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1:12:43of these halos and the normal matter that's inside of these halos. So let's talk about something called hydrostatic equilibrium. Normally we think about hydrostatic equilibrium in the sense of why are stars stable? Like why isn't every star a black hole? >> Yeah. >> Okay. Because if gravity is just like pushing everything inside, >> then everything should just collapse into a black hole. Well, the reason is there's a bunch of fusion going on and that fusion creates an external pressure outward that is counteracting the gravitational pressure, right? And that's what keeps a star >> stable. >> Yes. >> Okay. Now let's think about dark matter halos. >> Okay. >> Okay. With dark matter halos, remember I told you that during reionization, this cosmic dawn is what they call it, when
1:13:25there [clears throat] was a bunch of ultraviolet radiation, that ultraviolet radiation is going to heat up >> normal matter. Okay? So that normal matter is inside of a gravitational well of dark matter. So it's getting pushed in because of gravity, but it's also hot. >> Yes. >> And the hot is going to push out. >> And so there itself is a kind of hydrostatic equilibrium. The hotness is not coming from fusion. It's just coming from being bathed by this early ultraviolet light. >> Yes. Got it. Got it. >> Understood? Yes. >> Okay. So, there we get into a very cool thing >> which is there's actually a size limit to how big a dark matter halo can get before it starts having stars inside of
1:14:08it. Because imagine a very large dark matter halo. Okay? It's got a bunch of matter inside. the very large dark matter halo is going to start collapsing. The matter inside is big enough that it's going to start collapsing and it's going to start creating galaxies. But if it's really small, >> yeah, >> the matter inside is just going to sort of evaporate out >> because the hotness is too big. >> It's sorry, the hotness is too hot. The the particles are moving too fast for the gravitation of that small tiny little dark matter halo to keep everything inside. So there there comes this critical mass threshold >> and they calculate it. the the paper actually quotes it to be around 10^ the 9.7 times the mass of the sun. The Milky
1:14:49Way for context is around 10 12. So this is much smaller than the Milky Way. But at that critical mass threshold, the dark matter halo is big enough where it's like keeping the stuff inside. Okay? Anything bigger and the stuff that's inside is going to coales to become a galaxy and become stars. anything smaller and it's not enough and the and the gas is just going to leave. But right at that threshold, we've got this sweet spot where >> the gas is going to retain inside of my halo, >> but it's not enough to create stars and galaxies. >> You see, there's a sweet spot. It it it
1:15:29is it is this it is this like buffer line >> where it can just contain a lot of this is why like I'm thinking about this is why the cloud metaphor becomes interesting here because it doesn't it doesn't it's at a threshold where it can't materialize ma like this physical large scale matter the way we think about it and it's just these gas particles all >> yeah just moving around >> hanging out >> hanging out yeah and and so this there was a term that was coined in 2017 in monthly notices it was called the relic which is I think it stands for reionization limited H1 clouds. H1 meaning neutral hydrogen. Okay, it was [snorts] a paper out in monthly notices and what they said was there should be these halos right at the critical mass,
1:16:12right? That should be massive enough to retain neutral gas, >> neutral hydrogen gas, but not massive enough to cool down and condense and form stars. >> Yes. Okay. Okay. Yes. >> And these are called relics. So, finding one of these relics would be awesome. >> I just want to quickly pause. Yeah. >> And say back engineering. I can't remember what the name for it was. Back engineering. The acronym from what you want it to sound like. >> Relics. I mean, it's CL. >> That's a good one, dude. Scientists do this all the time. So, relics is a really good one. Reionization limited H1 clouds. >> That's pretty good. >> Okay. So, finding these relics would be really awesome. >> Yes. But finding it is very hard because
1:16:53this thing has no stars. >> Right. Right. So how do you >> The whole point like how do we see something? We we see it using light. But this thing has no stars which >> and dark matter is invisible. >> Which is Yeah. Right. Which is what makes any kind of discovery around this subject of dark matter extremely difficult. Yes. >> Because our we are only as good as the instruments we build in order to understand the world around us. And we have not yet maybe >> figured out a lot of this but maybe >> maybe maybe and what comes to the rescue is the hydrogen 21cm line. >> Yes. >> Okay. This is um our favorite >> line 1420 MHz. This is the hyperfine transition. This was used in the Pioneer plaque to tell aliens what our standard
1:17:37time and our standard unit of measurement is for all of the, you know, diagrams that we have on the Pioneer plaque. In case the aliens found the Pioneer plaque and they wanted to see where Earth was. All of the standard measurement on that pioneer plaque is made using the hyperfine transition of hydrogen, which is effectively hydrogen has a proton and an electron. If they're spinning parallel to one another versus when they're spinning opposite to one another, that difference in energy corresponds to a particle of light, a photon that is exactly the wavelength 21 cm or around 1420 MHz, right? Um, sidebar, at Princeton Physics, one of my
1:18:19favorite classes was the undergraduate advanced physics lab where we actually used the hydrogen 21 cm line to map out the Milky Way. There was a radio telescope on the roof of the physics department that we could use to point it in different directions and we could point it >> at the galactic disc of the Milky Way and you could see the 21 cm and you could actually figure out what part of the galactic disc was moving away from us versus moving towards us by pointing