(theme music plays)

John: Hi, I'm John Green. Welcome back to our walk through the entire history of the universe from beginning to end. In this episode, we've arrived at the moment when the stars turn on. Katie literally calls it that, just staggering to me. And we'll also see how the universe became visible to itself. But first , after our last episode, I can't help but ask Katie about free will. I think everyone should be proud of me actually, that I waited until four episodes in to ask about free will. So, here's our conversation. 

(theme music plays)

John Green, Very Curious: So, Katie, I was listening to our last episode and something occurred to me belatedly that I wanted to ask you about. It seems like a lot of this stuff was inevitable that, that each thing proceeded from the previous thing, right? 

Dr. Katie Mack, Astrophysicist: Yeah, I mean, you know, there was a sort of series of events where, I mean, it's all just kind of following the same laws of physics once you set it going. 

John: Yeah. That's starting to make me feel a little bit like once you set it going, there was always gonna be a galaxy called the Milky Way. Not called the Milky Way, but where the Milky Way is the size and shape of the Milky Way. And it's starting to make me think that there was always gonna be a sun. And it's starting to make me think there was always gonna be an earth, once we set this stuff in motion. And just to be clear, we didn't set it in motion. And that's starting to make me think, and I'm really hoping you can correct me on this one, that there was always gonna be a me. 

Dr. Mack: Uhm... I mean, the thing is right, like

Dr. Katie Mack: when you're looking at it from the perspective of now, all you can do is trace back all of the events along the timeline that we have followed. But there were a lot of different places in the past where things could have branched one way or another. Like it's true that once you set down the primordial fluctuations in the plasma of the hot Big Bang, those kind of have to grow into galaxies and clusters of galaxies and, and galaxies have to make stars and planets. But, you know, whether or not they had to make the earth and us, like there, there are a lot of things that are just random chaos that come into that. I mean, things like, you know, in the formations of planets, you have collisions between large rocky bodies and asteroids and, and things and stuff coalescing from the protoplanetary disk. And that could have gone lots of different ways. 

John Green: So what you're saying is, and I guess what I'm asking is, is free will still... in the equation?

Dr. Mack: Uhm...(hehe) That - I feel like that's above my pay grade somehow?

(John laughs)

Dr. Mack: Like, I feel like, free will is a, a very human question. And in the work that I do, humans are so insignificant that like, 

John: Yeah. No, I'm getting that feeling. 

Dr. Mack: it's not, I wouldn't even know, I wouldn't even know where to begin with that. On the cosmological scale, we seriously don't matter at all. Right? But whether or not we feel like we can, you know, make our own little choices in our own little space that I don't feel is something that I am qualified to weigh in on. 

John: Okay. Okay. 

(Dr. Mack laughs)

John: Now, when you say we don't matter at all, that's a little bit of. stinger for me. I have to confess because

Dr. Mack: Yeah. 

John Green: I know that we don't matter much, but don't we matter a little bit? But here's, here's my argument for why we might matter a little bit. 

Dr. Katie Mack: Okay. Okay. 

John: We are the ones doing this observing, and that's interesting. 

Dr. Mack: Right. I mean, we matter to our own... ability to gather information that- that's, that's true. 

John: Right. But we, we're not affecting the cosmos. 

Dr. Mack: No, there's this thing that I sometimes bring up when I'm talking about dark matter and dark energy and, and the nature of the cosmos in, in the talks where, you know, the famous image of the Earth as the pale blue dot, right? So this is Carl Sagan, 

John: Yeah.

Dr. Mack: and they, it was Voyager 1, they took a picture of the Earth from billions of kilometers away, and it just looks liek a speck of dust. Carl Sagan gave this beautiful speech about how, you know,  on that speck of dust, you know, every, every nation and every powerful ruler and every, you know, king and, and peasant, and you know, we're all on that little speck of dust. None of us matter, you know, I mean, we're, we're not, you know, we're, we're a tiny, tiny speck of dust in the universe. You know, I think about that picture sometimes.

And then I think about, well, in the bigger picture, you know, most of the matter in the universe is dark matter, like 85% or something, and we're gonna talk about dark matter and dark energy later on. But most of the matter in the universe is dark matter (in) that we, we can't interact with except through gravity. And most of the rest of the universe, if you just kind of add up the energy of the stuff in the universe is dark energy.

And dark energy is this even more mysterious stuff that has to do with the expansion history of the unvierse and, and how the universe is expanding and, and so on. And we don't know what dark energy is, but we can't do anything with it. It's just kind of a, apparently a property of space.

And so if you add up the sort of contribution of both dark matter and dark energy in the universe in terms of the energy density

Dr. Katie Mack: of the stuff in the universe, that makes up something like 95% of the stuff in the universe is either dark matter or dark energy. 

John Green: Oh no. 

Dr. Mack: And so the stuff that we're made of regular matter, what we call baryons in, in astrophysics, that regular matter and, you know, electrons and protons and neutrons, and, and you know, even, even radiation, all of that together makes up about 5% of what the universe is made of. And so, if you think about it that way, like the dark matter has the strongest role in the formation of structure in the universe. Dark energy has the strongest role in the evolution of the universe. We're just kind of the, the sort of window dressing, like our kind of matter is kind of just sort of along for the ride. Like it's important on the scales of galaxies, but beyond that, it really isn't. And so, you know, even the stuff that we're made of is unimportant in the universe is, is insignificant in the universe. You know, even our kind of matter is kind of, kind of an afterthought in the universe in some ways. And so, like the idea that we as humanity (chuckles a bit before continuing) can be significant to the universe when even the stuff we're made of is kind of not very significant in the universe. It's very humbling. It really is very humbling.

John: It's humbling. It's, in some ways, it's liberating, right? Because it also means that, uhm... did I ever tell you about the time I ate a sandwich that belonged to the New York Knicks?

Dr. Mack: No...

John: So, one time I was invited to the New York Knicks locker room by a friend of mine who was a reporter, and there was a big plate of sandwiches there in the locker room. And there was all these professional athletes and everything, and these reporters asking questions. And I was, I, I don't know if you have been able to gather this, but I'm not a professional athlete or, 

(Dr. Mack chuckles.)

John: or a particularly good question asker. And so I was just kind of hanging back and I, and I saw the sandwich board and I, I took a sandwich, I started eating it, and then the head of player operations for the New York Knicks said, "who are you?" And I said, "I'm john Green. I'm a, I'm a novelist and 

John Green: video blogger." And he said, "Those are for the players."

Dr. Katie Mack: Oh, oh no. 

John: And like all your mortification, you know, I, I think about it regularly at night, 

(Dr. Mack chuckles)

John: uhm... but with this particular mortification, it's worse. Because usually what you tell yourself is well, but they don't think about it. Nobody else thinks about it.

(Dr. Mack: Mhm.)

John: It's just something that I think about. 

(Dr. Mack: Hmm.)

John: But actually the next time, my friend, who is the reporter, saw the head of player operations for the New York Knicks, he said, "Man, I think all the time about your friend 

Dr. Mack (while laughing): Oh God. 

John: who ate one of our sandwiches."

Dr. Mack (while laughing): Oh no. 

John: So what you're saying is that actually, that's not very important. 

Dr. Mack (laughs a bit before talking): Yeah, yeah. I'm saying, I'm saying that, that none of that matters to the universe and whatever we do, there's a limit to how much we can screw things up because we have so little power. 

John: Yeah. Which is also kind of encouraging, right? Because I don't trust us on any level. 

Dr. Mack: No, not at all. Can you imagine if we, if we did control the evolution of the cosmos?

John: Oh God, no. No.

Dr. Mack (while laughing): That would be so bad. 

John: Yeah. Like if we could agree to the rate of expansion, 

Dr. Mack: Mhmm. Mhmm.

John: that would be very bad.

Dr. Mack: Yeah, yeah, exactly.

John: We would do a terrible job of that. 

Dr. Mack: Yeah. But, but you know, again, like it's, it's also sort of inspiring because, because as you said, we, we, we can learn so much, right? And we have this amazing power of, of knowledge of, you know, maybe we're just, we're just kind of observers in this bigger cosmos, and maybe we're just along for the ride, but we know so much about it, we can learn so much about it from looking at the distant galaxies and, you know, observing the past evolution of the cosmos and understanding the Big Bang, all of these kinds of things, like we can, we can learn a lot. 

John: And we have a lot in common with the universe on some level, because we're temporary. And everything that we've gathered about the universe is that it's temporary. And everything within the universe is temporary. 

John Green: And this is something that blows my mind every time I think about it. I tend to think of myself as being an observer of the universe. For whatever reason, like my whole life, I've been like somebody who watches more than participates. So I'm not uncomfortable with that idea. 

Dr. Katie Mack: Ok.

John: But we're not actually observers of the universe because we are the universe. Like, we're made out of the universe. So we're the universe observing itself. 

Dr. Mack: Right. Yeah. 

John: And I, I find that kind of like lovely and encouraging too, that I'm not separate from this thing that I'm looking at. I'm part of it. 

Dr. Mack: So there's this, this famous quote by Carl Sagan, right? "We are a way for the universe to know itself."

John: Mm. Mhmm.

Dr. Mack: To out knowledge, we are the only part of the cosmos that is studying the cosmos? I mean, there- it's very possible there's other creatures out there who, who are learning something about the cosmos who are doing the same observations. But, but we, we can't talk to them. We don't know what they're doing. But, but we are able to learn that we are able to study our own past, our own creation through the processes of the cosmos. 

John: Yeah. 

Dr. Mack: I think that's very special, really. 

John: Yeah. No, it's something that differentiates us from rainbow trout,

Dr. Mack: Yeah. 

John: but we still don't matter in the scheme of things. And so it doesn't really matter that I ate a New York Knick sandwich. 

Dr. Mack (while chuckling): Right.

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John: How do we hold these competing ideas in our mind? That we aren't very important in the scheme of things, and yet, that we are also the only part of the known cosmos and knows much about the cosmos. Things look different when you zoom all the way out, then when you zoom all the way in. And so, yes it is true that we are just creatures confined to a single solar system, confined to a single galaxy, and so on. But we nonetheless matter, because we are 

John Green: part of the cosmos seeking to deeply understand the cosmos, and also because we matter to each other. Which is quite literally why there is life insurance. And with Policygenius you can find life insurance policies that start at just $292 per year for one million dollars of coverage. Some options are 100% online and let you avoid unnecessary medical exams. Policygenius helps you easily compare your options from America's top insurers with just a few clicks. So, get peace of mind by finding the right life insurance with Policygenius. Head to policygenius.com/crashcourse or click the link in the description to get your free life insurance(s) quotes and see how much you could save. That's policygenius.com/crashcourse. Policygenius: Because we don't matter... but also, we do. 

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John: Whatcha goona teach me about today?

Dr. Katie Mack: I think when we last spoke, the first stars had just formed, we were at cosmic dawn. We had just exited the dark ages of the cosmos, and we were at the point when the first stars were beginning to shine. And those stars may have been very, very massive. They were very different from modern day stars because they were forming in this pristine gas that had no heavy elements in it. It had nothing that we as astronomers call metals, where we use the term metals to mean everything that's not hydrogen, basically, (starts chuckling) which is kind of, 

John: Oh, wow. That's helpful. That would be a great moment to rename something. 

Dr. Mack: yeah,

John: That would be great to have a new name for that.

Dr. Mack: yeah, yeah. 

John: I'll work on that. 

Dr. Mack: It's, it's quite silly. But, so before that, there were none of these heavy elements. We just had these, these big stars forming from this pristine gas. And that timeframe was somewhere around maybe a hundred million years, maybe a little earlier than that when the first star formed. We don't know exactly the numbers, 

Dr. Katie Mack: but it's somewhere in that range. Those stars formed in, you know, clumps. I mean, generally speaking stars and galaxies form in clumps because you get a big clump of matter, and that matter sort of has these little variations in it. And, and, and those little variations kind of form smaller clumps.

The way that things form in the cosmos, usually there are like fluctuations on really large scales. So there'll be a, a big area with a little bit of extra matter, and then there'll be little areas within that with more matter. And then there'll be sort of larger areas with sort of less matter.

And it, it ends up making things clumpy. So it ends up causing galaxies and stars and things to form in clusters. And so at this point, let's say the first star turns on, right?

The first star turns on, it's, it's burnign hydrogen into helium in its core. And as it's burning, what it's doing is it's, so, it's, it's fusing elements in its core, but it's also heating the gas around it. And there's a process that happens when these first stars are, are forming when the first galaxies are forming, where it's starting to really change the, the nature of the gas around in that environment.

So you had this transition from this sort of hot plasma to this cool neutral gas when the universe cooled down after the sort of hot Big Bang phase. So you had this neutral gas, and then stars started to form inside that. But when the star forms and gets really bright and really hot, it sort of ionizes all the gas around it.

It creates a little bubble of ionized gas just from dumping so much energy into that environment. Imagine this sort of, there's this kind of dense fog throughout the universe, right? And inside that fog at certain places, these little lights start lighting up like street lights in the fog, and around those little lights, it sort of burns these little, these bubbles

Dr. Katie Mack: of ionized gas around those lights. So around the first stars and galaxies, you get these little bubbles of ionized gas. And when that starts to happen, it makes it so that light can kind of pass through and, and it makes it sort of easier to see through that fog. So that gof of the sort of neutral dense gas of the dark ages, it turns out that, that that fog is kind of opaque to certain kinds of light.

It's opaque to visible light, ultraviolet light. Because essentially what happens is you, you shine light into that dense hydrogen fog and it'll just kind of excite the hydrogen or maybe ionize it a little bit, but the light will be absorbed by the hydrogen. Whereas once the hydrogen is ionized, then visible and ultraviolet light can pass through because it's this sort of diffuse ionized gas and it can't excite the atoms anymore.

It can't ionize the atoms anymore. And so the light can kind of pass through. And so it makes little regions around those first stars and galaxies transparent to visible light.

So when you, when this process first starts, when the first stars and galaxies turn on, each one of them is kind of like a streetlight, you know, embedded in the fog, you know, invisible to the other streetlights, right? Because from each perspective there's a little light and then there's just fog and you can't see throught it. But as this goes on, as more stars form, as more galaxies form as the cosmic dawn gets going, those bubbles of ionized gas, those bubbles of transparent gas start enlarging, start growing.

And so when we visualize this timeframe in this phase of the universe, we can make these visualizations, these simulations of, of how that, that proceeded. And it ends up looking like, like sort of this Swiss cheese structure of little bubbles around each big clump of stars and galaxies. And then those bubbles like expand until they overlap.

And the universe goes from being mostly opaque

Dr. Katie Mack: with little bubbles of light to being sort of fully transparent in the way that, that we see the universe today. 

John Green: So the universe became visible over time.

Dr. Mack: Yeah. Visible to itself. Yeah. 

John: Wow. Beautiful. 

Dr. Mack: And that process where it goes form being mostly neutral gas in the universe to mostly ionized gas in the universe, we call that process reionization. (laughs)

John (groans): Oh, that's so disappointing.  

Dr. Mack (chuckling before answering): I'm sorry. 

John: There was so many opportunities there. 

Dr. Mack: I know, I know. But at least this one is better than recombination because it is becoming ionized again after it has been ionized the first time. 

John: Yeah, no, I get that. I get that. I understand it. But like, what an opportunity to be like the visualization, the visual ability. The, I, I'd have to think about it. I acknowledge the size of the challenge here, but there's no way reionization is the best term. 

Dr. Mack: Yeah, it is, it is a little bit of a, yeah. 

John: Because you got, you got cosmic dawn right there.  

(Dr. Mack chuckling)

John: And so this is really, it's kind of like, uhm... it's cosmic- it's cosmic morning. 

Dr. Mack: Mm. Yeah. 

John: It's cosmic, it's cosmic 10:30 AM. It's cosmic brunch. 

Dr. Mack: It's, it's when the fog burns off, you know, when, 

John: Yeah. Which is around, you know, for me here in Indianapolis, usually around 10:30, like, you know, it's usually when I'm starting to think like, oh, you know, I could go for a bagel. 

(Dr. Mack chuckling)

(theme music plays)

John: Just to recap our discussion from last episode, following the Big Bang as things got less dense and less hot, there was a moment when neutral atoms were able to form for the first time, and this is know as recombination. I'm gonna spare you my disappointment in the name they chose here, but I do think they could have done better. Recombination was followed by a long period of time where the universe was mostly cold, neutral hydrogen gas, and that's known as 

John Green: the cosmic dark ages. And then with the help of dark matter, clumps of this gas started to come together and this led to the formation of the first stars. This was the beginning of what we call cosmic dawn. And part of cosmic dawn, as Katie just described, was the universe becoming visible to itself by breaking the dense fog of the the cosmic dark ages through the process of reionization.  But what exactly is reionization?

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John: Katie, I'm gonna confess what I really don't like about the phrase reionization? I think I know what ionization is, but it's straining my capacity for high school chemistry a little bit. So walk me through these, you know, we've got, we've got these early stars. They're throwing off new stuff, and that's reionizing this gas that's around these new stars.  

Dr. Katie Mack: So neutral gas, in this case, neutral hydrogen, if something is neutral, it just means that the positive charges and the negative charges balance. 

John: Okay. 

Dr. Mack: So in the case of hydrogen, that means it's got one proton in the nucleus. So that has a positive charge and it has to have an electron orbiting it or bound to it. That's a negative charge. And those charges balance, if, if you have one proton and one electron. That's neutral hydrogen. 

John: Right. And then it gets ionized because somehow in the throwing off of this energy, the electron is no longer bound to that particular proton?

Dr. Mack: Yeah. So, ionization is the process of removing electrons from something. So in the case of hydrogen, if you have a proton in your - and electron bound together, that's neutral hydrogen. If you separate the proton and electron, if you unbind them, then now you have ions, you have positive ions, the, the protons, and you have negative ions

Dr. Katie Mack: that that's the electrons. And so now you have an ionized hydrogen gas. 

John Green: And now, now much more interesting stuff can happen

Dr. Mack: Yeah, 

John: because now like light can pass through and other interesting stuff can happen. And that's why we, obviously, we don't wanna pick favorites,

(Dr. Mack is chuckling.)

John: but that's why we like ions. 

Dr. Mack: Well, yeah. So ionized hydrogen is transparent to visible light and ultraviolet light. I mean, assuming thta it's not super, super dense, it's transparent to this light. There's something here that gets a little bit tricky that bothered me when I was first learning about this, and I don't know if this is getting too technical or not, but when I was talking about the hot plasma of the very early universe before the surface of last scattering, before the cosmic microwave background happened, that hot plasma was so dense. So plasma is just an ionized gas, really. That's all that really means. It's hot ionized gas. So that hot plasma was made of protons and electrons and helium nuclei and more electrons. So it was just ionized hydrogen and helium and a bunch of electrons and a tiny smattering of like lithium and stuff. So that plasma was just ionized gas, just hot, dense ionized gas. But the light couldn't move around in that either. In some sense that was opaque too because, 

John: Because it was so dense. 

Dr. Mack: because it was so dense. Yeah. And so when that cooled down enough that those atoms were able to join together and make neutral atoms, when you went from that hot plasma to neutral hydrogen and neutral helium, so those electrons were able to find thsoe nuclei, the, the electrons and the protons bound together to make neutral hydrogen. The electrons found their helium nuclei and made neutral helium. When that happened, that became transparent to some kinds of light. So like radio waves could pass through that gas, even - that was a neutral gas kind of dense. So visible light still couldn't pass through that because it would get absorbed 

Dr. Katie Mack: by those atoms. But radio waves could now pass through. So it's funny 'cause when you talk to early universe cosmologists, they talk about the universe becoming transparent when the surface of last scattering happened and the universe became neutral. But then if you talk to like distant galaxy cosmologists, they talk about the universe becoming transparent when the universe became ionized again. 

John Green: Because one group is talking about when it became transparent to radio waves, the other group is talking when it became transparent to visible light. 

Dr. Mack: Yeah. And, and ultraviolet light. Yeah. And so it's all about like what kinds of light can pass through, what kinds of gases. And it's, it's this kind of complicated process. So at this point, once, once the gas is a bunch of neutral hydrogen, mostly then when the stars turn on, it becomes ionized hydrogen again. But first in sort of these, these bubbles of ionized gas. 

John: Yeah. And then the bubbles expand, and then eventually we have this situation where the whole universe is visible to itself. Yeah. Yeah. And, and, and it's visible because it's low density ionized gas because it's not like a hot, dense plasma. So there are a couple of really interesting things about that process. One of them is that we're starting to get to the point where we can, we can kind of see that process occurring in some ways. So one way is that when we look at really, really distant quasars - so a quasar, which we'll, I think, we'll talk more about later, a quasar is a, it's a super massive black hole in the center of a galaxy that creates jets of radiation. And it has matter swirling in creating this really bright glow. Those turn out to be extremely bright objects in the distant universe. So quasars are some of the brightest things that we can see in the universe. And so we can see them very, very far away into the, into the universe within the first sort of billion years of the universe. We can see some of these quasars. So sometimes when we look, when we look at the spectrum

Dr. Katie Mack: of the light from those quasars, we can see based on how the light has been absorbed by the gas between here and that quasar, we can see like for a certain region around the quasar, the light is not being absorbed very much. So we can see that the light is kind of passing through a little bit right around the quasar. And then we see this, this other region of the, of the spectrum of the quasar that shows that all of the light was absorbed in that part of space.

John Green: So we can see the streetlight and the litle bubble around the streetlight, but we can also kind of infer the fog. 

Dr. Mack: Yeah, yeah, exactly. And we started to be able to see that a few decades ago where we started to be able to see that we were looking at quasars that were within taht timeframe of reionization when, when the universe was not fully ionized yet when there was a lot of neutral gas in the universe. And understanding that transition is, is really important. And I'm, I'm talking a lot about this partially because this has been a big part of the research that I've done personally, has been thinking about how to better understand the epoch of reionization and what happened there and to learn about that early process of galaxy formation.

But if we really understand how that process happened, how the universe went from being fully neutral to fully ionized over that timeframe, which was a timeframe of probably from around half a billion years to around a billion years, somewhere in there is when that occurred. And it, it, you know, it took a little while. It wasn't instantaneous.

We can use that to learn about what those first stars were like, what those first galaxies were like, how bright they were, what they were doing to the gas around them. We can learn about those early quasars. We can learn about how the temperature of space was changing over time from that lighting up the universe.

And we're starting to get to the point where we can use radio telescopes to actually look at that neutral hydrogen gas as well. So usually with a telescope,

Dr. Katie Mack: what you do is you look at a bright thing, right? Like you just, you see a, a optical telescope to look at bright stars and galaxies. You can use a radio telescope to look at processes that create a lot of radio waves. So things like quasars can make these like bright radio sources in the, in the sky, pulsars, stuff like that can make bright radio sources.

JWST is looking at infrared light because it's looking at galaxies that are really, really far away and their light has been stretched out by the expansion of the universe. And I'll, I'll say a little bit more about JWST in a moment 'cause we'll talk about what it knows about these early galaxies. But one of the things that you can do with radio telescopes is you can look at a sort of weird property of hydrogen gas, of neutral hydrogen gas.

Normally the way an atom emits radiation is you have the nucleus of the atom and then you have electrons going around the nucleus of an atom in, in sort of energy levels, right? So they have different energy levels. So there's the sort of lowest energy levels and then there's the higher energy levels just has to do with kind of how the electron is going around the atom.

And normally the way that an atom emits light is that the electron will drop from a higher energy level to a lower energy level and that'll emit a photon. Or if it absorbs a photon, it can go from a lower energy level to a higher energy level, or it can even leave the atom entirely if the photon is energetic enough. I kind of imagine these electrons going up and down and either emitting or, or absorbing light.  

John Green: Okay.

Dr. Mack: With neutral hydrogen, if the atom is neutral and unexcited so the neutral hydrogen is just a proton with an electron going around it, right? There's another kind of transition it can do even in the ground state. So the, the electron is, is in its lowest energy state. It's just hanging around this hydrogen.

The hydrogen is, is cold and boring, but there's, there's a transition that the electron can do where the electron has a spin, which...it's a weird concept 'cause it's not really spinning, but there's this property of, of electrons