Five ways the universe might die — including one that could happen at any time

Originally published on September 26, 2020.

With wildfires, extinction events, rising sea levels, and, oh yeah, the COVID-19 pandemic, sometimes it can truly feel like the universe is coming to an end.

Scientists do mostly agree that the universe is indeed ending — although those other events have nothing to do with it.

And while we're not yet sure exactly how the universe will meet its demise, or even when it will happen, astrophysicists like Dr. Katie Mack are trying to figure all that out. 

She's been gathering data from the latest astronomical observations and particle experiments to figure out the messy, chaotic, and mind-bending ways that the cosmos could come to a close.

Bob McDonald spoke with Katie Mack about her book The End of Everything (Astrophysically Speaking).

This interview has been edited and condensed for clarity. 

How do we even know that the universe will come to an end? 

We have a pretty good idea. We know the universe had a beginning and we know that because we can actually see the leftover light from the big bang itself. And we know the universe has been evolving over time. We know that it's been changing, that the way that stars form, the way that galaxies evolve, that's all been changing over time. We know that the expansion is continuing and it either continues, or it turns around, or it stops. 

And there's no sense in which it just kind of stays as it is. And all of those changes lead to different kinds of possibilities for the end. But they all do have an end. 

Take me through the main ways that you see the universe ending. 

There are five different possibilities I cover in the book, and I use those five because they are a reasonable selection of the kinds of things that we talk about as physicists. So there's the Big Crunch where the universe collapses on itself, the Heat Death where it expands forever and just gets colder and kind of dies out. There's the Big Rip where the universe rips itself apart. There's Vacuum Decay, which is a wild idea where the universe basically succumbs to an instability built into space itself. And then there's Bouncing Cosmologies where you have some kind of ending and beginning cycling over and over again. 

Do you have a favourite version? 

I do. My favourite is Vacuum Decay. I should say the one that's most likely, based on our current data and the one that's most accepted by physicists, is the Heat Death, where things just kind of fade away in the future. And that seems like it's likely to happen. It would be a very long time from now. So it's a very gentle kind of fading away, but it's not the most exciting. 

And I think that the Vacuum Decay scenario does bring some possible excitement to the story because it's a very sudden and very complete ending, and it comes out of our understanding of particle physics in this way. That brings together weird things we're learning about particle physics, and something that has an effect on the entire cosmos. And so it's a very cool thing from a physicists perspective. But it's also cool because it's very dramatic and sudden and in principle could happen sort of at any time. We don't think it's going to happen soon, but it could take a while. It's very unlikely. And that brings some more excitement. 

In this very violent finale of the Big Rip, it would actually rip apart the fabric of space itself.- Dr. Katie Mack

Now, another ending to the universe that you bring up in your book is called The Big Rip. Take me through that. What happens? 

Sure. The Big Rip is basically a worst case scenario about dark energy. Dark energy is this mysterious stuff that's making the universe expand faster. So right now, we know that the universe is expanding, and that means that galaxies are getting farther apart from each other. There's more empty space and every galaxy in the universe is going to get more and more isolated over time until everyone is kind of alone in their own little pool of light and the rest of the universe is dark. 

But if dark energy is a little bit more powerful than we expect, then not only will galaxies be isolated, but eventually that dark energy is going to stretch out more than just empty space. It's going to stretch out matter itself. It's going to build up and sort of pull apart galaxies, pull stars away from their galaxies, pull planets away from their stars. And in this kind of very violent finale of the Big Rip, it would actually rip apart the fabric of space itself. That's a scenario that is probably not very likely. If that's going to happen, we're really sure it's not going to happen for at least about 200 billion years. 

What could happen after our universe has ended? 

It depends on how it ends, and it depends on whether it's embedded in some larger space, some larger universe or multiverse that's out there. There are a few ideas for a universe that comes out of a larger space, where maybe our universe and other universes could have sort of sprouted out of this much larger space, then those other universes would be separate enough from ours that we wouldn't interact with them, but they could exist sort of alongside or before or after our universe. And then there are these cyclic models where a previous universe ended, and in the process of it ending, it set the scene for our universe and sparked the beginning of ours, and then ours might end and spark a new one. 

But there are also possibilities where basically when it ends, our universe could be destroyed in such a way that we don't know how something could come after that. 

Can humans have any effect on the way the universe ends, either by making it happen sooner or could we even stop it from happening and live longer? 

Humans affecting the evolution of the universe seems very unlikely for a couple of reasons. One is that we don't have technology that can influence the shape or behaviour of space time at the moment. But also when we look at what the universe is made of, most of the matter in the universe is this invisible dark matter that we don't understand, and we can't see. And most of the stuff just in general of the universe is this dark energy, which is also invisible. And when you add up dark matter and dark energy, that's 95 percent of what the universe is made of. 

Being as insignificant as we are, we have an amazing comprehension of the cosmos, and we've learned so much. And I think that's a really amazing thing about us as a species.- Dr. Katie Mack

And so only five percent of the universe is even stuff that we can see or touch or interact with. Even the kind of matter that we're made of is only about five percent of the universe. And then we're just a tiny speck on top of that. We're just one species on one planet around one star and one galaxy. And there are maybe two trillion galaxies in the observable universe. So the idea that we're significant enough to affect the evolution of the cosmos seems very unlikely. 

But I think that insignificance is actually kind of inspiring in some ways, because even though we can't do anything, we can't affect the evolution of the cosmos, we have this amazing power to understand it. And even being as insignificant as we are, we have an amazing comprehension of the cosmos, and we've learned so much. And I think that's a really amazing thing about us as a species. 

Produced and written by Amanda Buckiewicz