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This is a snippet of a larger conversation taking place on Crash Course Pods: The Universe. Over the next 11 episodes, John Green and Katie Mack will walk through the entire history of the universe… even the parts that aren’t written yet.

Episode 1 is out now and can be streamed on the Crash Course channel and wherever else you get your podcasts. Subscribe at

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(John Green) Take me back... (Katie Mack) Okay.

Okay. (John Green) To the very beginning of the universe. And then after you tell me the story of what? the first second? the first nanosecond? (Katie Mack) I'll get into the first minute or so, yeah. (John Green) How the heck do we know what happened in the first minute of the universe 13.8 billion years ago? (Katie Mack) Okay.

Okay. So, I'll start with the Big Bang Theory. When people talk about the Big Bang Theory, usually what they mean is like, they're like, “oh yeah I heard, you know, the universe was a singularity it was a tiny, infinitesimal point, then it exploded in all directions.” And that's not that's not really what we as astronomers mean when we say the Big Bang theory.

When astronomers say the Big Bang theory, we actually mean something a lot closer to the theme song of the TV show The Big Bang Theory, I use this example ‘cause it's actually pretty good, um, In that theme song it says “the whole universe was in a hot, dense state, then nearly 14 billion years ago, expansion started.” Then the song goes on to other things, right? But that's, that's it! So the Big Bang Theory is just the idea that the universe was hot and dense in the beginning. 13.8 billion years ago it was hot and dense and it's been expanding and cooling since then.

The origin of that theory is the idea that currently the universe is expanding, right? So we observe that because we see all the distant galaxies are moving away from us. Essentially what's happening is that we see the light from all these very, very distant galaxies.

That light is being kind of stretched out by the expansion of the universe. So what that does is it moves it from sort of visible light to infrared light as the wavelength is kind of stretched out. And it's a similar effect to like if a siren goes past your house and it goes into lower pitch you know, (imitates siren sound), like that the same kind of thing happens with light.

When things are moving away from you, they get redder, or to longer wavelengths. When they're moving toward you, they get bluer, to shorter wavelengths. And this happens at all the different wavelengths of light, you know, from radio to gamma rays and so on.

So anyway we see that distant galaxies are moving away from us. They're moving away from each other. There's more and more empty space happening all the time.

The universe is expanding. It doesn't mean that, like, objects are expanding. It just means that there's, like, empty space in between objects that's expanding.

And we've known that the expansion is happening, we've known that for a long time since like the, I guess, twenties, 1920s. (John Green) That’s not that long (Katie Mack) Well, I mean, since we started be able to know that, like, there are other galaxies, essentially we started to see that the the ones that are far enough away are moving away from us. (John Green) Right. (Katie Mack) The conclusion you get from that is that if the universe is expanding now, it must have been smaller in the past. Like if all those galaxies are getting farther away now, they must have been closer together. And, you know, if you push things closer together, it makes them hotter, you know, makes them denser.

Like, you can squeeze things and they get hot and dense. And so you can just kind of extrapolate and say, well, the beginning of the universe, things must have been hot and dense and really close together, right? Then you kind of keep going with that extrapolation.

You you arrive at the idea that the universe was this kind of hot, dense soup of energy in the very beginning. And that idea has been around for a long time, has been kind of floated in different ways. And the kind of confirmation of that came in the 1960s when we started to actually see the light of that hot, dense soup. (John Green) So we know that the universe is expanding both because we can tell that galaxies are getting farther away from us, but also because we can glimpse this hot, dense soup that the universe was at the very beginning.

So we have two independent ways of knowing that the universe used to be a hot, dense place. (Katie Mack) Yeah, essentially. (John Green) That was a clip from The Universe, a new limited series podcast from Crash Course, where Dr. Katie Mack, a theoretical astrophysicist, walks me through the entire history of the universe, including the parts that haven't been written yet. It's available now both on the Crash Course YouTube channel and wherever you get your podcasts.