The Hydrogen 21-cm line

Waiting, waiting. — It I'm sending content. I promise you I am sending content. — Okay. Uh you are not on screen. — No, I'm working on it. Um, so for those wondering what the heck happened, I I got a cool new teleprompter from my husband for my birthday. And I don't know why, but it's causing my video to have like a 3se secondond delay unless I screen capture it. So, I am setting up a screen capture to get my uh video into uh our software cuz I don't I can't even with what's going on today. — Screen capture. So, you're just like just grabbing a chunk of the screen which has your video on it. — Yeah. Because pulling the video directly in is like I shall do a 3se second, [snorts] — right? Let's loop. Let's loop through the dark places of your Mac. — Yeah. Um, cool. Yeah. So, I'm back. Uh, we got back in Thursday night. — So, you're 50% of the way back from jet lag. Jet leg is a bear. — It really is. — You like you always think you're going to like you're just going to white knuckle your way through it. I'm just going to go to bed at my normal time and then I'll get a good night's sleep and then I'll be back to normal. — So I have been passing out on the couch at 2:00 in the afternoon. I have been wide awake at 2:00 in the morning. Um I So last night was pretty good. I How did this go? It was about 4:00 and I passed out on the couch. had a little nap, woke up at 5:30, — and then we made dinner, watched TV, Deep Space 9, and then Yeah. And then I fell asleep at 8:00, woke up at 9:30. — Uh said, "I'm going to bed. " Went to bed, woke up at 3, and I have been up since 3. — So, you know, that's like 3 hours off of my normal. Like, normally I'm up at 6:00. So now we're kind of, you know, we're in the strike zone and that's whatever four hours of four days of me trying to recover from my jet lag. So yeah. Um Thailand was fantastic. We had a really good time. Yeah. Um you know, it's always amazing to me when you have this place in your mind where lots of people said, "Oh, I went to Thailand. It was really great. " Or, you know, I went been to Japan. I had a really good time. Right? But you have no concrete actual understanding of what it's like to go there. You just have this mental model. Yeah. And then you go and then the practical reality is replaces this model in your mind of what it's going to be like. — Yeah. — And so now I have this very realistic understanding of what Tai, you know, we were there for a month, right? We were in Bangkok, we're in Chiang Mai, we were in smaller towns, bigger towns, big cities. We — we used the local transportation. We ate at the various restaurants and now I sort of now that chunk of sort of mystery terra incognita has been replaced, you know, the cloud on my g my map has been replaced, right, with the reality. And uh and so that was great. So we uh started in Bangkok and it's like a big loud city. Yeah. Um but it but it's — it's very modern. Um — the cars are all probably within the last 2 or 3 years. — Yeah. — Uh lot of electricity city. Yeah. — City city. Yeah. And we were in sort of a very touristy area and so it was very geared towards western tourists. So there were grocery stores. Grocery stores had Nutella, you know, like make sure the German tourists are happy. Um the prices were reasonable in Bangkok. you know, sort of like, oh yeah, you know, maybe that's half of what I would pay for going out to a restaurant back home. — Yeah. — Um, — everybody drives in motorcycles and it's so it's just mayhem. Like you're just on the road and people are weaving their motorcycles in and around you. Uh, just [snorts] and Carla, you know, you can walk up to those like motorcycle drivers sitting at the side of the road waiting for you. — Yeah. — And you walk up to them and go like, "I'm going here. " And they're like, "That'll be — a dollar. " And you get on the back of their motorcycle and they take you to your location. I'm like, absolutely not. I am. There's no way I'm getting on the back of a motorcycle. Like, like the laws of physics still apply in this country. They have not been rescended.

That sounds like a death trap to me. Carlo was like, "Woohoo. See you later. " So, she was like on these [snorts] motorcycles taking I was like, "I'm walking home. " So, I would walk home and she would take the motorcycle uh and really enjoyed it. Um and then I was like looking up motorcycle death statistics in Thailand and they're very high. Yes. Uh, so we did that and we were there for about 7 days and that was way too long. Bangkok is — Yeah. big crazy city. — Loud. — Uh there's no sidewalks so you're just you're walking essentially on the road wherever you go. And so we kind of had enough of it within about 3 days. We're like I you know unfortunately we booked our place so we wanted to go. — Yeah. And they went to Chiangg Mai which is actually went to place called Kang Chanambururi which is a little sort of rural region to the west of Bangkok near the border with Myammar. Burma. Um anyway and — Burma is now Myanmar. — Yeah. Yeah. So a lot of really cool temples and we so we sort of spent a day templing and it was the bridge over the river Quai. I still got pictures of me walking on the bridge over the river Quai and uh it was a cool little town. So, we really enjoyed that. — Um and then went to Chiang Mai, which is uh sort of this city in the north. — Mhm. — And much better. It was sort of more relaxed, not as bad pollution. Uh still a good services for tourists. We had some of the best vegan meals we've like ever had. They've got like they're really well equipped to handle tourists. So, it's got this kind of mix of old city charm with sort of the Thai experience. Um, food is much cheaper there. Like now you're looking at a restaurant meal for $5, $4, $3. — Yeah. — Um, and [snorts] so that was great. I really enjoyed it. Uh and then we went to the south to the sort of to the ocean and this town called Krabby which is like lots of islands and get on boats and go to various little islands and it was beautiful but it was also like a real touristy city and like I heard Phuket is kind of the same thing which just it was just filled with crowds of tourists who are like looking to have a good time like they're looking to party and we just wanted to just have a nice quiet place and contemplate, you know, nature. And so, uh, I think we kind of regretted going there and would have gone to a smaller town or even just like something that was totally off the beaten track. — Uh, and that's what we did for sort of the last town that we went to. So, we went to this place called Pacchong, which is right beside this — the national park in Thailand where you can see elephants in the wild. And so we stayed in this town and it felt very much like just small town Thailand where you know very clean while a lot of the other places were very a lot of trash to the side of the road. This felt very clean, felt like just you know people just live here and so the restaurants were ludicrously cheap. Um but it was also much harder to find vegan food. Um took a tour into the park try and see elephants. Failed but saw lots of other animals which was great. monkeys and porcupines and deer and all kinds of stuff. — Were you mugged by a monkey? — Uh, I was not, but uh Carla was sitting down watching a troop of monkeys and they just came over and like they were crawling all over her and getting into her stuff and she got bit a couple of times by them. — Yeah. Very gently like No, didn't break the skin, but it was like, you know, it was like a little unnerving. So the pictures of her where you like it looks [snorts] like she's smiling and then when you zoom in it's like it's a panicked — fearful look. — The I'm going to die now, but I'm going to try and look good doing it. — This escalated way beyond where I want to be. And there's like stupid idiots around — feeding them and — Oh yeah. — And but also like roughousing with them. [snorts] — Well, like you know how you like rough house with a cat? — Yeah. — And then the cat will learn to claw you and bite you. Uh-huh. And — you're like, "Oh, I really wish we hadn't done that when you were a kid, cuz now you're an asshole. " Yeah. So, they were the guy was doing that with a monkey. I'm just like, — "You asshole. " So, anyway, — um Yeah. And then we came home. So, I really loved it, but not I want to live there. Loved it in the way that I felt that way about Japan. Like, Japan, I'm like, "Oh, yeah. I could live here. Costa Rica, I could live here. " Um, but Thailand is like nice place to visit. I don't think I want to live here. — There's the density of humans is too much for me. — Well, the I mean the density is like that in Japan. It's just — but — and the people are so sweet. Like they're just so nice. Like you go to Japan and you are a ghost, right? You are just this ethereal entity that is moving through their space and they are

just Yeah. And they're just ignoring you, right? They have no interaction with you whatsoever. Uh Thailand, they're like really friendly and you know, people are really helpful. Like we came to this small town, we didn't know where our Airbnb was. We stopped our car. We asked some people and they we were trying to puzzle it together and we finally figured out where it was. Got back in our car and Carlo was like a little stressed out by this situation and they followed us in their car to make sure that we got to our place. And so they were just so people were so nice there — in a way that you just don't get in Japan. Like in Japan you are just this yeah like I said you're like it's like you're in limbo. You're moving through this you're out of phase with society — as a tourist. The the way I look at it is Japan has that same we know how to queue that you get in Northern Europe and Thailand has the same you're on your own dude that you get in Southern Europe. — 100% 100%. Yeah, you're in Thailand. It is you are fend for yourself but everyone's nice about it. But you are you know there are no rules in traffic. Like you literally just go I've decided I'm going to cross the road now and I may or may not make it across to the other side. Well, in Japan, you know, there's the there's like a walk sign and a stop, you know. Yeah. — And there's no cars in all directions and everybody's patiently waiting for the little walk sign, — right? While in Canada, we sort of we — You're in the middle of that. Yeah. In between that, right? So, we'll go like there's no car, so I'm just going to cross. Right. That that's the way we are. So, so North America has its own thing going. Uh, and then as you migrate south, you get more and more of the and we're done with queuing that occurs, — right? And I like when I was in Venice for a conference, I was unable to make it all the way to the window to buy a water taxi card because like little babushka Italian versions were jumping in front of me and like my brain's like, "You do not cut off a little old lady. " But the little old ladies have no qualms. They're totally cutting me off. And — yep, — a friend from Jordan rescued me and was able to get the two of us tickets cuz I was a coward is what I learned. I need to be a ghost in a country that cues, — right? So, it was great. We had a great time. I'm glad to be home. Uh the weather, you know, atmospheric rivers are awaiting awaited us, but it's been really warm. — And you live uphill. — Yes. — So, the atmospheric river is — Yeah. No, I can't. Our property can't — Flood. Yeah. I what's going on in northern Washington? — I'm getting pictures from Third Rock Astronomy and — Yeah. That's happening in BC, too. Like, we're getting ludicrous amounts of rain in the lowlands. Yeah. — All right. There you go, everybody. There's your update. Um and I'll have to do that again for my my question show later on tonight. Um so, let's do our jobs. — All right. I am going to admit to frustration. So I rearranged my desk because husband got me a new teleprompter and I have glasses that work for the distance that my monitors are at, but not the distance the teleprompter is at. And they're they just don't work. So — everything is just slightly out of focus. Okay, fetching done. I'm hitting record. Whoa, my audio is overwhelming. I'm going to try that again with — I've also pressed record. — How is my audio that overwhelming? What have I done? — I'm going to stop my recording. — Yeah, hold on. Let me test this. Testing testing. Why am I saturating my audio? All right, let's turn you down just a little bit because dear god. My mic has decided to mic. Testing, testing. Are we better? It's still saturating. Um, let me make sure I have the correct mic. — Cuz you sound fine to me. You sound exactly the same as you normally do. — Yeah. Hold on. — So, your mic isn't hotter. — Yeah. Um trying to figure out what the heck is happening. — That sounds right though, that it's it's using cuz I'm sure there's like seven devices using the wrong microphone. — Yeah. Probably the one on your Mac. — Yep. That's exactly what's happening.

— Good lord. — Technology humans technology. There will be there will come a day when we will be able to record a podcast and the audio won't be a pain in the ass. That day is not today. — No, — but there will be a day. Yeah. In the far future [sighs] — as the robots crunch across the landscape, stepping on human skulls, firing their pulse rifles. — Testing, testing. All right. — Deploying proper microphones to everybody. I am now recording all the places. — Okay, I'm going to record too. I am also recording. — Thank you, Rich. — Here we go. Astronomycast episode 774, the 21 cm line. Welcome to Astronomycast, our weekly facts based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I'm Fraser Kane. I'm the publisher of Universe Today. With me as always is Dr. Pamela Gay, senior scientist for the Planetary Science Institute and the director of CosmQuest. Hey Pamela, how you doing? — I am doing well enough. Uh I am currently fighting. I have new technology which is absolutely amazing but nothing works. So uh everyone thank you for your patience as there is ludicrous hacking that went into putting together this episode — right — now. Now, normally after the amount of time that I've been gone, I would say like, "Hey, it's great to be back and did you all miss me? " But because you in your infinite wisdom said, "Let's just record all these shows, get them in the can, and then you don't have to think about this anymore. " Um, we recorded all the shows, we got them in the can, and then I didn't have to think any about them anymore. And I am so glad. You were so smart. You were so right because I because it is, you know, being on the road, it's a gone show and trying to then set up internet of different time zones. It was so nice to go, oh yeah, all those astronomy casts are done. So, uh, and now we continue uninterrupted, which was just great. And — and you escaped the Cosmoquest Hangouton this year. There was no live recording of Astronomy Cast. You did not have to be part of our wild fundraising. By the way, if anyone wants to donate money, please join both Fraser's Patreon and my Patreon. We both need your support to keep doing what we do [clears throat] as independent journalists. — All right, that's done. [snorts] — Yeah. Uh and this will turn into a rant, so I don't want anyway. I am finding that journalists are reaching out to me and saying, "Do you have any work? " — Yeah. It's really bad right now. And that is telling me that the sort of copyrightiting apocalypse is starting to roll out. And fortunately, because we're Patreon funded and we don't use AI for our writing, uh we are going to be this island of stability as the rest of this industry erodess all around us. So, thank you everybody who supports us financially. Uh you are allowing me to pay everybody's salaries. All right, let's get into this week's episode. Molecular hydrogen is the raw material for stars. But there's a problem. It's cold and dark, but it can do a very rare trick, releasing a photon in a very specific wavelength known as the 21 cm line. And thanks to this wavelength, astronomers have mapped out star forming regions across the Milky Way, the universe, and it will allow them to peer into the dark ages of the universe. We'll talk about it in a second, but it's time for a break. And we're back. All right. 21 cm line. That's it's a very obscure sounding topic or very a very nerdy topic, but it is like just one of the most useful tools that astronomers have at their disposal. And it's kind of weird that we haven't talked about this up until now. I mean, we've mentioned it, but I think, you know, let's give it the the, you know, the appropriate amount of conversation. And I have to admit, I had to go back and rewrite the show promo I initially wrote because I was quite certain that not only had we recorded an episode about this, which I determined we hadn't, I was also quite certain that a Nobel Prize had been given to the 21 centimeter discovery humans, which it hadn't. This is a line that's like super important and just doesn't seem to get the love it deserves, — right? It will. — It will today. — Maybe after the show. We're giving it the astronomy cast bump. — It's true. And yeah, go ahead. — Okay. So, I guess let's talk about I'm trying to think. Let's talk about molecular hydrogen. I guess the raw material for stars. — So, so molecular hydrogen and just to be clear, the 21 cm uh hydrogen line comes from atomic hydrogen. It comes from the atom of hydrogen.

— Yeah. So, uh, molecular hydrogen take two hydrogen atoms. They each have one electron going around them in normal cases. And it turns out that the electron shells and atoms really are completionists. And I understand this as someone who is a completionist, — right? You and hydrogen, — get all your tasks in. — Yes. Exactly. And so the s shell wants to have two electrons in it. So two hydrogens, they get close enough together or like we can complete our shell if only we share our electrons. And so they come together, they share their electrons, they complete their shell, and they're much happier this way. — So this is the stuff of the cold, dark universe. — Right. And I think it's really important to sort of understand like when hydrogen receives a lot of radiation then it starts to warm up. It glows. Those are nebula, right? We see them. — Um but they don't want to turn into stars. They're too hot, — right? And so with 21 cm line, this is something that you don't encounter in anything vaguely warm. So you've now taken me in a direction I was not prepared for. Where are we going? — Well, right. So the I guess the point here is that if you want to find — clouds of hydro clouds of hot hydrogen — Yes. — all you have to do is look out there with a telescope. — Yeah. Hydrogen alpha. So, so there's two major series of hydrogen lines that we look for depending on what red shift we're looking at. So, in the local universe, we have the bomber series, which is electrons jumping from higher energy levels down to the second energy level. Then at the ultraviolet locally, we have the Lyman alpha the Lyman series where Lyman alpha is 2:1. uh and so it's higher energy level into the first uh energy level — and as things get redshifted further and further that lime and alpha eventually migrates into the visible wavelengths and it allows us to see hydrogen at the highest uh red shifts out there up until the point when there's no light going through the universe when we have this foggy period uh that before the universe reionized. — Right. And like I can look I have nice dark skies here. I can look towards — and I can see the Orion Nebula. — Yes. — With my eyes. Right. There is this little glowy spot uh in Orion Scabbard and if you look in pair of binoculars or a telescope then you definitely can see it. And then you take a picture and you can absolutely see it. And so to see where the clouds of hydrogen are that are ionized, — yeah, — that are bright, uh, glowing, you can it's not that challenging a problem. The what is the challenging problem is to find the hydrogen that is cold. has not uh been ionized, is put pumping out radiation that is cold. And yet it is that cold hydrogen which is the raw material for stars. And that's where I'm going is that astronomers need a technique to find the cold hydrogen. — Yeah. And so what we're looking for is the stuff that isn't so dense that it's blocking the light behind it. So it's fairly easy to spot molecular clouds of super dense hydrogen. They are great walls of blocking the rest of the galaxy. — Yeah. — Uh so these are things like the boach globules. I to see warm stuff you have all these wonderful transitions in hydrogen that are quite happy to transition for you. So if you have hydrogen atoms that are not getting collisionally excited, heated up by surrounding light, that are just cold, non-interacting, so diffuse, this is like collisions are not a thing that an atom can expect to experience. This is where you start to be able to imagine and it turns out that it's actually there that you can start to see what are called forbidden transitions. These are transitions that statistically we just should never have a chance of seeing, — right? And the p the specific forbidden line that we're looking for is if you have a proton that has a spin up and you

have an electron that has a spin up that has a higher energy in that alignment than if you have them anti-aligned. So if the electron flips from spin up to spin down or if you had the proton down and the electron up and sorry if you have proton down electron down and it flips to up that flip between being anti-aligned sorry aligned and being anti-aligned gives off a tiny amount of energy And the smaller the amount of energy, the longer the wavelength of light, — right? — And in this case, that length of light is 21 cm is your wavelength. So you're going from something that is like hair's breadth to can measure it with your hands. — Yeah. Yeah. I mean like when we talk about wavelengths of light, we're talking, you know, often it's like, oh, it's 500 nmters or you're thinking about visible light and it is like we don't have any practical experience to understand how small that is. — We talk about even infrared light. We're looking at things that are in the micrometers to sub millimeter. — Yeah. But the you know this thing the 21 cm line you know it's like about that. — Yeah I'm podcast listeners I'm holding my hands out you know about 23 of a foot right 20 cm. — What wavelength like what regime is that in? Is that in the microwave? It's part of the Lband of microwave. Radio is where my brain puts it because all of that is something that — you can measure with basically a radio dish. — Um it's just what is the horn you're using. So this is part of a atmospheric hole that there's various wavelengths that our atmosphere is like no you're not allowed to observe that. And this luckily falls into one of the bands that we can completely see from the surface of our world. And so when folks were starting to get a handle on quantum mechanics, we're these are all the different ways that energy can get released as protons and electrons flip and interact. Um it was predicted in 1944. This is a fairly new realization. Um it was predicted in 1944 that this could [snorts] be something that might be observable. And then in 51 they finally put together the set of observations to detect this super faint line. And the reason it's faint is the alignment that we're looking for with the aligned proton and electron that flipped to be anti-aligned. That atomic situation is stable for 11 million years. — Right? [gasps] So no hold on. So, so I take a proton — Mhm. — with its electron. — Mhm. — And I leave it — Uhhuh. — And then if I wait 11 million years, that's about how long it's going to take for it to do that spin flip. — That That's halflife is the wrong word for this. But — the probability of it flipping is probabilistically 11 million years. — It'll probably happen. — Yeah. in 11 million years, right? And so I if you have any one individual proton of hydrogen, — it's not going to do this in, you know, in a thousand lifetimes. But you get a cloud that is large enough, — then some number of them is giving off this signal. — So you need to have a cloud that is excessively cold, so things aren't moving around very much. excessively diffuse. So, what little motion you're going to have just because of the base temperature of our universe isn't going to let these things collide with each other on time scales of tens of millions of years. And then you need enough atoms enough of these flips are occurring that we're receiving enough light in our direction that it's detectable. — All right. So, we're going to talk about why this is important. We've talked about the how it happens. Now we're going to talk about why it's important.

But it's time for another break. And we're back. Okay. Why Why is this important? Why is this this weird behavior of atomic hydrogen? Why does this matter? [gasps] — I there's a variety of different reasons. The first is it allows us to map out the least dense corners of our galaxy. the outer parts of the disc. Uh it's like the line that we can catch from diffuse clouds of hydrogen uh that are just barely gravitationally held onto. And it's from these 21 cm measurements of our galaxy and other galaxies that folks like Vera Rubin were able to start saying, "Wait, these motions don't what's going on here? " And — the person not the telescope. — Right. The human being. Sorry. That's now a requirement to say, isn't it? — Yes. Um Yeah. Um so the human being who studied dark matter um right — along with other human beings who studied dark matter — were able to spot this flattening of the rotation curve of our galaxy which isn't something anyone expected. It was expected that as the visible material dropped off, we'd see the velocities decreasing with distance, — right? — And they're not, which says there's a whole lot of stuff out there that — isn't gassy enough to have forbidden lines, — right? So that we look out in space and we see a galaxy and we see all of the stars. We see all the star forming regions. bright stuff. — Yeah. — But that is not the galaxy. There is more galaxy around that galaxy. There is it is also surrounded by clouds of hydrogen — that will maybe eventually get pulled into stars or maybe get spun out or be sucked away through tidal tails through interactions with other galaxies. How big is that galaxy really? — Right? by mapping out this cold hydrogen which is I guess more dense than just the than just inter galactic space. Yes. — Right. There's more stuff in that than there is just in intergalactic space. You can map out the real shape of the actual galaxy and all of the clouds of hydrogen that are surrounding it. And this is something that we can do locally. And a standard homework assignment at uh many universities that have small radio telescopes is to just assign a senior lab where you go out and you measure the 21 cm uh line in clouds of gas around the Milky Way and you do your own rotation curve repeating this historic uh work. — Um that's amazing. Yeah, it's it's one of those things of it's fundamental, but it's repeatable in a way that you can't deny that there's unseen stuff that is out there when you see the data for yourself as a student. — But I think it's important to like qualify that's not dark matter. Like that's dark, matter, right? It is regular matter. regular hydrogen your, you know, your — Yeah. — Whatever 75% made of the stuff. — Yeah. — Right. Or whatever. — But dark matter is different stuff that does not react with the electromagnetic force. So, — but and so like as an astronomer, you might be able to ask questions like how much gas is left in that galaxy that can form more stars? Where are the reserves of gas in that galaxy? Do they line up with the spiral arms? How do they transition between just clouds of gas to star forming regions? What is the potential of that galaxy? That's where you're you're mapping out using the 21 cm line. And that is the next place to go with this is so first you have for me the most interesting part which is the discovering that not all the stuff that makes up a galaxy can be detected through gravity allows us to see that other stuff is out there. And then what is the potential for star formation? continued life? What is the stuff available to feed super massive black holes? There's amazing maps of our own galaxy that allow you to see all throughout the disk of the Milky Way the presence of 21 cm emission. And that's telling us there is still gas and dust

out there. Not so dark that it blocks all the light, not so hot that it glows in bomber or uh lyman lines of hydrogen transitions. It's just out there being diffuse and not colliding. — It's our war chest. It's our gas reserves — that the Milky Way can draw on for trillions of years into the future to make new stars. And the question, you know, astronomers will ask this question, how many stars can this galaxy make? It comes from the cold sort of inert hydrogen that's just sitting there, not glowing, not interacting, not blocking light, just [snorts] being. — All right. Oh, go ahead. — No. — Okay. All right. We're going to continue this conversation and sort of think back in time, but it's time for another break. And we're back. All right. So, you take your microwave telescope, you tune it to 21 cm, you point it in the sky thanks to the atmosphere allowing that wavelength to get through and then you just move around and you map out blobs here, blobs there, and so on. How does that then change as we want to look out into the cosmos, which of course is looking back in time? So we have two ways to see the cold blobs of gas that haven't bothered to get themselves into galaxies uh as we look out. So, one of those is uh we see what are called the Lyman Alpha forest where the light from background galaxies uh passes through clouds of gas that are between us and those galaxies and at the red shift of those galaxies we see the hydrogen lines of absorption. Now the other side of that is sometimes we are lucky enough to see the everlengthening hydrogen uh 21 cm emission from those gas clouds. And once you start getting out to around 50 cm, you're starting to look at cosmological distances where there really isn't much light to give us a clue as to what's going on. The only way we're ever going to be able to detect light from the dark ages of our universe is to look for this extremely faint background light. This — right and you mentioned sort of 50 cm. So in other words that you know the universe has been expanding. the wavelengths have been redshifting in the same way that what was once red light after the cosmic microwave background has turned into microwave. Uh this light started out in the microwave and has now been redshifted into much longer wavelengths. So — and there's two different effects that does this. one is just as the universe expands it expands the light with it and the other is just the cosmological expansion of the universe adds its own red shift. So it's a really ugly calculation. Um but it means that while interesting things like lime and alpha land in the visible uh things that started out long ended up even longer, — right? And so that takes a very special kind of telescope to see redshifted 21 cm line. And [clears throat] we haven't gotten to the point yet that we're starting to detect this age before reionization from this light. It is something that we dream of doing that we plan on doing, — right? — Um but — Yeah. And like I think it's really important for people to understand, right? like you had the beginning of the universe, you have the cosmic microwave background, the whole universe is kind of red — and then it's becomes transparent for the first time and then it cools down, — but the first stars haven't formed yet. And so now we talked about those clouds of gas that are in the Milky Way. Imagine if the whole universe was that, right? Where is the stuff, — right? — Well, you need the 21 cm line to show you where the stuff is. So, so that is the key to us understanding how those first galaxies, those first stars came together at a time when everything is obscured and you can't see it. Wasn't until all of those galaxies got going. The stars got going, they cleared out

all the rest of that material and we could see them again. That's the reionization you're mentioning. So, um, so what are the sort of best ideas people do this? You mentioned we're kind of at the cusp like we really are at this point in the history of astronomy where this is a technique that is just within reach for us to be able to try and observe the first you know the to map out this initial cold hydrogen. So you know what can we sort of count on do you think? — Well first of all we need to get more detectors off our planet. That's one of the big frustrations is as we get to this particular set of wavelengths, we're fighting tooth and nail against the atmosphere, — right? — There are atmospheric poles shortward of this, longward of this, — right? — This is a cursed wavelength, — right? So you mentioned that if it was just a regular 21 cm line, then it gets to go through the atmosphere, but now it's been redshifted. So now the atmosphere is not playing nice with it anymore. — Correct. So building radio telescopes in space is something we have the capacity to do. But there's other things that are a whole lot more interesting that like the James Web Space Telescope is capable of looking at myriad different problems. A longwavelength radio telescope is going to be difficult to build. You have to have really big dishes to get any kind of resolution or you have to have an interferometric system to get any sort of resolution because your resolution is dependent on the diameter divided by the wavelength. Your wavelength goes up. You need a bigger diameter to get the same wavelength, — right? But it's also faint, — right? Like it's faint, too. So, so an interferometer doesn't get only gets you so far because you also need a telescope that can handle you need a lot of just antenna space. So, you need something that is has a large amount of resolving area and has a large baseline ideally. — Right. — So, so have you like looked at some of the cool lunar telescopes to try and solve this problem? — They are dead to me until they're funded. — Right. Of course. Yes. All right. Well, then I am going to explain this to you. Yes. — Uh which is that there are a bunch of teams that are working on ideas for moonbased telescopes that would be on the far side of the moon. So it would be blocked by the the moon and so you wouldn't get the radiation coming from the earth. All of our stupid, you know, radio traffic, you would have this pristine dark radio environment and then they could build really big telescopes. There's these ideas where you would say land a spacecraft on the moon. You have a rover on board and it would reel out an antenna because you know you need to have you don't need to have like a big dish. You can actually just have a wire that you put down on the surface of the moon in a shape that you need. Um, and so you could have this sort of central lander and then rovers that are crawling out in all directions from it, laying down antenna onto the regalith that would form this gigantic antenna that's blocked from the surface of the Earth. There's one called Far Side. There's one that NASA is working on. Um, the Europeans are working on some ideas. Uh, the Chinese are actually going to be doing a test in a couple of years. They're going to send a uh radio telescope to the far side but orbital uh of the moon and try to make some detections of the 21 cm line at the dark ages of the universe. — It's super important to put this stuff on the far side of the moon because our atmosphere does leak these radio frequencies because we are literally using radio and television in this frequency band. Yeah, we're yelling in this frequency band and we would uh corrupt the results from a radio telescope, — right? So, we have to block our own shouting. Um — yeah, — as we look for this — and there are enough plans now that one of these is going to happen like there are uh there was a prototype experiment that was on one of the lunar landers that toppled over. — Yeah. — And so they were going to try and make those observations. uh you've got like some tentative observations with things like the merches array which is the precursor to the square kilometer array. You're probably going to get some detections using the square kilometer array but it's sort of not its main job. So, it's really going to be, let's put a telescope on the far side of the moon, a big giant antenna or like a wire spooled out across the moon that will get us these observations cuz they like

the other ideas like little Christmas trees like what they've got with the merchants array. Uh there's been a lot of like really cool ideas like it doesn't have to look like a telescope. Doesn't even big radio dish. It can be this very simple, very robust telescope and yet it will do this job and detect and then we could there could be this time when astronomers are able to start to just get a sense of the density mapping out — you know how thick was this hydrogen early on when what was the separation between the clouds of hydrogen and the initial galaxies that were forming do we see the super massive black holes forming first pulling in material from around them so there's a lot of, you know, physics. — Yeah. I mean, it's called the dark ages for a reason, — right? And there's one other obscure usage of this line that we haven't talked about and that's the idea lots of different space observing civilizations would probably want to protect just as we've protect uh protected this line from being used for everyday transmission. So we don't have radio stations or television stations using this wavelength because it's reserved for astronomy. Now you can start to imagine that if that is a common habit preserving wave bands for science that uh there could be civilizations out there that decide they're going to transmit purposefully making their existence known at this particular wavelength. So — um it's been proposed that the 21 cm line also works for SETI potentially. — I love that. Yeah. So, I particularly love the idea that wanting to do science is something we should expect to be a universal idea of civilizations. And I really hope it's true. I really hope — That's awesome. Wonderful. Thanks, Pamela. — Thank you, Fraser. And thank you to everyone out there in our Patreon audience. You are all amazing. And [snorts] this is where Rich inserts the previously recorded uh names. — Great. All right. Thanks everyone and we will see you next week. — Bye-bye everyone. And then they saved — 774. — I thought it was 775. — Uhoh. I said 774. I don't have a 775 in my list. I may be off by a number — and off by one error. — Yeah, that's kind of a thing for me. — Does the array start with zero or does it start with one? [sighs and gasps] — Uh, okay. — No, this is 775. How does bad science happen with 774? — Why do I not have that in my Trello? — I do not. — That's weird. Okay. So, let me re-record my intro then. — Okay. — Yeah. Um, and would you rather I use atomic hydrogen than molecular hydrogen? That would be better, right? — Yeah. — Okay. Let me just do that. — Astronomycast episode 775, the 21 cm line. Atomic hydrogen is the raw material for stars, but there's a problem. It's cold and dark, but it can do a very rare trick, releasing a photon in a very specific wavelength known as the 21 cm line. And thanks to this wavelength, astronomers have mapped out star forming regions across the Milky Way, the universe, and into the dark ages. Bill Nash has shared a churro emote that looks strangely like a satellite because it's a churro with just like a napkin around only the center, so it looks like a core. the square with massive solar panels off to both sides. But no, I'm told it's a churro and I simply see satellites where there are none, which is a new form of paridolia. I am not amusing Fraser. — Whoops. [snorts] — That was a mistake. — Whoops. Okay. — Please tell me you didn't just delete the episode. — Nope. — Okay. — No. I just accidentally pressed play on something. Okay, so let me fix this. [snorts] Everything's fine. Just — All right. Not having used my computer for a month, everything is requiring me

to log in again. — Oh, I feel that pain. — I am annoyed. Okay. All right. So, that will give Rich everything he needs. Um, and then let me change this on the Trello. Are you sure it's — okay? Cuz I don't see the seven. Oh, I see the problem. I see. We did. I did two 772s. Okay, we're fine. Everything's fine. — Um, all right. We got five minutes. Any questions? — You want churros now? Yeah, because of Bill Nash, I want churros. — Okay, I'm going to reveal something. — Okay. — I've never had a churro. — Have you had a beaver tail? — Well, yeah. Obviously, Canadian, but like a chereo is a like a — kind of — It's a stick of fried — tubular. — Yeah. — Right. A donut and it's cinnamony and sugary. Right. — Okay. All right. It's related to a beaver tail is what I'm saying. — I mean, a beaver tail is kind of related to a cinnamon bun, but it's like in donut form, — but it's kind of related to a apple fritter. — Um, so, so do I have it kind of right though? Cuz like I've had like a crunchy thing. I'm surprised that you you've like taken your kids to Disneyland more than once and avoided churros. — Yeah, I've Disneyland. — I've visited my wife in Texas, — right? TMEX and swore that we eat for every single meal and did — so at some point come across a — churro. But Fla is superior to churro. So if you were at a restaurant, Fla was the superior dessert choice. Yeah, we had beignes. — Yes. — Which are the sort of — That's French though. — Well, it's Louisiana. — It's Cajun — donuts. Yeah. Cajun donuts. Man, they were good. — Uhhuh. — Um but uh yeah, I've never had a churo. I mean, like I can wrap my mind around what a churo is and I sort of like I can see it and sort of envision what it tastes like to eat a churo. — Uh but I've never actually purchased a churro. And no, I didn't do it at Are you kidding? The prices at at Disneyland, I wouldn't let my kids eat food. We packed lunches. — So, I'm trying I'm pretty sure it was when I was in Lisbon there. There were shops that sold fries and just had a sign that was a cup with fries coming out of it. And there were shops that sold churros and had signs that were just a cup with churros coming out of it. And it was just one of those things where I neither got fries nor churros, but the signs stayed with me, — right? — But every civilization, it feels like every part of the world has their own — carbohydrate fried. — Yes. Thailand didn't — really. — Yeah. Well, like they had taken to western carbohydrates fried like you — you could get tons of French fries, hash browns, uh waffles are really big there. And of course there's the famous banana pancake trail. — Yeah. — In Thailand where you sort of follow a trail of hostels where they like back in the 70s — Uhhuh. made banana pancakes for the tourists. Um, but yeah. Anyway, uh, like I know in Japan like they fry things like crazy. Was there like deep fried stuff there? No, there like there wasn't a lot of deep fried stuff when we were in Thailand. It's weird. Like a lot of meat on a stick. — Yeah. — Which, you know, obviously we avoided. Um, — they didn't have dumplings — like rav a ravioli. — You would have like Yeah. You would have Chinese they would have like bow buns like Chinese — restaurants or ch you know that kind of thing. But I'm but I — I thought bow buns were Korean. — Well ba the word bao means treasure bun like treasure bun in Chinese. — Okay then I know nothing. I just eat it. — I mean no like everybody like as you said the — the dumpling is the universal — uh creation. Yeah. But anyway, um what we got? One minute left. What's going on? — Come on up. — I am uh yeah, just madly playing catchup after our hangout and uh looking forward to not focusing

on fundraising. I think cleaning my house is the continued priority for today. — That day will never end. come. — I know — you will always be obsessed with fundraising. has been the who you are since I've known you and it will be what we will be in the future. — Yeah, Patreon I have to say that is where our commitment is right now because they're going to be there for us no matter what. And if any of you out there aren't using the Patreon app yet, the they have created a basically scroll of creativity. So instead of doom scrolling the socials, you can uh mine feels like D and D scrolling because — what a great way to approach that. That's Yeah, I like you can follow people for free on Patreon. You don't have to pay them. And so as you Man, that's so great. Yeah, you can just fill your feed with creative people doing creative things and nobody — is trying to rage bait you. Nobody is trying to get you to hate your common uh citizen. It's just people making fun things and you being able to see the results of that — 3D printing stuff. Oh my goodness. — Yeah. And like just follow a ton of it — and just enjoy as opposed to someone trying to raise your hackles and make you hate your common man. — It's amazing. Yeah, it's mine is 3D printing and DND and painters and — Okay, I'm gonna do that. I sort of always have it installed, but I never I haven't customized it or like I don't really use it. — I do everything on the computer, but I think you're exactly right that if you're going to doom scroll, — creativity, — doom scroll. Yeah, totally. Yeah, good call. Like I think we're at the point now where this stuff is so infested in our brains that we need to be able to wait to pick up our phone and go, you know, scroll. — I'm making coffee. I need to scroll. — Yeah, exactly. I'm on the bath. Yeah, I'm on the toilet. Yeah. Now, — so let's swap out the bad hat. Like I just deleted Reddit off my phone. Like I was using it while I was traveling just as because you're like killing time in an airport or whatever. — Uh but I I can see my the dopamine is making me reach for my phone to see what's on Reddit. No. — Get lost, Reddit. — I'm going to replace you with this habit. No. Good one. — Awesome. Okay. Well, I'm going to be back in five hours. So, those of you who want to join me for my live stream, I've put the event on the channel and I will see everybody then. — All right. See you all next week. Bye-bye everyone.