I follow the work of physicists and write about their most interesting theories and discoveries.
Asking the Final Question
When we talk about space and time, we often talk about how the universe began. We know that it "started" with a Big Bang—everything that makes up what we see on Earth, in our galaxy, and beyond had once existed in an extremely dense state, or as a single point, as some scientists like to say. Then it all began to expand into its current state (and it's still expanding—even faster than before, physicists think).
But there's a different question that doesn't enter the conversation as often. It's a question that's arguably more pressing than that of the universe's origins. And the answers to this question can seem frightening, depressing, or awe-inspiring.
That is—how will the universe end?
And if we're still around to witness the final chapter, what will happen to us?
Shifting Our Gaze Toward the Future
Katie Mack, a theoretical cosmologist who obtained her PhD in astrophysics from Princeton University, wrote a book titled The End of Everything (Astrophysically Speaking) that tackles this apocalyptic puzzle.
"There’s so many books about the beginning," Mack said in an interview on the Mindscape podcast hosted by theoretical physicist Sean Carroll. "I felt like [. . .] we really got to cover the other side."
Mack argues that although people are eager to learn about the future, physicists "just don't talk about that enough."
What Do We Mean by "End," Anyway?
On this complicated subject, it's important to get definitions right. Saying the universe will end someday is a broad statement. Are we saying the universe will die? Will it explode? Implode? Maybe it'll just run out of gas, so to speak.
Carroll, who holds his own PhD in astronomy from Harvard University, says it's not guaranteed that the universe will "literally cease to exist." And why should it? After all, it's difficult to imagine the universe going from something to nothing. Try picturing nothing and you're guaranteed to picture something. The absence of the universe—the absence of anything and everything—is a non-image that eludes our minds. No matter how hard we try, our creative powers just can't grip nothing.
But Carroll puts the universe's end in terms we can play with.
"[The universe] might either change so dramatically that life itself would be impossible, or it could just fade away," Carroll said in a preface to the Mack interview. "The universe could end with a bang or with a whimper."
"Change" is the operative word here. When we say the universe will end, perhaps what we're really saying is that it will become something totally unrecognizable—and totally inhospitable to people. It may even change so much that it stops changing at all.
A Critical Competition: Attraction vs. Repulsion
In order to understand the different possible outcomes of the universe, we must take two forces into account: the attractive force of gravity and the repulsive force that's expanding the universe—the latter being tied to "dark energy."
Dark energy is a simplistic term for an advanced concept. If you can accept that the universe is expanding, you're one step away from obtaining a rough understanding of dark energy. Originally, we thought the expansion of the universe would either slow down or reverse itself. But our measurements of supernovas have showed us that the universe's expansion is, in fact, accelerating. Why? We don't know. And so we coined the term dark energy, which is not so much a precise description of this acceleration as it is an expression of our bewilderment.
Astrophysicist and author Neil deGrasse Tyson calls dark energy a "placeholder" term.
"Some mysterious pressure in the vacuum of space, acting opposite the force of gravity, we don't know what it is, but we can measure its effect [. . . .] We call it dark energy," Tyson said in an interview on Moyers & Company. "That's our placeholder term to describe what we observe—the acceleration of the universe."
It may be a mistake to say that dark energy is the direct cause of the universe's expansion. This is the kind of talk a layman can toss around while physicists shake their heads. But the bottom line is that the universe's expansion and dark energy must be understood together, whatever the true nature of that relationship might be.
The two forces—attraction and repulsion—are naturally at odds with each other, and how the universe will end largely depends on how this competition resolves itself.
The Big Freeze
The Big Freeze comes into play when the repulsive force continues to dominate. In this scenario, the universe expands until eventually things become so far apart that it's as if everything exists on an island.
"There will be so much space between galaxies that we won't be able to see other galaxies anymore [. . . .] People in this very, very distant future will not be able to figure out that the universe had a beginning [. . . .] Nobody will be able to check this stuff from first principles," Mack said.
Galaxies will dissolve and, ultimately, the universe will reach a maximum entropy state in which no more "work" can be performed. Nothing of significance will be created. Time will lose meaning, since between "now" and a million years from now, nothing will have changed. (Of course, all of this is also subject to head-shaking by physicists. This is a very general picture being offered, and the finer details are highly complicated.)
So in the event of a Big Freeze, the universe does sort of run out of fuel. If you ever hear a physicist mention the "heat death of the universe," this Big Freeze state is roughly what they're referring to.
"That points to a kind of sad, bleak future for the universe," Mack said. "Basically what happens is the universe kind of fades to black in this really long, drawn-out way."
The Big Crunch
If attraction defeats repulsion, something very much like the opposite occurs. Instead of expanding eternally, everything begins to compress: galaxies merge, stars crash into each other, and the universe gets very, very hot.
"At some point it gets so compressed that the surfaces of stars catch fire [. . . .] So stars will start to be cooked from the inside out," Mack said. "At that point, nothing is survivable. That’s it."
A Big Crunch sounds far more spectacular than a slow, lonely freeze-out. As far as we know, we don’t have a choice in the matter. But if, hypothetically, you were handed an "End of the Universe" menu, what might you order?
"The question is, you know, is it better to burn out or fade away?" Carroll said. "Either way, your goose is cooked, but in the heat death, you slowly slide into oblivion, and in the crunch, you get roasted to death and you burn."
Your personal preference may vary.
If a Big Crunch offers any comfort, perhaps it's in the implication that the universe might re-expand, or experience a "bounce." Depending on who you ask, this may result in a repetition of the same unfolding of events we see now. In other words, it's possible that you'll re-live your life over and over again (and you may have already lived this life many times before). But not all physicists—nor philosophers, for that matter—agree with this prognosis. The details, again, are complicated.
Mack speaks of a Big Crunch in terms that seem to exclude us from the universe's potential re-expansion.
"Some of these Big Crunch models have a bounce, so where there's a Big Crunch, and then there's a new Big Bang afterward. Again, not our universe. We're still dead. But some people like that idea," Mack said.
The Big Rip
A third result involves the force of repulsion not only winning in the end but going on what essentially amounts to a rampage. If we may extend the analogy of a competition, imagine repulsion defeating attraction and then kicking it while it's still down. The fight goes from dramatic to mean—until it finally gets ugly.
When gravity loses in this way, galaxies are pulled apart and stars are shattered. Individual atoms cannot survive, either. In a Big Rip, everything suffers.
So how, and why, is this different from a Big Freeze? The answer has to do with something that Mack calls "phantom dark energy."
"It means that suddenly the dark energy is able to not just move galaxies apart from each other but start pulling galaxies apart themselves and ripping stars off galaxies, ripping planets away from their stars," Mack said. "And so as the density of dark energy increases and increases over time, you get to a point where it rips the entire universe apart."
This might be what Carroll meant when he said the universe could end with a bang as opposed to a whimper.
Vacuum Decay: The Bomb You’ll Never See Coming
You're perusing the menu, trying to decide between a Big Freeze, a Big Crunch, and a Big Rip. But you notice each option has an asterisk next to it, and then you see the footnote at the bottom: "Unless vacuum decay happens first."
Another definition is needed here. In physics, there is something called a "Higgs field." In rough terms (that is, not to be taken literally), the Higgs field is responsible for giving mass to subatomic particles, such as quarks and leptons. A more literal explanation of how this works would exceed the scope of this article, but for now, just understand that the structures of the universe are tied to the way in which particles interact with the Higgs field.
Like everything else in the universe, the Higgs field wants to reach its lowest energy state, where it is stable. This state is called the "true vacuum." If the Higgs field currently resides in the true vacuum, there's nothing to worry about. If, however, the field lives in a false vacuum—in which it assumes a higher, less favorable energy state—then the field can discover or "tunnel" into the true vacuum, thus changing its value and causing the mass of particles to change with it.
To illustrate the concept of this tunneling event, Mack uses the analogy of a ball making its way to the bottom of a valley.
"You have a ball sitting in a valley, and that's pretty stable, but if that valley is not really a valley, it's just a little divot in the side of a much deeper valley [. . . .] Imagine the edge of that divot is kind of soft, and [the ball] could fall through, and then it'll just end up rolling all the way down," Mack said.
Such an event could upend the laws of physics. The universe could, in theory, wipe itself clean of all the life and structure we observe today. It'd begin with a vacuum-decay bubble, and that bubble would grow at the speed of light, annihilating everything it touches.
"Because this bubble is expanding at the speed of light, you absolutely cannot see it coming," Mack said. "So if it started on the other side of the room, by the time the light gets to you from the bubble, the bubble is on top of you and is eating you up."
Mack adds that you wouldn't feel any pain when the bubble incinerates you. You would be destroyed within a fraction of a second. You'd never know what hit you. For some people, this is comforting. If your death is imminent and unpreventable, you may prefer not to receive an advance warning.
Fortunately, it's highly unlikely that this will occur anytime soon. The tunneling time, according to Mack, is very long—so long, in fact, that you’ll almost certainly be long dead when the bubble launches.
"Estimates I've seen give us something like ten-to-the-one-hundred or ten-to-the-five-hundred years before it's likely to happen in our observable universe," Mack said.
But this vacuum decay model allows for an additional terrifying possibility: the bubble might be heading for us right now. And there might be more than one. Again, it's unlikely. But the thought is stimulating. Luckily for us, the accelerating expansion of the universe works in our favor here: if vacuum decay has already initiated, it may have started so far away that the expansion of the universe will prevent it from ever reaching us. So while vacuum decay is indeed scary, worrying about it isn't practical.
What to Bet On
With a Big Freeze, a Big Crunch, and a Big Rip on the table (along with that footnote about vacuum decay), how should we expect the universe to end?
At the time of writing, Mack and many other physicists believe the most probable outcome is a Big Freeze.
"[The] stars die and sort of fade out [. . . .] You don't make new stars, and then a bunch of things just start collapsing into black holes [. . . .] The black holes will start to evaporate, particles will decay, and we'll be left in this cold, dark, empty universe," Mack said.
Carroll then asked Mack how long this Big Freeze state will last.
"Forever," Mack said.
Searching for Meaning at the End of the Universe
Some of us confront the inevitability of death more often than others, and we handle it in different ways. The thought of leaving behind a legacy—children, photos, art, our impact on future generations—can make our deaths easier to accept. But with the end of the universe comes the end of everything you leave behind and everything you affected.
"If ultimately you send the future far enough, nothing you do will ever matter again," Mack said. "That's a little confronting. And then you have to decide is there a way to assign meaning to your life, where that life and that meaning and all of your impact disappears."
This is a puzzle for people to solve on their own terms. There's no single correct answer.
One solution is to direct our focus to the present moment. Carroll advances the sentiment:
"It’s the YOLO universe. You only live once," Carroll said. "So enjoy the moment, not the future, because the future is depressingly finite."