This is How The Universe Will End, According to an Astrophysicist

“Our contemplations of the cosmos stir us,” the late atronomer Carl Sagan once said. “There’s a tingling in the spine, a catch in the voice, a faint sensation, as if a distant memory of falling from a great height.” If reflecting on the universe gives you a shiver, thinking about its end can make you quake.

In her new book, The End of Everything: (Astrophysically Speaking), theoretical astrophysicist Dr. Katie Mack starts with the Big Bang — the theory of how the universe began. Its start can tell cosmologists like her a lot about its inevitable end. She cheerfully takes readers through five astrophysics apocalypses: The big crunch, heat death, the big rip, vacuum decay, and the big bounce. To keep you from getting stuck in the quark-gluon plasma (don’t worry, she explains it), Mack keeps everything accessible and conversational. It’s much more fun than you’d expect a book about the end of the universe to be. Don’t let the universe-is-ending existential dread get you down, she seems to say.

We spoke to Mack about dark energy versus dark matter, how telescopes let us literally look into the past, and the weirdness of space.

(This conversation has been lightly edited for clarity.)

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Digital Trends: What prompted you to write this book?

Mack: Over the years, I’ve studied a lot of different things in the area, broadly, of cosmology. So, cosmology covers, you know, the universe as a whole and its components and its evolution. I’ve worked on the early universe. I’ve worked on dark matter, black holes, galaxy evolution, all of those kinds of things. And lately I’ve been very interested in the end of the universe. and that’s kind of how this book arose.

Your book offers a very accessible explanation for how we’re able to observe the Big Bang. Can you walk us through that?

So, the idea is that if the universe is currently expanding — which we observe, we see galaxies moving apart from each other — then it stands to reason that in the past, the universe was more compressed. So everything was closer together. And you can kind of dial that extension back and you get to a point where everything was kind of on top of each other.

So, as the universe expands, it gets cooler, matter gets more diffuse, energy gets more diffuse. In the past, it must have been hotter and denser and, in some sense, smaller than today. So that’s basically the Big Bang Theory. That’s the most simplistic statement of the Big Bang Theory, just that the universe was hotter and smaller and denser in the past.

end of everything book cover by kate mack

And if that’s the case, then it also stands to reason that if you look far enough away, you are looking farther and farther back in time, because of the time it takes light to travel to you. And so you should be able to get to a point where if the universe really was hot and dense everywhere — if the Big Bang was something that happened throughout the whole cosmos — then you should be able to see parts of the universe that are so far away that they’re still in that hot, dense state, that they’re still in the final stages of that kind of primordial fireball existence.

And in order to get there, you have to assume that the universe is large and always was an extended thing, which we do believe. We think that the Big Bang is something that happened everywhere. There’s no single origin point. And so if you follow that reasoning, then there should be background light. There should be light coming to us from every direction, from the most distant reaches, the farthest we could possibly see. There should be the light that’s the leftover light from the final cooling of that fiery state of the cosmos.

The book explores five possible ways that the universe could end. Why are there are so many different ways it could go?

Well, it comes down to a few things. One is that we don’t fully understand what’s making the universe expand in the way it is right now. There was a time when we thought it would be quite simple because we had a good theory of gravity, general relativity, and we could measure the expansion rate of the universe, and we knew how all the stuff in the universe should be slowing down the expansion. And so then, it was just a matter of figuring out the balance between the expansion and the gravity.

So, if the expansion were too fast for all the gravity, then it wouldn’t slow it down enough and it would expand forever. And if the expansion were not fast enough or if there was too much gravity, then the gravity would win, slow the expansion down, stop it, and we’d have a recollaspe — the big crunch. And so for a time, those were the only options that made any sense.

One is that we don’t fully understand what’s making the universe expand in the way it is right now.

But then, when it was discovered that the universe was actually accelerating in its expansion, we had to add a new component to the universe. We had to revise our understanding…

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