John Green, Very Curious: So Dr. Mack,

Dr. Katie Mack, Astrophysicist: Yes.

John: today we are gonna learn about, and I like this phrase, Cosmic Noon.

Dr. Mack: Cosmic Noon.

John: That involves us, right? Like, that involves the creation of, this one, the Milky Way.

Dr. Mack: Yeah.

John: Ours. 

Dr. Mack: Yeah, it involves the creation of most of the stuff like, that we see.

John: Great.

Dr. Mack: Yeah. Yeah.

(Theme music plays)

John: Okay, so as I just mentioned, in this conversation we're gonna talk about Cosmic Noon. Cosmic Noon is interesting because one, we are nearing the point in our journey through the entire history of the universe, where, like, we exist. And also, two, it involvs the mystery of what I'm calling a secret cosmic wind that has caused the accelerated expansion of the universe, something that Albert Einstein may have recognized unknowingly. So we'll talk about that mystery and we'll even discuss a theory for how our solar system came to be. We'll explore the ways cosmologists have posed and attempted to answer big questions about our origins on a grand scale. But first, we need to talk about where we are in our timeline and also what time even... is? So here's out conversation. 

(Theme music plays)

Dr. Mack: So let's kind of go back, because we did, we did a few departures from the timeline because we were talking about the cool, weird stuff with dark matter and black holes. So, we got to the Epoch of Reionization. We went through the Dark Ages, Cosmic Dawn, Epoch of Reionization. And Epoch of Reionization is when the stars just turn on enough to ionize the intergalactic medium to take all of the neutral hydrogen gas out there and split it into ionized hydrogen. So proton, electrons no longer connected. Right? That's ionized hydrogen. And so from that point on 

Dr. Katie Mack: most of the gas in the universe, most of the intergalactic medium is ionized gas, which is just very low density ionized gas. And that allows the universe to be transparent to visible light. So that means starlight can flow through the universe unimpeded by the gas, right? That's the period that we're in now, still, like the universe is mostly ionized gas and the starlight can go through.

But over time, like the process of the formation of galaxies and the formation of stars has kind of varied as stuff comes together, right? So the matter comes together via gravity, and you get these clouds of gas and those clouds of gas form stars and, you know, they're in galaxies and just bigger clouds of gas that often sort of form into discs. And then you get star formation in the discs of these galaxies.

And all that stuff is kind of condensing and swirling around and galaxies are like being attracted to each other with their mutual gravity and sometimes colliding with each other. And there's this constant sort of churning and colliding of stuff in the universe. And when that stuff comes together, you get the formation of new stars and galaxies and so on.

And that process just kind of keeps going on. But there's, there's a sort of balancing thing that happens between, like, the gravity is pulling everything together on, you know, galaxy scales, galaxy cluster scales. Things are, are kind of slowly condensing over time, but the universe on the whole is getting bigger, right?

So the space between galaxies is on average getting larger. And so there's, there's a kind of a balance where in the real early universe, there hasn't been enough time for all of the gas to condense for, for a lot of the galaxies to form because it takes some time for everything to gather together. But if you wait too long, then the galaxies are being pulled apart from each other on large scales.

And so you don't have as much sort of structure formation

Dr. Katie Mack: if the universe is too big and there's like, you know, there's too much space in between everything. This kind of sweet spot is somewhere around 2 to 4 billion years after the beginning. So kind of in the universe's adolescence maybe is when there's, you know, there's a lot of stuff coming together and, and the expansion of the universe has not pulled too much stuff apart. 

John Green: Right. Okay, that makes sense. It's a bummer to hear that we're not still in the sweet spot. I was hoping to be amid the sweet spot,

Dr. Mack: Yeah, 

John: but we're not. 

Dr. Mack: yeah, yeah. So, so that time period, which we call Cosmic Noon. 

John: Great phrase. 

Dr. Mack: Yeah, it's, it's good. I always feel like I hear the phrase Cosmic Noon and it, like, it conjures up this image of some kind of like Wild West ranch. 

(John laughing.)

Dr. Mack: Like, I don't know, somehow the, the idea of like, it's, it's a hot day in the, in the, in the West and anyway. 

John: Right. Yeah. Cosmic Noon would not be a bad name for a sort of psychedelic rock band. 

Dr. Mack: That's true. Yeah. 

John: It also wouldn't be a bad name for an, like an artist residency, you know, like, 

Dr. Mack: Hmm.

John: have you applied to Yaddo and Cosmic Noon?

Dr. Mack: Yeah, yeah, yeah. 

John: So I like that. By the way, Katie, there are times when you are talking to me, and this was just one of them where you're describing this sweet spot between, you know, the gravity of all this stuff being pulled together and the universe itself expanding. People are always like, "oh, you don't know that the earth is moving." Well, I do, when you talk like that.

(Dr. Mack chuckles.)

John: I suddenly feel it. I'm going 25,000 miles an hour and I'm spinning on a rock that's spinning around a rock that's spinning around a freaking black hole at the center of the galaxy. I'm feeling it right now. I'm feeling it pretty intensely. 

Dr. Mack: Yeah, yeah. There's, there are all these like, just huge forces out there, you know, and they're, they're altering how the universe works. They're altering what the universe is made of, like all the dynamics of everything. And we are part of that, you know, like we're just sitting there, we're just part of that.

John Green: Yeah. We're like little protons on the tide of empire.   Dr. Katie

Mack: Yes. 

(theme music plays)

John: Oh boy. Okay. So

(Dr. Mack chuckles.)

John: 2 to 4 billion years ago, we've got this Cosmic Noon. 

Dr. Mack: Yeah. And so, so a few things are kind of happening more then, than at any other time in the universe. And the, I should say the way we know about this time period is through like, directly looking at it, which, which I, I, I've said this a few times that, that we learn about the past and the universe just by looking at it. And it sounds very simplistic, but it's absolutely what we're doing.

And I, I still find it kind of wild because, you know, if you talk to like an archeologist or paleontologist or something, they're looking at like, remnants. They're piecing together a timeline from debris from, you know, remains like all this kind of stuff. Like, you know, you have to kind of put all the clues together and figure it out.

Like we're just- we can just look at the past. We can just directly see it. 

John: Yeah. Because of the gift of light. 

Dr. Mack: Yeah. 

John: It's amazing. 

Dr. Mack: Yeah. It's wild. We can look at galaxies at different times in the past. And, and at this point, I'm gonna have to talk about Redshift again because as a cosmologist, this is just, this is how I think about the timeline.

And I will get too confused if I try and do it another way. So we talked about Redshift a few times. Redshift is a term for how the expansion of the universe is stretching out the light as it's traveling to us from distant objects.

You can also have redshifts from something moving away from you, either moving away from you or the universe expanding the light. It's kind of an equivalent looking thing. The, the light gets stretched out.

And so things that would be shining in ultraviolet or visible light might be shining in instead visible and infrared light. Because the light is sort of stretched to the redder or longer wavelength part of the spectrum. And if you look at different epochs of the universe, because the universe

Dr. Katie Mack: has been expanding, all of the galaxies at a, at a certain time will be redshifted by the same amount, right? And so we can, we can label time periods by the redshift by the degree to which the light has been stretched out. 

John Green: So 2 billion years ago might be another way of saying that might be redshift six or something.

Dr. Mack: Yeah. So 2 billion years ago is not that high a redshift, 

(John guffaws.)

Dr. Mack: but yeah, like, so like, 

John: Okay, But I don't, I, yeah, yeah-

Dr. Mack: no, no, but like-

John: Do-Do you genuinely say like redshift and then a number?

Dr. Mack: Yeah, yeah, exactly. Yes.

John: So, I was making a joke, but it turns out that you actually do say redshift and then a number to represent what I would think of as billions of years. 

Dr. Mack: Yes. Yeah. And it's connected to the size of the universe at that time, also. 

John: Right.

Dr. Mack: So, I think I mentioned this before, but the, there's this concept of the scale factor, which is like, if you take the scale factor to be, like, for example, like something related to the average distance between galaxies at a particular epoch, then, you know, at another epoch that scale might be twice as big. And so we, we use a number called the scale factor, which we write down as "a", and we, we define the scale factor to be 1 today. So whatever that distance is, it's one now. And at previous times in the universe it was maybe like half of that or a third of that or whatever. And the redshift is related to the scale factor in an inverse way. So, so "a" equals one over one plus "z", where "z" is the redshift. So today is redshift zero. So the light coming from today is not redshifted at all. 

John: Mhm.

Dr. Mack: And so the scale factor today is one over one plus zero, which is one. 

John: Okay. 

Dr. Mack: So at redshift one, the scale factor is one half 

John: Mhm.

Dr. Mack: because it's one over one plus one. And so that means the universe was half as big as redshfit one. 

John: Got it. 

Dr. Mack: And it turns out that redshift one was when the universe was about 6 billion years old. 

John Green: Okay.

Dr. Katie Mack: So it's, it's about 13.8 now. So, you know, you can kind of count back. 

John: Okay.

Dr. Mack: Redshift two is when the universe was a third as big as it is now. And the universe was about 3 billion years at that time. Redshift three, the universe was a quarter as big as it is now. And that was when the universe was about 2 billion years old. And then you can go all the way to like, redshift ten, the universe was 1/11th as big as it is now. And the age of the universe was about 500 millions years old. So about half a billion years old. 

John: Okay. Okay.

Dr. Mack: So the reason we think of these, these epochs in terms of redshift is because the redshift is directly observable,  we can look at a galaxy and we can see that all of the absorption lines that should be at certain frequencies are shifted as a block to different frequencies. And so we can see what that shift is. And so we know exactly how much the universe has stretched out since the time the light left that galaxy. And that's a monotonic function, right? So it means that like if the thing is older, the redshift is gonna be higher, the thing is farther away. Like that's, it's all, it's always gonna be in that direction. 

John: Right. And so when you look at a distant galaxy, you can tell how far away it is, approximately. You can tell how old it is and how far away it is and how old it is are always gonna be related. 

Dr. Mack: They're related. Now the physical distance, that gets real complicated real fast. 

John: Okay. I'm sorry. I apologize 

(Dr. Mack laughs.)

John: for bringing up physical distance.

Dr. Mack: No, no, no, no. Don't, don't apologize. It's- 

John: I'm always trying to understand this stuff 

(Dr. Mack chuckles.)

John: as if I were like, looking out my window at the universe, and I need to understand it a little differently, I think. 

Dr. Mack: Well, yeah, no, the, the, the reason physical distance is a problem is because, (sighs before continuing) because the, in an expanding universe, you can never just like lay down your measuring tape