. 344 00:17:26.640 --> 00:17:28.039 Ram pressure stripping. 345 00:17:27.759 --> 00:17:30.400 Yeah, which effectively blows all the loose gas right out 346 00:17:30.400 --> 00:17:31.160 of the dwarf. 347 00:17:30.880 --> 00:17:32.440 Galaxy, just strips it clean. 348 00:17:32.640 --> 00:17:36.720 Right, So you are left with a pristine, ancient environment. 349 00:17:37.319 --> 00:17:42.119 No supermansive black holes, no active star formation, no pulsars. 350 00:17:42.240 --> 00:17:47.039 They are practically empty of ordinary high energy astrophysical background noise. 351 00:17:47.200 --> 00:17:48.480 The perfect clean room. 352 00:17:48.680 --> 00:17:51.319 So, returning to our analogy, trying to find dark matter 353 00:17:51.359 --> 00:17:54.319 in the Milky Way was the heavy metal concert. Pointing 354 00:17:54.359 --> 00:17:57.799 the Fermi telescope at a dwarf galaxy like reticulum texts 355 00:17:57.920 --> 00:17:59.920 is like trying to hear that acoustic guitar in a 356 00:18:00.039 --> 00:18:02.279 perfectly soundproof empty library. 357 00:18:02.400 --> 00:18:04.880 Yes, there is zero background noise. 358 00:18:05.119 --> 00:18:08.440 And because we know, based on their gravitational dynamics, that 359 00:18:08.480 --> 00:18:11.599 these dwarf galaxies are absolutely packed with dark matter, the 360 00:18:11.640 --> 00:18:13.640 math is incredibly. 361 00:18:12.880 --> 00:18:14.680 Straightforward, very straightforward. 362 00:18:14.759 --> 00:18:16.960 If dark matter is annihilating in the Milky Way and 363 00:18:17.039 --> 00:18:20.799 producing that massive JEV gamma ray glow, then it must 364 00:18:20.839 --> 00:18:23.319 be annihilating in the dwarf galaxies too, right. 365 00:18:23.799 --> 00:18:27.240 The community formalized this expectation by calculating something called the 366 00:18:27.319 --> 00:18:29.960 j factor for each of these dwarf galaxies. 367 00:18:30.000 --> 00:18:30.960 The J factor. 368 00:18:31.400 --> 00:18:35.559 Yes, it's essentially a mathematical integral of the dark matter 369 00:18:35.640 --> 00:18:38.599 density squared integrated along our line of sight. 370 00:18:38.720 --> 00:18:41.200 Okay, so a measure of how dense the dark matter is. 371 00:18:41.279 --> 00:18:44.640 Basically, it tells us exactly how bright the gamma ray 372 00:18:44.640 --> 00:18:47.759 signal should be if annihilation is occurring. Right for dwarf 373 00:18:47.839 --> 00:18:51.359 galaxies like Segue one are Reticulum two, the J factors 374 00:18:51.400 --> 00:18:54.720 are incredibly high. The target is painted clearly. We know 375 00:18:54.759 --> 00:18:56.440 where to look, We know exactly where to look, we 376 00:18:56.480 --> 00:18:59.319 know exactly what energy spectrum to look for, and we 377 00:18:59.400 --> 00:19:02.960 know there is no background radiation to confuse us. 378 00:19:03.079 --> 00:19:04.079 So what did we find? 379 00:19:04.640 --> 00:19:07.839 We point fermi at these quiet little galaxies, open the 380 00:19:07.839 --> 00:19:10.920 digital shutter, and wait to capture the undeniable glow of 381 00:19:11.000 --> 00:19:12.160 dark matter annihilation. 382 00:19:12.359 --> 00:19:15.759 But here is the massive, unsettling reveal. When we point 383 00:19:15.759 --> 00:19:19.759 our telescopes at those incredibly dense, dark matter rich dwarf 384 00:19:19.799 --> 00:19:23.880 galaxies year after year, we don't hear the acoustic guitar. No, 385 00:19:24.079 --> 00:19:28.440 we don't hear a whisper. We hear absolute, terrifying. 386 00:19:27.960 --> 00:19:29.359 Silence, total silence. 387 00:19:29.440 --> 00:19:31.559 There is a complete and total absence of a gamma 388 00:19:31.599 --> 00:19:32.440 ray signal. 389 00:19:32.279 --> 00:19:34.720 In an astrophysics that silence is deafening. 390 00:19:34.839 --> 00:19:35.519 I can imagine. 391 00:19:35.599 --> 00:19:37.680 We call it a null result, but it is a 392 00:19:37.759 --> 00:19:40.559 null result that carries the weight of a sledgehammer. It 393 00:19:40.640 --> 00:19:44.640 creates a massive kinematic paradox that threatens to break our 394 00:19:44.640 --> 00:19:46.799 foundational understanding of how the universe is. 395 00:19:46.799 --> 00:19:49.720 Constructed, because you simply cannot have it both ways. 396 00:19:49.880 --> 00:19:50.559 You really can't. 397 00:19:50.680 --> 00:19:54.119 If the fundamental dark matter particle is exactly the same 398 00:19:54.400 --> 00:19:57.759 everywhere in the universe, which is the bedrock assumption of 399 00:19:57.799 --> 00:20:01.680 the cold dark matter paradigm, right, and it is annihilating 400 00:20:01.720 --> 00:20:05.519 to create that massive glowing sphere in the Milky Way. 401 00:20:06.359 --> 00:20:09.759