WEBVTT 1 00:00:03.439 --> 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 00:00:27.000 --> 00:00:29.280 So, if you look at the absolute oldest light in 8 00:00:29.320 --> 00:00:33.159 the universe, the grand cosmic math says, reality is thirteen 9 00:00:33.200 --> 00:00:34.560 point eight billion years old. 10 00:00:34.759 --> 00:00:35.039 Right. 11 00:00:35.240 --> 00:00:37.280 But and this is where it gets crazy. If you 12 00:00:37.320 --> 00:00:40.039 look at your cosmic window right now, at the stars 13 00:00:40.079 --> 00:00:43.920 and galaxies relatively close to us, the local math insists 14 00:00:44.479 --> 00:00:46.799 the universe is a billion years younger. 15 00:00:46.960 --> 00:00:48.679 Yeah, it's a massive discrepancy. 16 00:00:48.719 --> 00:00:50.640 Like, let that sink in for a second. We haven't 17 00:00:50.679 --> 00:00:55.119 just misplaced your car keys. We have somehow misplaced an 18 00:00:55.359 --> 00:00:59.000 entire billion year era of cosmic history and. 19 00:00:58.920 --> 00:01:01.479 Losing a billion year I mean that isn't a mere 20 00:01:01.560 --> 00:01:05.000 rounding error. It fundamentally shatters our entire timeline. It throws 21 00:01:05.040 --> 00:01:08.640 into question really our understanding of how galaxies assembled, when 22 00:01:08.640 --> 00:01:11.079 the first star is ignited, and even how the very 23 00:01:11.079 --> 00:01:14.480 fabric of space and time behaves on a macroscopic scale. 24 00:01:14.640 --> 00:01:17.079 We are dealing with highly precise data sets here, and 25 00:01:17.120 --> 00:01:19.920 they are pointing in two completely contradictory directions. 26 00:01:20.280 --> 00:01:24.519 It is a massive crisis in modern astronomy. They often 27 00:01:24.560 --> 00:01:27.840 refer to it as the Hubble tension. So for everyone listening, 28 00:01:28.319 --> 00:01:31.159 our mission today is to break down a really fascinating 29 00:01:31.200 --> 00:01:35.079 new approach to solving this exact standoff. There's this massive 30 00:01:35.120 --> 00:01:37.879 research effort led by dron Old Bannock at the University 31 00:01:37.879 --> 00:01:40.920 of Portsmouth, and they submitted a study this past July 32 00:01:41.040 --> 00:01:42.599 that tries to finally settle. 33 00:01:42.359 --> 00:01:44.719 The debate, which is incredibly exciting, right but. 34 00:01:44.799 --> 00:01:46.920 Instead of peering out to the very edge of the 35 00:01:46.920 --> 00:01:51.040 observable universe, they are looking inward. They're hunting for the 36 00:01:51.120 --> 00:01:55.519 absolute oldest, most pristine cosmic fossils right here in our 37 00:01:55.560 --> 00:01:58.480 own galactic backyard, the Milky Way. So we are going 38 00:01:58.519 --> 00:02:01.439 to explore these two conflict tape measures of the cosmos, 39 00:02:01.719 --> 00:02:04.359 break down how a staggeringly massive senses of over one 40 00:02:04.439 --> 00:02:06.959 hundred and fifty five thousand ancient stars is acting as 41 00:02:06.959 --> 00:02:11.599 a tie breaker, and explore the incredibly wild possibility that 42 00:02:11.599 --> 00:02:14.759 our local neighborhood of space might just be completely distorting 43 00:02:14.759 --> 00:02:15.599 our view of reality. 44 00:02:15.680 --> 00:02:18.400 The shift in methodology here is crucial to understand because 45 00:02:18.439 --> 00:02:21.439 for decades cosmology has relied really heavily on measuring the 46 00:02:21.439 --> 00:02:26.000 stretching of space itself, exactly the container. This new approach 47 00:02:26.240 --> 00:02:29.479 entirely abandons the attempt to measure the container, so to speak, 48 00:02:29.840 --> 00:02:33.520 and instead focuses on finding the oldest possible objects sitting 49 00:02:33.520 --> 00:02:34.560 inside that container. 50 00:02:34.759 --> 00:02:36.759 But let's start with the container, though, because I feel 51 00:02:36.800 --> 00:02:38.879 like you need to understand the standoff before you can 52 00:02:38.919 --> 00:02:43.680 appreciate the solution. The core of this crisis revolves around 53 00:02:43.759 --> 00:02:47.360 measuring how fast the universe is expanding, which is a 54 00:02:47.400 --> 00:02:50.199 metric known as the Hubble constant. Right, And the tension 55 00:02:50.240 --> 00:02:53.639 exists because we have this nine percent mismatch between two 56 00:02:53.879 --> 00:02:57.919 historically bulletproof methods of measuring that expansion. Let's talk about 57 00:02:57.960 --> 00:02:59.879 tape measure number one, the early universe. 58 00:03:00.039 --> 00:03:02.439 So the early universe method relies on something called the 59 00:03:02.439 --> 00:03:06.439 cosmic microwave background, or the CMB. To picture this, you 60 00:03:06.520 --> 00:03:09.000 have to imagine the universe about three hundred and eighty 61 00:03:09.120 --> 00:03:10.759 thousand years after the Big Bank. 62 00:03:10.840 --> 00:03:13.120 Okay, pretty early on, very early. 63 00:03:12.879 --> 00:03:15.439 Before this point. The universe was so intensely hot and 64 00:03:15.560 --> 00:03:18.479 dense that it was essentially a thick fog of plasma. 65 00:03:18.759 --> 00:03:21.280 Photons of light couldn't travel freely. They just you know, 66 00:03:21.599 --> 00:03:23.599 constantly smashed into free electrons. 67 00:03:23.800 --> 00:03:27.639 So the universe was effectively opaque, right, just a giant, glowing, 68 00:03:27.840 --> 00:03:31.439 superheated cloud where light was totally trapped precisely. 69 00:03:31.639 --> 00:03:34.919 But as space expanded, it naturally cooled, and right around 70 00:03:34.919 --> 00:03:37.639 that three hundred and eighty thousand year mark, the temperature 71 00:03:37.719 --> 00:03:41.120 dropped just enough for electrons to finally bind to protons, 72 00:03:41.159 --> 00:03:43.759 which formed the first neutral hydrogen. 73 00:03:43.360 --> 00:03:45.039 Atoms, and that's when the fog lifted. 74 00:03:45.159 --> 00:03:49.599 The fog lifted, light could suddenly travel unimpeded, and that 75 00:03:49.680 --> 00:03:53.120 first burst of free light has literally been traveling through 76 00:03:53.120 --> 00:03:57.199 space for billions of years. As the universe has expanded, 77 00:03:57.520 --> 00:04:00.319 those ancient light waves have been stretched out into the 78 00:04:00.360 --> 00:04:01.560 microwave spectrum. 79 00:04:01.919 --> 00:04:05.599 So when we point speralized radio telescopes at the sky today, 80 00:04:05.879 --> 00:04:08.400 we can actually see the afterglow of this exact event. 81 00:04:08.680 --> 00:04:10.879 It's like we're looking at a baby picture. 82 00:04:10.599 --> 00:04:13.759 Of the universe we are, and that baby picture, interestingly, 83 00:04:14.080 --> 00:04:18.160 is not perfectly uniform. It has microscopic temperature fluctuations these 84 00:04:18.199 --> 00:04:21.639 tiny hot and cold spots. By analyzing the physical size 85 00:04:21.639 --> 00:04:24.959 and distribution of those temperature ripples, cosmologists can apply our 86 00:04:25.000 --> 00:04:26.759 standard model of the universe. 87 00:04:26.439 --> 00:04:28.959 Which is known as the Lambda CDM model. Hold on, 88 00:04:29.079 --> 00:04:32.639 let's not let that acronym just slide by Lambda CDM. 89 00:04:33.000 --> 00:04:34.959 What physically are we talking about there, because I know 90 00:04:35.040 --> 00:04:36.920 that model is doing a whole lot of heavy lifting 91 00:04:36.959 --> 00:04:37.600 in this measurement. 92 00:04:37.720 --> 00:04:41.720 Sure, so, Lambda represents dark energy. This is the mysterious 93 00:04:41.759 --> 00:04:45.000 intrinsic energy of space itself that is causing the expansion 94 00:04:45.040 --> 00:04:46.319 of the universe to accelerate. 95 00:04:46.480 --> 00:04:47.600 It pushes things apart. 96 00:04:47.720 --> 00:04:50.759 Yes, it acts like a repulsive gravity, pushing space apart. 97 00:04:51.439 --> 00:04:56.839 Then CDM stands for cold dark matter. This is the invisible, 98 00:04:57.040 --> 00:05:00.160 slow moving matter that doesn't interact with light at all, 99 00:05:00.560 --> 00:05:04.480 but it exerts a massive gravitational pull. It acts as 100 00:05:04.519 --> 00:05:06.720 the invisible staffolding for galaxies. 101 00:05:07.040 --> 00:05:09.879 So we basically have dark energy pushing everything apart on 102 00:05:09.879 --> 00:05:12.319 one side, in cold dark matter trying to pull it 103 00:05:12.319 --> 00:05:13.160 all together on the other. 104 00:05:13.360 --> 00:05:17.199 Exactly. The LAMB to CDM model perfectly balances those two 105 00:05:17.199 --> 00:05:20.920 opposing forces by plugging the temperature fluctuations of the CMB 106 00:05:21.040 --> 00:05:25.240 baby picture. Into this mathematical model, cosmologists can calculate exactly 107 00:05:25.279 --> 00:05:28.160 how fast the universe was expanding in its earliest moments, 108 00:05:28.319 --> 00:05:31.399 and then they can project that math forward to the 109 00:05:31.439 --> 00:05:34.639 present day. When you run that simulation all the way 110 00:05:34.680 --> 00:05:38.120 to its conclusion, the math dictates that the universe has 111 00:05:38.120 --> 00:05:41.279 been expanding for thirteen point eight billion years. 112 00:05:41.439 --> 00:05:44.279 Okay, so that is tape measure number one, giving us 113 00:05:44.279 --> 00:05:47.720 the universe that is thirteen point eight billion years old. 114 00:05:48.319 --> 00:05:52.399 It is an incredibly sophisticated deduction based on the oldest 115 00:05:52.680 --> 00:05:55.519 light in existence. But then we have tape measure number two, 116 00:05:55.959 --> 00:05:59.079 and this one doesn't look at ancient stretched out microwaves. 117 00:05:59.120 --> 00:06:01.920 It looks at the local, present day universe right outside 118 00:06:01.920 --> 00:06:03.000 our cosmic front door. 119 00:06:03.360 --> 00:06:07.120 Right The local measurements rely on a completely different astrophysical tool. 120 00:06:07.399 --> 00:06:11.560 They use standard candles. Standard candles, specifically, astronomers look at 121 00:06:11.600 --> 00:06:15.720 pulsating stars called cepheides and exploding stars known as type 122 00:06:15.759 --> 00:06:19.439 ia supernovae. The fundamental principle of a standard candle is 123 00:06:19.439 --> 00:06:22.879 that we know its exact intrinsic true brightness. 124 00:06:23.319 --> 00:06:24.839 Let me make sure I'm wrapping my head around this. 125 00:06:24.920 --> 00:06:26.279 If I look out my window at night and I 126 00:06:26.279 --> 00:06:28.759 see a dim street lamp, it might be a tiny 127 00:06:28.759 --> 00:06:30.560 flashlight one hundred feet away, or it could be a 128 00:06:30.560 --> 00:06:34.120 massive stadium floodlight a mile away. Without knowing its true brightness, 129 00:06:34.160 --> 00:06:35.800 I can't determine its distance at all. 130 00:06:35.959 --> 00:06:40.120 That is the fundamental problem of all astronomical observation. You 131 00:06:40.160 --> 00:06:42.920 can't just take a literal tape measure to the stars, 132 00:06:43.319 --> 00:06:47.399 but standard candles solve this. Back in the early twentieth century, 133 00:06:47.519 --> 00:06:52.759 astronomer Henrietta swan Livett discovered that cephed variable stars pulse 134 00:06:52.879 --> 00:06:55.839 at a rate that is directly tied to their true brightness, 135 00:06:56.279 --> 00:07:00.160 so they basically blink yes, and a slower pulse means 136 00:07:00.160 --> 00:07:03.120 a brighter star. Once you measure the pulse rate, you 137 00:07:03.160 --> 00:07:06.399 know exactly how bright the star truly is at its source. 138 00:07:06.720 --> 00:07:10.000 So, going back to my streetline analogy, the star essentially 139 00:07:10.079 --> 00:07:13.040 tells you, hey, I am exactly as bright as a 140 00:07:13.120 --> 00:07:15.160 hundred watt bulb. So when you look at it in 141 00:07:15.160 --> 00:07:17.959 the telescope and it looks incredibly dim, you can use 142 00:07:18.000 --> 00:07:21.519 a simple inverse square law of light to calculate precisely 143 00:07:21.519 --> 00:07:23.720 how far away it must be to appear that dim 144 00:07:23.720 --> 00:07:24.079 to us. 145 00:07:24.160 --> 00:07:26.600 You've got the distance. That's half the puzzle. Now you 146 00:07:26.639 --> 00:07:28.759 need the speed because the Hubble constant is all about 147 00:07:28.800 --> 00:07:31.839 distance and speed. To get the speed, astronomers measure the 148 00:07:31.839 --> 00:07:35.680 red shift of that star's light. Yeah, as the star 149 00:07:35.800 --> 00:07:38.519 moves away from us, due to the expansion of space, 150 00:07:39.079 --> 00:07:42.160 the light waves it emits get stretched out. Over that distance. 151 00:07:42.319 --> 00:07:45.319 The wavelength becomes longer, shifting toward the red end of 152 00:07:45.319 --> 00:07:46.240 the light spectrum. 153 00:07:46.279 --> 00:07:48.439 Oh, it's like the visual version of the Doppler effect. 154 00:07:48.759 --> 00:07:50.839 Like when an ambulance speeds away from you, the sand 155 00:07:50.879 --> 00:07:54.279 waves stretch out and the pitch drops in space. As 156 00:07:54.319 --> 00:07:57.199 a galaxy speeds away, the light waves stretch out and 157 00:07:57.240 --> 00:07:58.120 the color reddens. 158 00:07:58.199 --> 00:08:01.839 That's a perfect analogy. Combining the precise distance from the 159 00:08:01.879 --> 00:08:04.839 standard candle with the precise speed from the red shift, 160 00:08:05.160 --> 00:08:08.600 astronomers can calculate the local expansion rate of the universe today, 161 00:08:09.120 --> 00:08:12.040 and here is where the crisis erpts the tension. The 162 00:08:12.160 --> 00:08:16.079 local measurement reveals a noticeably higher expansion rate than the 163 00:08:16.079 --> 00:08:19.040 early universe. CMB measurement predicts. 164 00:08:18.759 --> 00:08:21.240 The universe right around as appears to be flying apart, 165 00:08:21.319 --> 00:08:24.439 roughly nine percent faster than the early universe. Baby pictures 166 00:08:24.439 --> 00:08:27.480 said it should be, and that brings us to the 167 00:08:27.600 --> 00:08:31.519 mathematical nightmare here. If space has been expanding at this 168 00:08:31.560 --> 00:08:34.799 faster rate for almost all of cosmic history, it wouldn't 169 00:08:34.759 --> 00:08:37.159 have taken thirteen point eight billion years to reach its 170 00:08:37.159 --> 00:08:38.759 current size, not at all. 171 00:08:39.200 --> 00:08:41.639 It would have reached its current volume much much faster. 172 00:08:42.279 --> 00:08:46.039 If you apply the local expansion rate retroactively, the universe 173 00:08:46.120 --> 00:08:49.320 is suddenly only twelve point five to roughly twelve point 174 00:08:49.440 --> 00:08:50.440 nine billion. 175 00:08:50.200 --> 00:08:54.360 Years old, thirteen point eight billion on one side, twelve 176 00:08:54.399 --> 00:08:57.000 point five billion on the other. That is a gap 177 00:08:57.039 --> 00:08:59.480 of over a billion years. Now I have to ask 178 00:08:59.519 --> 00:09:02.279 the obvious question that anyone listening is probably you know, 179 00:09:02.279 --> 00:09:04.559 shouting out to speakers right now. Why can't we just 180 00:09:04.559 --> 00:09:06.960 split the difference? Why not just average the two numbers, 181 00:09:06.960 --> 00:09:09.279 say the universes, I don't know, thirteen point two billion 182 00:09:09.360 --> 00:09:11.000 years old, and call it a day. 183 00:09:11.240 --> 00:09:14.799 Splitting the difference is intellectually dishonest. In physics, it is 184 00:09:14.840 --> 00:09:17.679 equivalent to taking a thermometer that says water boils at 185 00:09:17.679 --> 00:09:20.120 one hundred degrees and another that says it boils at 186 00:09:20.200 --> 00:09:23.080 ninety degrees, and just deciding water boils at ninety five degrees. 187 00:09:23.159 --> 00:09:24.240 Right, It doesn't work that way. 188 00:09:24.360 --> 00:09:27.799 No, these aren't sloppy approximations. The instruments measuring the CMB, 189 00:09:28.080 --> 00:09:31.480 like the Plank satellite, are total marvels of engineering. The 190 00:09:31.519 --> 00:09:34.840 telescopes observing the cepheeds, like the Hubble Space telescope, have 191 00:09:34.919 --> 00:09:39.039 been calibrated relentlessly for decades. The nine percent discrepancy is 192 00:09:39.120 --> 00:09:43.240 significantly larger than the combined margin of error for both methods, so. 193 00:09:43.159 --> 00:09:46.720 We absolutely cannot chalk this up to like a smudge 194 00:09:46.720 --> 00:09:48.799 on a telescope lens or a bad line of code 195 00:09:48.799 --> 00:09:52.039 in the software. A nine percent discrepancy on a cosmic 196 00:09:52.080 --> 00:09:54.480 scale means there has to be a physical reason for 197 00:09:54.519 --> 00:09:59.320 the mismatch, which leaves us with two pretty terrifying possibilities. 198 00:09:59.399 --> 00:09:59.600 Yeah. 199 00:09:59.679 --> 00:10:03.600 First, there could be some entirely new physics, some exotic 200 00:10:03.799 --> 00:10:07.159 energy field or particle behavior that shape the universe's expansion 201 00:10:07.200 --> 00:10:09.799 from its earliest moments that our LAMB to CDM model 202 00:10:09.879 --> 00:10:14.600 is completely blind to. Or conversely, this fast local expansion 203 00:10:14.679 --> 00:10:17.200 rate is an illusion created by something weird in our 204 00:10:17.200 --> 00:10:18.000 local neighborhood. 205 00:10:18.120 --> 00:10:20.919 The scientific community has been at a total impasse over this. 206 00:10:21.360 --> 00:10:24.279 Are we missing a fundamental piece of the cosmological puzzle? 207 00:10:24.600 --> 00:10:26.799 Or is our tape measure skewed by where we live? 208 00:10:27.279 --> 00:10:29.519 And to break a deadlock like this, you cannot keep 209 00:10:29.600 --> 00:10:32.080 using the same tools. You can't just run another CMB 210 00:10:32.240 --> 00:10:35.360 scan or catalog another thousand supernovae. 211 00:10:35.399 --> 00:10:38.759 You need a totally independent referee, which brings us to 212 00:10:38.840 --> 00:10:42.080 Indronobanic and the University of Portsmouth team. They realize that 213 00:10:42.120 --> 00:10:44.440 you have to stop measuring the stretching of the container. 214 00:10:44.840 --> 00:10:46.440 If you want to know how old a house is, 215 00:10:46.759 --> 00:10:49.279 you don't necessarily measure how far the wooden beams have 216 00:10:49.360 --> 00:10:52.159 warped over time. Right, you look for a newspaper stuffed 217 00:10:52.159 --> 00:10:55.440 in the walls during construction. You date the objects inside 218 00:10:55.440 --> 00:10:55.879 the space. 219 00:10:56.279 --> 00:10:59.960 This relies on what is formally called the minimum age principle, 220 00:11:00.519 --> 00:11:03.159 and the logic is as robust as it is simple. 221 00:11:03.720 --> 00:11:06.879 The universe cannot possibly be younger than the oldest object 222 00:11:06.960 --> 00:11:07.399 within it. 223 00:11:07.679 --> 00:11:11.600 I love this concept. It's like paleontology, but for the cosmos. 224 00:11:12.279 --> 00:11:14.399 If a geologist is trying to determine the age of 225 00:11:14.399 --> 00:11:17.159 a rock formation, they look for fossils. If they dig 226 00:11:17.200 --> 00:11:19.919 into a canyon wall and find a trilobyte fossil, that 227 00:11:20.039 --> 00:11:23.320 carbon and radiometric dating proves as say, five hundred million 228 00:11:23.360 --> 00:11:27.120 years old. Then they know for an absolute, undeniable fact 229 00:11:27.360 --> 00:11:29.720 that the canyon rock must be at least five hundred 230 00:11:29.720 --> 00:11:32.799 million years old. Exactly, the rock cannot be younger than 231 00:11:32.799 --> 00:11:34.080 the fossil trapped inside it. 232 00:11:34.200 --> 00:11:37.480 Stars act as these exact cosmic fossils. If you can 233 00:11:37.559 --> 00:11:41.399 accurately isolate and date the absolute oldest star in our galaxy, 234 00:11:41.840 --> 00:11:44.320 you establish a hard physical floor for the age of 235 00:11:44.320 --> 00:11:46.440 the universe. Of course, you have to add a small 236 00:11:46.440 --> 00:11:48.480 buffer of time to account for how long it took 237 00:11:48.480 --> 00:11:51.159 that start of physically form after the Big Bang, but 238 00:11:51.200 --> 00:11:52.840 it gives you a definitive baseline. 239 00:11:52.960 --> 00:11:56.200 And this methodology is a direct attack on the younger 240 00:11:56.440 --> 00:12:00.799 twelve point five billion year estimate. Right If the local 241 00:12:00.799 --> 00:12:03.679 expansion rate is correct and the universe is only twelve 242 00:12:03.679 --> 00:12:06.639 point five billion years old, then it should be physically 243 00:12:06.679 --> 00:12:09.960 impossible to find a star that is thirteen billion years old. 244 00:12:10.159 --> 00:12:13.600 Finding a thirteen billion year old star would immediately bust 245 00:12:13.679 --> 00:12:16.799 the local expansion theory. At least as a universal constant 246 00:12:16.840 --> 00:12:17.879 rule for all of space. 247 00:12:18.519 --> 00:12:22.759 The theory is flawless. The challenge is the execution. The 248 00:12:22.799 --> 00:12:25.679 Milky Way contains anywhere from one hundred to four hundred 249 00:12:25.759 --> 00:12:28.720 billion stars. You are searching for the absolute oldest members 250 00:12:28.759 --> 00:12:31.679 of a population that vast. You cannot merely point a 251 00:12:31.679 --> 00:12:33.919 telescope and estimate the age of a star just by 252 00:12:33.919 --> 00:12:36.080 looking at its color size on the main sequence. 253 00:12:36.399 --> 00:12:39.279 Main sequence being the long stable middle age of a 254 00:12:39.279 --> 00:12:41.600 star's life right like what our sun is doing right now. 255 00:12:41.759 --> 00:12:44.399 Yes, for the vast majority of its existence, a star 256 00:12:44.519 --> 00:12:48.320 is in hydrostatic equilibrium. Gravity is constantly trying to crush 257 00:12:48.399 --> 00:12:51.159 the star inward, but the immense heat and pressure from 258 00:12:51.240 --> 00:12:53.879 nuclear fusion in the core where it's fusing hydrogen into 259 00:12:53.919 --> 00:12:55.080 helium creates an. 260 00:12:55.039 --> 00:12:56.679 Outward pressure it pushes back. 261 00:12:56.919 --> 00:12:59.840 Exactly as long as the star has hydrogen to burn, 262 00:13:00.360 --> 00:13:04.120 those forces are perfectly balanced. It remains stable, its brightness 263 00:13:04.120 --> 00:13:08.120 barely changes, and its temperature remains relatively constant for billions 264 00:13:08.159 --> 00:13:08.600 of years. 265 00:13:08.759 --> 00:13:11.039 So if I'm trying to find a star's exact age, 266 00:13:11.559 --> 00:13:14.600 looking at it during this phase is basically useless. It's 267 00:13:14.639 --> 00:13:16.240 like trying to tell the difference between a thirty year 268 00:13:16.240 --> 00:13:18.399 old human and a thirty five year old human just 269 00:13:18.440 --> 00:13:21.879 by looking at them sitting on a couch. The physical 270 00:13:22.000 --> 00:13:24.759 changes over that five year span are too subtle to 271 00:13:24.799 --> 00:13:25.679 measure accurately. 272 00:13:26.000 --> 00:13:28.720 That's a great way to put it. To accurately data star. 273 00:13:28.840 --> 00:13:30.840 You need to catch it at a moment of extreme 274 00:13:30.960 --> 00:13:34.519 structural transition. And this brings us to the specific focus 275 00:13:34.600 --> 00:13:38.200 of Banick's team, a very precise evolutionary stage known as 276 00:13:38.279 --> 00:13:39.360 the subgiant phase. 277 00:13:39.440 --> 00:13:42.679 Okay, walk us through the astrophysics of a subgiant. Why 278 00:13:42.679 --> 00:13:45.919 do we care about this specific kind of awkward teenage 279 00:13:45.960 --> 00:13:49.120 transition phase of a star? What is physically happening inside 280 00:13:49.120 --> 00:13:51.159 the star that makes it a perfect clock. 281 00:13:51.440 --> 00:13:54.399 A star enters the subgiant phase the very moment it 282 00:13:54.480 --> 00:13:58.240 exhausts the hydrogen fuel in its core, the fusion engine 283 00:13:58.279 --> 00:14:01.440 at the very center just shuts down because that outward 284 00:14:01.440 --> 00:14:06.120 thermal pressure disappears. Gravity instantly starts winning the battle. Oh wow, Yeah, 285 00:14:06.200 --> 00:14:08.879 the helium core begins to collapse in on itself under 286 00:14:08.879 --> 00:14:09.440 its own weight. 287 00:14:09.600 --> 00:14:11.159 That sounds catastrophic for the star. 288 00:14:11.320 --> 00:14:15.559 It is a massive physical crisis. As the core collapses inward, 289 00:14:15.799 --> 00:14:19.200 the extreme compression generates an immense amount of new heat. 290 00:14:19.720 --> 00:14:23.320 This heat radiates outward and actually ignites a fresh shell 291 00:14:23.399 --> 00:14:27.440 of unfused hydrogen that surrounds the dead core. So now 292 00:14:27.480 --> 00:14:30.320 you have fusion happening in a shell around the core 293 00:14:30.519 --> 00:14:34.000 rather than right in the center. Yes, shell burning, and 294 00:14:34.080 --> 00:14:36.600 the energy generated by this new shell burning is so 295 00:14:36.799 --> 00:14:39.960 intense that it forces the outer gaseous layers of the 296 00:14:39.960 --> 00:14:43.159 star to rapidly expand and cool. The star begins to 297 00:14:43.200 --> 00:14:46.679 balloon in size. It has left the stable main sequence 298 00:14:46.720 --> 00:14:48.919 and is well on its way to becoming a massive 299 00:14:48.960 --> 00:14:52.840 red giant. The subgiant phase is that incredibly brief vauldatle 300 00:14:52.840 --> 00:14:54.279 window caught right in between. 301 00:14:54.600 --> 00:14:57.720 But wait, I'm confused here. You're telling me a star 302 00:14:57.840 --> 00:15:01.679 going through a massive structural crisis. Its core is collapsing 303 00:15:02.159 --> 00:15:05.600 a new shell of fire, igniting its outer layers ballooning 304 00:15:05.600 --> 00:15:08.279 out of control. That makes it easier to date. If 305 00:15:08.320 --> 00:15:10.000 I'm trying to figure out how old a car is, 306 00:15:10.240 --> 00:15:13.200 I don't photograph it while the engine block is actively exploding. 307 00:15:13.519 --> 00:15:15.720 Doesn't that volatility make it a terrible clock? 308 00:15:16.159 --> 00:15:19.879 It's a completely fair question. But the instability is exactly 309 00:15:19.919 --> 00:15:22.799 the feature we need. Because the star's structure is changing 310 00:15:22.879 --> 00:15:26.320 so rapidly during this phase, its surface temperature and its 311 00:15:26.440 --> 00:15:31.039 overall luminosity, its true brightness changed dramatically over a relatively 312 00:15:31.080 --> 00:15:32.480 short period of cosmic time. 313 00:15:32.639 --> 00:15:34.399 Ah I see it's moving fast on. 314 00:15:34.360 --> 00:15:37.000 The graph exactly When a star is on the stable 315 00:15:37.039 --> 00:15:40.440 main sequence, its brightness changes at a glacial pace, but 316 00:15:40.559 --> 00:15:44.080 during the subgiant phase, the luminosity becomes hypersensitive to the 317 00:15:44.080 --> 00:15:47.919 star's precise mass and its age. By mapping the exact 318 00:15:47.960 --> 00:15:52.039 surface temperature and the exact luminosity of a subgiant, astrophysicists 319 00:15:52.039 --> 00:15:56.080 can run stellar evolution models backward with incredible precision. The 320 00:15:56.159 --> 00:15:59.159 rapid physical changes give us a much tighter temporal window. 321 00:16:00.039 --> 00:16:03.240 Subgiants are the perfect cosmic clocks because they are in 322 00:16:03.240 --> 00:16:06.720 the middle of a highly visible, mathematically predictable growth spurt. 323 00:16:07.360 --> 00:16:10.000 Now we need the data, and you can't just comb 324 00:16:10.000 --> 00:16:13.519 through a few hundred stars to find the absolute oldest fossils. 325 00:16:13.519 --> 00:16:16.639 The researchers pulled data from two of the most monumental 326 00:16:16.720 --> 00:16:21.240 astronomical surveys ever conducted, Gaya and la Most. 327 00:16:20.720 --> 00:16:24.320 These two surveys provide complementary data sets that are fundamentally 328 00:16:24.360 --> 00:16:27.759 altering our understanding of the galaxy. Let's start with Gaya. 329 00:16:28.159 --> 00:16:31.240 The Gaia Space Observatory is engaged in the most ambitious 330 00:16:31.279 --> 00:16:36.879 astrometry mission in human history. It is precisely measuring the positions, distances, 331 00:16:36.879 --> 00:16:39.559 and proper motions of over a billion stars. 332 00:16:39.679 --> 00:16:42.759 Astrometry is essentially the three D mapping of the galaxy. Right, Yeah, 333 00:16:43.000 --> 00:16:44.919 but how does it actually know how far away a 334 00:16:45.000 --> 00:16:47.039 star is? Without using those standard candles we. 335 00:16:47.080 --> 00:16:50.600 Talked about earlier, Guya uses a geometric principle called parallax. 336 00:16:51.039 --> 00:16:53.279 As the Earth orbits the Sun, our vantage point in 337 00:16:53.360 --> 00:16:55.799 space shifts by about one hundred and eighty six million 338 00:16:55.840 --> 00:16:57.720 miles from one side of the orbit to the other. 339 00:16:58.000 --> 00:17:00.320 Oh like, if you hold your thumb up in front 340 00:17:00.320 --> 00:17:02.639 of your face and close your left eye, then open 341 00:17:02.679 --> 00:17:04.839 it and close your right eye, your thumb appears to 342 00:17:04.960 --> 00:17:07.119 jump back and forth against the background of the wall. 343 00:17:07.279 --> 00:17:08.039 That's parallax. 344 00:17:08.200 --> 00:17:12.880 Precisely, Gaya takes incredibly precise measurements of a star's position 345 00:17:13.000 --> 00:17:17.240 against the background of distant, unmoving galaxies. Six months later, 346 00:17:17.279 --> 00:17:19.440 it takes the measurement again from the other side of 347 00:17:19.440 --> 00:17:23.519 the Solar System. By measuring that tiny apparent shift, which 348 00:17:23.559 --> 00:17:27.000 is microscopically small equivalent to measuring the width of a 349 00:17:27.119 --> 00:17:30.920 human hair from miles away, Guya can calculate the exact 350 00:17:30.920 --> 00:17:33.680 distance to the star using simple trigonometry. 351 00:17:33.759 --> 00:17:36.599 So Gaya gives us the perfect three D position and distance. 352 00:17:36.839 --> 00:17:40.440 What does LAWMOS do. That's the large sky area multi 353 00:17:40.599 --> 00:17:45.160 object fiber spectroscopic telescope. A total mouthful of an acronym. 354 00:17:44.880 --> 00:17:48.319 Lams provides spectroscopy. If GUYA is the map, most is 355 00:17:48.359 --> 00:17:51.839 the chemical laboratory. A spectroscope takes the light emitted from 356 00:17:51.839 --> 00:17:54.160 a star and splits it into its component colors like 357 00:17:54.160 --> 00:17:56.599 a highly advanced prism. But when you look at that 358 00:17:56.640 --> 00:17:59.359 resulting rainbow, there are tiny dark lines. 359 00:17:59.039 --> 00:18:01.039 Missing from the spectrum, the absorption lines. 360 00:18:01.319 --> 00:18:05.279 Yes, different chemical elements in the star's atmosphere absorb very 361 00:18:05.319 --> 00:18:08.799 specific wavelengths of light. By looking at which specific lines 362 00:18:08.799 --> 00:18:12.440 of color are missing, astronomers get a literal chemical barcode. 363 00:18:12.640 --> 00:18:15.400 LUMOS reads that barcode to tell us exactly what elements 364 00:18:15.400 --> 00:18:16.279 a star is made of. 365 00:18:16.480 --> 00:18:19.319 Which is absolutely crucial for dating a star. I want 366 00:18:19.359 --> 00:18:21.920 to really dig into this because how does reading a 367 00:18:22.000 --> 00:18:24.240 chemical barcode of a star actually tell you how old 368 00:18:24.319 --> 00:18:24.599 it is? 369 00:18:25.039 --> 00:18:28.079 This involves the concept of metallicity, and it requires a 370 00:18:28.119 --> 00:18:31.359 really quick history of the elemental universe. In the immediate 371 00:18:31.400 --> 00:18:34.000 aftermath of the Big Bang, during the first few hundred 372 00:18:34.039 --> 00:18:38.400 million years, the universe consisted almost entirely of hydrogen and helium, 373 00:18:38.680 --> 00:18:41.240 with just trace amounts of lithium. That was it. 374 00:18:41.400 --> 00:18:44.359 The periodic table was basically just two blogs correct. 375 00:18:44.559 --> 00:18:47.519 So the very first generation of stars, which astronomers call 376 00:18:47.599 --> 00:18:51.680 population third stars, were forged purely out of this pristine 377 00:18:51.799 --> 00:18:56.119 hydrogen and helium gas. Inside the extreme pressure of their cores, 378 00:18:56.279 --> 00:19:00.079 these massive early stars fuse those basic elements into heavier 379 00:19:00.119 --> 00:19:03.559 elements carbon, oxygen, nitrogen, all the way up to iron. 380 00:19:03.799 --> 00:19:06.519 And when astronomers say metals, they don't just mean iron 381 00:19:06.599 --> 00:19:10.960 or gold. In astrophysics, anything heavier than helium is lazily 382 00:19:11.000 --> 00:19:12.680 referred to as a metal, right. 383 00:19:12.920 --> 00:19:17.839 A charming quirk of astronomical jargon. Yes, oxygen, carbon, neon, 384 00:19:17.880 --> 00:19:22.119 astronomers consider them all metals. Now, when those massive first 385 00:19:22.160 --> 00:19:27.599 generation stars dyed in spectacular supernova explosions, they violently scattered 386 00:19:27.640 --> 00:19:30.599 all those newly forged heavy metals out into the surrounding 387 00:19:30.640 --> 00:19:32.440 interstellar gas clouds, so. 388 00:19:32.359 --> 00:19:34.200 They seated the universe with the heavy stuff. 389 00:19:34.319 --> 00:19:37.799 Exactly. The next generation of stars, population two stars formed 390 00:19:37.799 --> 00:19:40.640 from those seated gas clouds, so they incorporated a small 391 00:19:40.680 --> 00:19:43.799 amount of those heavier metals into their structure. They lived, 392 00:19:43.839 --> 00:19:47.920 they died, They exploded, seating the universe with even more metals. Finally, 393 00:19:47.960 --> 00:19:50.559 you get two population I stars like our own Sun, 394 00:19:50.599 --> 00:19:54.640 which formed relatively recently from heavily enriched gas. Our Sun 395 00:19:54.680 --> 00:19:57.160 has a high metallicity because it is essentially made from 396 00:19:57.200 --> 00:20:00.519 the recycled, polluted douts of multiple dead star GA generations. 397 00:20:00.880 --> 00:20:03.400 This is an incredible detective story. So if you were 398 00:20:03.480 --> 00:20:06.319 hunting for an ancient cosmic fossil, you do not want 399 00:20:06.359 --> 00:20:08.839 a star with a lot of carbon or iron. You 400 00:20:08.880 --> 00:20:11.599 want a star that is fundamentally devoid of heavy metals, 401 00:20:11.960 --> 00:20:14.200