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This is how the universe will end (according to a theoretical astrophysicist)

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“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.)

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.

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 and put in this thing called dark energy. And dark energy is something that’s making the universe expand faster. But because we don’t really understand dark energy, we can’t say with much certainty that that’s where things are going. So, that’s why things like the big rip or the big crunch are still on the table and heat death is the one that we seem to be heading toward.

And then the two oddball ideas, vacuum decay and bouncing cosmologies, come from the fact that there’s a lot we’re still trying to understand about the very early universe and particle physics. So the bouncing cosmology has come out of the idea that maybe our current best guess at the very early universe, this inflationary phase, maybe that’s not the whole story. Maybe that didn’t happen. Maybe there was some other evolution in the very early universe that led to the conditions that we see today. And if that’s true, then some of those ideas can lead to these stranger cyclic cosmologies.

And then the idea of vacuum decay really comes out of the fact that our particle physics knowledge is incomplete, and the current best understanding our particle physics knowledge suggests that the way particle physics works right now is just not fully stable, which leaves the universe vulnerable to this decay process. So if we better understood particle physics, if we better understood the very, very early parts of the universe, then we’d be able to say something about those two models. But for now, we can’t either rule them out or say with certainty that they’re the way things will go.

Of all the scenarios, which is the most probable?

Heat death is considered to be the most likely, partially because it requires the fewest additional weird things. So, in cosmology, we like to keep things as simple as possible. We like to not assume any new components of the universe unless we absolutely have to. And the heat death scenario has a kind of dark energy that’s this cosmological constant, which is something we don’t entirely understand. But it’s an idea that’s been around since Einstein, and it’s just a property of spacetime, that it has this little bit of expansion built into it.


So that gives us a very simple, straightforward evolution, where the universe continues to accelerate in its expansion forever and it all just fades away. And that makes a lot of sense if the universe has dark matter, regular matter, and a cosmological constant as the dark energy. You don’t have to assume anything more complicated than that. Implicit in that is that the inflation did happen, and that was the beginning of the universe. That’s part of the so-called concordance model of cosmology where everything is as simple and boring as you can imagine it being.

But the reason that we don’t just settle on that and say we’re done is because we really don’t know for sure that the dark energy is a cosmological concept. And so that does leave some room. We also can’t say for sure that we understand particle physics enough to say that vacuum decay won’t happen or that the early universe evolution wasn’t different enough to imply some cyclic phase at the end.

You mention some new radio telescopes that will allow scientists to watch the first structures of the universe form. What are experts hoping to learn from that? 

We’ll get a better picture of the evolution of the cosmos through this period between the background light and the modern universe where we’re, you know, the universe full of galaxies. There’s a reasonably sized chunk of time where we have little information about what was going on then. So we’ll learn a lot about the evolution of the cosmos. We’ll get just measurements of way more galaxies. So I mentioned in the book the Vera Rubin Observatory, which is going to map out something like billions of galaxies and show us how they’re moving through the universe and how they’re evolving over time and how they’re distributed. And that’ll give us a lot of information about just the layout of the universe and the evolution of the cosmos. So those will be important clues.

Rubin Obs/NSF/AURA

We also might learn about other aspects of physics. So I’m interested in some of these big radio telescope arrays, because they might be able to tell us something about dark matter, if dark matter is annihilating in the distant universe. That can change how first stars and galaxies are evolving, and that might give us some clues to the next step in particle physics. And so there are lots of things we might be able to fill in if we just have way more data about the distant universe, the early universe, other galaxies — the sort of dawn of the galaxy era, you might say. It’s really a matter of just getting a better map and getting a better history and looking for surprises. You know, we really hope that we see new and interesting phenomena as we get more and more data.

Can you explain the difference between dark energy and dark matter?

Yeah, so, dark matter and dark energy act on the universe in pretty opposite ways. Between the two of them, they are the most important aspects of the cosmos if you’re thinking about its long-term evolution. So dark energy is something that makes the universe expand faster. It sort of stretches out space. It’s really governing the evolution of the cosmos, from here on out. It started becoming really, really important about maybe five billion years ago. And it’s kind of taking over the universe now. And so we’re at its mercy in terms of the evolution of the cosmos from here on out.

Dark matter and dark energy act on the universe in pretty opposite ways.

But dark matter is sort of what’s responsible for all of the buildup of structure in the cosmos, so the growth of galaxies and clusters of galaxies. Those are all built on the scaffolding of dark matter. So dark matter is some kind of invisible matter, but it’s most of the matter in the universe, and it has some properties that makes it easier for it to come together and build the scaffolding upon which all of the other matter is built. And because it’s matter, because it’s most of the matter in the universe, it’s acting to try and slow down the expansion of the universe. And for a while it was slowing down the expansion of the universe, and it wasn’t until about five billion years ago that the universe got so big that dark energy just took over.

Your enthusiasm comes through a lot in the book, especially when you talk about things like the Hubble radius. What are some of the other things like that are just so exciting in your field that you just have share with other people?

I mean, just the fact that we can see the past very directly continues to blow my mind. The fact that we can see the final stages of the Big Bang directly with telescopes, with microwave receivers. We’re picking up the light from the final stages of the Big Bang in every direction. The fact that we can just look out into the universe and see the past and therefore learn about our own history, I think that’s super-amazing. That astonishes me all the time. And then, you know, there are just all these weird aspects of cosmology, weird things in physics that come up if you’re in a universe that’s expanding and governed by the relativity. So you mentioned this thing about at a certain point, galaxies stop looking smaller anymore. You know, a galaxy the same size would start looking bigger, which is weird, as you go farther and farther out.


Then there’s just the fact that we can see the expansion of the universe, that we can map that out over time, is amazing. I don’t even know if I really talked about this in the book, but when we look at very distant supernovae, solar explosions, they appear to be happening more slowly if they’re far away, because the expansion of the universe stretches out time as well in this really strange way. So the way that space and time interact with each other get very confusing and strange when you’re dealing with cosmology, and that’s really interesting. You know, relativity just does weird things to space and time in all sorts of contexts. That I find amazing and cool.

There’s a lot in cosmology that’s unknown. What’s one mystery you’d love to solve? 

Oh, there’s so many. The whole dark matter/dark energy thing is is huge. If we knew the nature of dark matter, that would definitely be a big help to just our understanding of physics in general.

But I think the thing that would be maybe most impactful would be really knowing if inflation occurred and then, like, how and why. So filling in just a tiny, tiny moment of time at the very beginning of the universe would really change everything about our picture of the cosmos. If we knew for sure that this happened, that would tell us something about the origin of the universe that would help us to be able to say something about its future as well. It would really let us have a handle on the fundamental structure of the cosmos. So, yeah, understanding inflation, dark matter, dark energy, those are the kind of the big ones, I think. And then, you know, there are things like figuring out how to connect general relativity and particle physics. But I think if we knew the answers to inflation, dark matter, and dark energy, I think that would give us a give us a lot of clues about how to put together like a more complete picture of physics.

You write a bit about existentialism and dread in the book. Is that just something that you’re reflecting on because you’re writing this book, or is it kind of always there?

Am I haunted? I mean, I think it’s definitely something I felt like I had to grapple with for the book, because I think that it is natural to ask the question of, you know, if we don’t go on forever, what does it even mean? Like, what’s the meaning of life? What’s the purpose of existence if it has an end date? So it was definitely something that came up in thinking about all these big questions. I’m not somebody who spends all my time pondering the meaning of life in general. I usually don’t get caught up in that stuff. And I also, I’m not somebody who likes to think about death. I tried very hard to avoid thinking about death because I find it very troubling. So it really was prompted by thinking about this book and trying to put some context around these big ideas, because the reality is that we do have an emotional response to the universe. Even if that seems irrational from a strictly practical point of view, it’s hard to avoid having that response.


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