The largest 3D map of our universe thus far, with Earth at the middle and each dot exhibiting a galaxy
DESI collaboration and KPNO/NOIRLab/NSF/AURA/R. Proctor
The universe is dead; long live the universe.
Not right now. Not yet. But someday, everything we know will be gone. The cities we build, the lakes we swim in, the planet we live on, the solar system we call home, the star we orbit, and every star we don’t—they’re all headed for an inescapable end.
So what actually happens at the end of it all?
Some scientists say our ever-expanding universe will eventually slow down, then do a cosmic U-turn, undoing everything that’s happened since the Big Bang. Eventually, everything gets crushed into the tiniest possible space, then explodes outward again in a wild rebirth. That’s the idea behind cyclic cosmology, or the Big Bounce. It’s been around for a while, and honestly, the theory itself has gone through a trajectory that mirrors its contents. It was popular for a bit in the mid-20th century, then fell out of favor. Now, it might be making a comeback thanks to new data from the largest 3D map of the universe ever created, made by the Dark Energy Spectroscopic Instrument (DESI).
Here’s the thing, though. As with most grand cosmological ideas, the people who like cyclic cosmology mostly like it because it’s beautiful. If the universe is cyclic, then we probably don’t have to worry about what caused the Big Bang or what came before it. Those nearly impossible questions already have an answer. There’s this lovely sense of symmetry to the whole thing.
Catherine Heymans, the Astronomer Royal for Scotland, said it really well during a recent New Scientist subscriber event I hosted. She told me, “It really gels with me that the universe sort of is created in a big bang, it expands, it slows down, gravity pulls it back in on itself, there’s a big crunch, there’s another big bang and it expands… This just makes me very happy.”
At the same event, Adam Riess—who won a Nobel for discovering dark energy—gave one of the more concrete reasons why cosmologists are fond of this idea. “We like it because it tells us that this is not a special time that we live in or the one-shot universe,” he said.
In other words, in a cyclic universe, it wouldn’t be such an unbelievable coincidence that we’re here at all. Personally, I don’t think the idea that times like this happen over and over—maybe not in every bounce, but definitely in more than one—with all the right conditions for life and trees and rockets to the moon, makes it any less special. But that’s just me. Maybe that’s more of an emotional, human-centered take than one based on physics.
For a long time, cyclic cosmology fell out of favor. Part of that was thanks to Riess’s own work showing that the universe is actually expanding faster and faster. If the space between stars is growing quicker and quicker, it seems pretty unlikely that it’ll ever shrink back down to nothing. Gravity just isn’t strong enough to overcome dark energy.
“Unfortunately, all of the measurements that we make tell us that there just isn’t enough mass in the universe to pull it back together,” said Heymans. “At the moment, the evidence is pointing towards a very cold and sad and empty death for our universe.”
That idea is called the heat death, and right now it’s the most accepted version of what’s to come.
There are plenty of other reasons the Big Bounce faded into relative obscurity. Most of them have to do with what happens when you try to figure out how matter, energy, and entropy could be recycled or destroyed in the moment between bounces.
The second law of thermodynamics is a big sticking point. It says that disorder—or entropy—in a closed system (like the universe, as far as we know) can never decrease. With an expanding universe, that’s easy to square. We’d just see a steady, gradual increase in entropy over the cosmos’s lifetime. But if the universe starts contracting again, entropy would have to start decreasing.
There are ways around this. Some involve pushing the problem off into the next cycle of expansion and contraction. If the universe gets bigger in each cycle, entropy is still increasing overall. But if you extrapolate far enough forward or backward, you end up in the same situation as before. You still start with a Big Bang and end with heat death. It’s just a more complicated, step-by-step path between the two.
Another way around the entropy problem was popularized in the 2010s by legendary theoretical physicist Roger Penrose—yes, the Penrose triangle guy. His model is called conformal cyclic cosmology. And here’s the wild part: it would look exactly like an ever-expanding universe… right up until the very end.
As the universe expands and everything gets farther and farther apart, matter decays into its component parts. Eventually, everything is just leftover photons floating in the void. That part isn’t really controversial. But what Penrose proposed next? That’s the head-scratcher.
His idea is that the extreme emptiness and uniformity of space-time at the end of one cycle—or “aeon”—is actually the same as the structure you’d expect at the very beginning of a new aeon. Because of this functionally identical structure (and some very complicated math), a brand new, expanding universe could kick off from the cold, dead remains of the previous one.
The idea is niche and difficult—bordering on impossible—to test. Penrose has proposed some potentially measurable bits of evidence for it, but overall, most cosmologists find them unconvincing. That said, it hasn’t been disproven either. And the fact that it manages to get around the entropy problem means it shouldn’t just be thrown out, even if most experts view it with skepticism.
So where does that leave us? Stuck, basically. Without much of a way to apply these ideas to the actual universe we live in.
The Mayall 4-meter Telescope at Kitt Peak National Observatory, which is utilized by DESI to survey the stars
DESI Collaboration/DOE/KPNO/NOIR
Then enter DESI.
Its massive map of the universe has shown something surprising: dark energy—which we thought would just keep getting stronger forever—seems to be weakening. In other words, the outward acceleration of the universe appears to be slowing down.
Now, as Heymans stressed during the event, this doesn’t mean the universe is coming back together. It’s still expanding faster and faster. Just… not quite as fast as before.
Still, this is a huge shift in how we understand dark energy. And it might kick off a whole new era of theories about how our cosmos will spend its final days.
Among those new theories? Cyclic cosmologies seem to be making a comeback.
“What could be causing dark energy to change,” Heymans said, “could mean that in another 10 billion years’ time, dark energy weakens so much that it does reverse and it does pull everything back in on itself, which would be lovely.”
The real problem with figuring out what any of this means is that we don’t understand a huge chunk of the universe. Dark energy makes up nearly 70 percent of all the matter and energy in the entire cosmos. It controls the ultimate fate of everything. And yet? We have no clue what it is or how it works.
On cosmological time scales, we’ve barely just met the stuff. Riess and his colleagues only identified dark energy less than 30 years ago.
“Without understanding the nature of the dark energy that’s driving the present acceleration,” said Riess, “it’s very difficult to extrapolate it into the future. Will it weaken? I would say all bets are off about the future.”
So here’s where we stand. The smart money is probably still on a cold, empty end for the universe. But for the first time in a century? It might actually be worth placing a long-shot bet on the Big Bounce.