The Rise, Fall and Rebound of Cyclic Cosmology

The universe is dead; long live the universe.

Not right now, not yet. But one day everything we know will be gone. The cities we build, the lakes we swim in, the planet we live on, the solar system we inhabit, the star we orbit, and every star we are not—all are heading toward an inevitable end.

At the end of it all, what happens? Some say that our ever-expanding universe will slow down and then one day do a cosmic U-turn, undoing all the growth that happened after the big bang. Eventually, everything will collapse into the smallest possible space and then erupt again in a riot of rebirth – this is the idea we call cyclic cosmology or the big bounce. It has been around for a long time, and the idea itself has encountered a trajectory that reflects its content. It was briefly popular in the mid-20th century, fell out of favor, and now may 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).

As is sometimes the case with grand cosmological hypotheses, proponents of cyclical cosmology prefer it mostly for its elegance: if the universe is cyclical, that means we probably don’t have to worry about what precipitated the big bang or what existed before it—those near-impossible questions are already answered. There’s a beautiful sense of symmetry to the whole thing. Kathryn Heymansthe Astronomer Royal for Scotland, put it well during a recent A new scientist subscriber event I hosted where she said, “I really love that the universe was created in a big bang, it expands, it slows down, gravity pulls it back in, there’s a big grip, there’s another big bang and it expands… It just makes me very happy.”

At the same event, Adam Reeswho won a Nobel Prize for his and his colleagues’ discovery of dark energy, pointed to one of the more specific reasons for many cosmologists’ attachment to the idea. “We like it because it tells us that this is not a special time we live in or a one-time universe,” he said. In other words, in a cyclical universe it wouldn’t be such an incredible coincidence that we’re even here to ponder these things. Personally, I don’t think the idea that moments like this happen over and over again – maybe not every bounce, but definitely more than one – with all the proper living conditions and trees and rockets to the moon makes it any less special, but I digress. Perhaps this is an anthropocentric, emotional position rather than one based on the laws of physics.

For a long time, cyclic cosmology fell out of favor, led in part by the work of Riess showing that the universe is expanding at an accelerating rate. If the space between the stars is growing faster and faster, it seems unlikely that it will eventually shrink to nothing again. Gravity is simply not strong enough to counteract dark energy. “Unfortunately, all the measurements we’re making tell us that there just isn’t enough mass in the universe to pull it back together,” Heymans said. “Currently, the evidence points to a very cold, sad and empty death for our universe.” This idea, called heat death, is now the most accepted version of what’s to come.

There are various other reasons why the big bounce faded into relative obscurity, largely related to problems that arise when we try to understand how matter, energy, and entropy can be recycled or destroyed in the time between bounces.

The second law of thermodynamics is an obstacle: it says that disorder or entropy in a closed system (like the universe as we know it) can never decrease. With an expanding universe, this is easy to balance out – we’ll just see a continuous slow increase in entropy over the lifetime of the cosmos. But if the universe begins to contract again, entropy will begin to decrease accordingly. There are ways around this, usually involving pushing the problem to the next expansion and contraction cycle. If the universe gets bigger in each cycle, entropy still increases overall. But if you extrapolate back or forward enough time, you’ll end up in the same situation as before. We still start with a big bang at the beginning of the universe and an eventual heat death at the end, it’s just a more complex, cascading path between the two.

Another way around the entropy problem was popularized in 2010 by the legendary theoretical physicist Roger Penroseof Penrose Triangle fame. His model is called conformal loop cosmology, and it would look exactly like an ever-expanding universe… right next to the very end. As the universe expands and everything moves further and further away from everything else, matter will break down into its constituent parts and eventually everything will be just residual photons floating in the abyss. This is not particularly controversial. But what Penrose proposed next is. His idea is that the extreme emptiness and uniformity of space-time at the end of a cycle, or aeon, is the same as the structure we would expect at the very beginning of a new aeon. The idea behind conformal loop cosmology is that, thanks to this functionally identical structure (and some very complex mathematics), a new, expanding universe can be created from the cold remains of the previous one.

The idea is niche and difficult (bordering on impossible) to test. Penrose suggested some potentially measurable ones pieces of evidence about it, but cosmologists generally tend to find them unconvincing. However, it is also not disproven, and the fact that it manages to circumvent the entropy problem means that it should not simply be dismissed, even if it is widely viewed with skepticism. So we are stuck without much way to apply these ideas to the real universe in which we live.

Enter DESI. His vast map of the universe showed that the dark energy that had previously looked like it would only grow in power forever seemed to be weakening. This means that the outward acceleration of the universe appears to be slowing down. As Heymans emphasized during the event, this doesn’t mean the universe is reuniting — it’s still accelerating its expansion, just not as fast. Still, it’s a radical shift in our understanding of dark energy and could usher in an era of new theories about how our cosmos will spend its final days.

And among these new theories, cyclical cosmologies seem to be rising again. “What could be causing the dark energy to change could mean that after another 10 billion years the dark energy weakens so much that it turns around and pulls everything back in on itself, which would be wonderful,” Heymans said.

The problem with knowing what it all means is that we don’t understand a huge part of the universe. Dark energy constitutes nearly 70 percent of all matter and energy in the entire cosmos; it controls the ultimate fate of everything and yet we have no idea what it is or how it works. On cosmological time scales, we’ve only just met—Rice and his colleagues only identified it less than 30 years ago.

“Without understanding the nature of the dark energy driving the current acceleration, it’s very difficult to extrapolate it into the future. Will it slow down?” Reece said. “I’d say all bets are off for the future.” The smart money may still be at the cold, empty end of the universe, but for the first time in a century, it may also be worth taking a long-term bet on the big bounce.

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