Apocalyptic visions have always percolated in humanity’s collective imagination, whether it’s the Rapture, Ragnarök, or a future asteroid impact like the one that killed the dinosaurs. In recent decades, however, scientists have managed to establish some rough parameters around the ultimate Doomsday: the death of the universe itself.
Katie Mack, a theoretical cosmologist at North Carolina State University, explores these terminal diagnoses for the cosmos in her new book The End of Everything (Astrophysically Speaking), which packs a huge amount of scientific research into forecasting the eventual fate of our universe.
“It doesn’t end well,” she warned in a call.
Before you scribble “end of universe” onto the list of things keeping you up at night, take comfort in the fact that the really bad stuff will happen in the far-future of the cosmos, at least tens of billions of years from now. You will be long dead, as will Earth and the Sun. The senescent universe will be a time and place totally alien from our own surroundings, far more fantastical than any the feverish apocalyptic visions of myth or fiction.
“I get asked a lot: How do you deal with thinking about these big topics, like ultimate destruction? How does it affect your outlook?” Mack said. “I think all you can do is go to the absurd in the sense that there’s no way to conceptualize this stuff with daily experience.”
“It’s like the universe is laughing at this idea that we can have an orderly and safe environment in which to live,” she added. “It very much upends our notion of stability in our world. I don’t know how to respond to that other than just laughing at it, because it’s not personal.”
We asked Mack to unpack a few of the juiciest apocalyptic scenarios in The End of Everything (Astrophysically Speaking), which is out from Scribner on Tuesday.
Heat Death: When time ceases to matter
The universe will most likely perish in a state of total disorder known as Heat Death, when the direction of time as we experience it ceases to matter and just about anything may be possible, according to cosmologists.
In this scenario, space just keeps expanding until galaxies fall apart, all the stars burn out, and even atoms decay and disintegrate. At this point, the universe will have reached a point of maximum entropy, or disorder, rendering the “arrow of time”—the difference between the past and the future—meaningless.
“Time still happens, but you have lost the directionality in some sense,” Mack explained. “It’s based on the fact that the way we define past and future, from a strictly physics perspective, is that the only thing we know about that really cares about the difference between past and the future is the second law of thermodynamics, which is entropy.”
“If you can get to a point where entropy is maximized, where you can’t create more entropy, then it’s hard to say that time is really meaningful in a global sense anymore,” she said.
Needless to say, some trippy stuff could end up happening in a universe that has maxed out on entropy and faded into a vast and eternal bath of thermodynamic equilibrium. For instance, Mack describes the Boltzmann Brain problem, which involves “disembodied sentient brains popping in and out of existence,” according to the book. This, and anything else—a whale materializing next to a bowl of petunias, or a piano assembling itself from nothing—becomes increasingly likely in such a scenario.
Such wild imaginings stem from the sheer slowness of the Heat Death, a decay that could take a googol (10 to the power of 100) years to really get rolling. Those huge timescales boost the odds of totally bizarre random events occasionally happening in a fizzled-out cosmos—including, potentially, the birth of a new universe.
“The nice thing about the Heat Death is that you have a lot of time,” Mack said. “If you want to make sure that you get a lot done in your universe before it goes out on you, then maybe the Heat Death is the best option.”
The Big Rip: When gravity breaks and the Earth explodes
The Heat Death is the probable outcome of the accelerated expansion of the universe, but cosmic expansion may also lead to a less likely, yet far more violent, end of everything: the Big Rip.
In this scenario, objects in the universe don’t drift apart and decay into maximum entropy. Instead, a point is reached at which the expansion of the universe ultimately tears apart the fabric of spacetime itself, like a cloth sheet that splits when stretched, causing the force of gravity to lose its trademark grip.
In a chilling section of the book, Mack describes exactly what this fate would look like to us on Earth if it was approaching in the near-future. “Our night sky begins to darken,” she writes, “as the great Milky Way swath across the sky fades. The galaxy is evaporating.”
“We begin to find that the orbits of the planets are not what they should be, but are instead slowly spiraling outward,” she continues. “Just months before the end, after we’ve lost the outer planets to the great and growing blackness, the Earth drifts away from the Sun, and the Moon from the Earth. We too enter the darkness, alone.”
It sounds lonely, I know, but take heart: we would only have to bear this isolation for a few hours before the Earth blows up.
The explosion of Earth due to shredded spacetime certainly makes for a cinematically exciting scene. But if we are fated for a Big Rip, it is not likely to happen for about 200 billion years. That’s a lot sooner than the standard Heat Death scenario, but it is well beyond the lifespan of our solar system, Earth, and (probably) humanity.
That said, there may well be alien civilizations in the future, or perhaps descendants of our own species, that could have to face this horrifying reality. “I think it’s entirely possible that there could be life still around on those timescales,” Mack said.
If there are still sentient beings at that point, and the Big Rip does come to pass, all we can do right now is offer our sincere condolences to them from the distant past.
Vacuum Decay: A sudden end
Heat Death would kill the universe slowly and softly, while the Big Rip is a much swifter assassin. But if you’re looking for the fastest end to existence, Vacuum Decay is the cosmic Doomsday for you.
“I have a special place in my heart for Vacuum Decay,” Mack said. “Partly, because it’s just so out of left field and such a bizarre possibility that has only really been very seriously talked about in the last few years.”
“But also because it’s quick and painless and you don’t notice it,” she added. “So that’s nice.”
This outcome is a bit of a dark horse, though its profile has been raised thanks to the recent discovery of the Higgs boson particle by the Large Hadron Collider in Switzerland. One of the implications of this breakthrough is that our reality may not be all that fundamentally stable in ways that could have rather abrupt consequences for life, the universe, and everything.
The universe could be a “true vacuum,” which means that objects in it are always able to find their lowest energy state, creating some level of cosmic stability. However, the Higgs field appears to be “metastable,” which raises the possibility that the universe may be a false vacuum.
Theoretically, this means that if the Higgs particle were to sense a true vacuum, it would be attracted to that environment. This is very bad for us, because it would trigger the spontaneous destruction of the universe in what Mack calls “a bubble of quantum death.”
Unlike the creeping dread of the Heat Death or the Big Rip, the Vacuum Decay apocalypse would kill us all in a snap. The death bubble would simply expand its borders at the speed of light, incinerating everything in its path with ruthless efficiency.
“In terms of the aesthetics of it, or the practical implications of what actually happens to you, maybe vacuum decay is a nicer option,” compared to the Heat Death or the Big Rip, Mack said.
In principle, Vacuum Decay could happen at any time, but cosmologists think it is far more likely to happen tens of billions of years into the future, similar to the other end-times scenarios. We will need to keep pushing the boundaries of particle physics and cosmology in order to develop the idea, but it’s not considered a probable end to the universe at this time.
“It’s just a super fun thing to work on because the implications are so big,” Mack said. “We can learn a lot about our cosmos by assuming that it could happen.”
The universe will likely continue to exist for several hundred billion years to come. But thinking about its ultimate end seems particularly resonant in 2020, a year that has taken on its own identity of apocalyptic mayhem.
It’s understandable to be exhausted by the horrors of the Covid-19 pandemic and the economic devastation it has wrought, or by the rapid onset of climate change with its myriad disasters, or by the threat of authoritarian leaders and brutal state violence.
Reading about the ultimate death of the universe may not assuage those fears, but it will immerse you in the astonishing weirdness of our wider surroundings, and remind you of the ingenuity of scientists who have spent centuries trying to read the cosmic tea leaves.
“There is a kind of luxury about being able to think about things that are disconnected from you and not just everyday survival,” Mack said. “It’s real and it’s destructive and everything is torn apart, but maybe that’s a way to displace some of the feelings of angst and desperation that you might have in daily life.”