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.