At the time, the work seemed to conflict with what physicists thought they knew about the universe, with its presumed cosmological constant. And some string theorists still argue that stable universes with positive cosmological constants can exist in string theory; one attempt at constructing such a solution appeared earlier this month. Nevertheless, in light of DESI’s “hint” that the dark energy density is indeed dropping, the conjecture from Vafa and his co-authors now looks prescient.

Trans-Planckian Censorship

There’s another thought experiment that calls into question the very idea of an eternally unchanging cosmological constant.

Let’s say that the expansion of the universe accelerates at a constant rate. Eventually, the expansion gets so fast that even really small things instantly blow up.

In particular, we can consider the smallest distance imaginable — the “Planck length,” which is thought to be the smallest spatial increment that’s observable even in principle. The opposite end of the cosmic ruler is the Hubble horizon, the distance to the edge of the observable universe, beyond which we cannot see. In a universe with a positive cosmological constant, it’s only a matter of time — a lot of time — before the Planck length should grow to the size of the Hubble horizon. According to the trans-Planckian censorship conjecture (TCC), formulated by Bedroya and Vafa in 2019 and updated last year, this should never happen.

It’s a commonsense kind of statement, Bedroya said. “Something that is sub-Planckian” — and hence fundamentally unobservable — “shouldn’t get to the scale where we can observe it.” Nor should it get to a scale that is too big for us to observe it. “The smallest length scale that makes sense in your theory,” he said, “shouldn’t stretch out to be bigger than the biggest length scales that make sense in your theory.”

Assuming that dark energy is the cosmological constant, a straightforward calculation can tell you how long cosmic expansion would have to keep accelerating before the Planck length would start stretching beyond the Hubble length. That amount of time, Bedroya and Vafa computed, is at most 2 trillion years. Dark energy could not behave like a cosmological constant beyond that point.

The TCC conjecture allows for two different scenarios. Dark energy could experience a slow, steady decline. The other possibility is that dark energy holds steady for a while, like a ball stuck in a dip or valley, midway down the hill. But in an instant — occurring somewhere between 10 billion and 1 trillion or so years from now — that will change. In a process called quantum tunneling, the ball will blast through the mound that has been confining it and immediately start rolling down the hill.

If DESI’s preliminary result is confirmed, that would align with option 1.

For Steinhardt, the allure of the swampland endeavor is how it draws on different ideas to make testable predictions about how the universe must be. We’re now entering the regime of measurement, he noted, “where we can actually test these predictions. If they are correct, we should see clear signs of time-varying dark energy very soon.”

‘Biggest Problem in the Universe’

Will DESI’s hint hold up? The team has already collected two more years’ worth of galaxy location data, and an analysis of this larger data set, according to Palanque-Delabrouille, could be out by spring.

Until then, many cosmologists are reserving judgment. “Based on the evidence that we have now, I am not willing to discard the [constant dark energy] model just yet,” said Licia Verde, a cosmologist at the University of Barcelona and a DESI member.

But if the initial DESI finding is confirmed, it will tell us something crucial about dark energy and its future. “Even more importantly,” Vafa said, “we can deduce that this is marking the beginning of the end of the universe. By ‘end,’ I don’t mean nothing happens after that. I’m saying something else happens that is very different from what we have now.” Perhaps dark energy will fall until it settles into a stabler, possibly negative value. With that, a new universe, with new laws, particles and forces, would replace the current one.

“We’re desperate for a clue,” Caldwell said. Dark energy is “literally the biggest problem in the universe.”