But there’s a limit to how much energy you can use for a single measurement. Put too much energy in a small space, and you’ll create a black hole. To make precise measurements without hitting this limit, physicists need to find another way to reduce the quantum fluctuations.

In flat space, physicists can do this by taking measurements from (effectively) infinitely far away — far enough away to shield their measuring devices from the fluctuations. In anti-de Sitter space, it’s even easier: Quantum fluctuations go to zero on the boundary of a boxlike universe, so you can make perfect sense of quantum measurements by setting up your experiment on the universe’s edge.

In de Sitter space, there’s a problem. As you go farther away from the particles you’re measuring, quantum fluctuations don’t get any smaller. “Gravity is fluctuating, quantum mechanically, everywhere,” Green said. “And there’s no place to shield yourself from that.” Without an accessible boundary to take a measurement from, it’s as if an experimenter in de Sitter space is always stuck inside their own experiment.

“The whole machinery of quantum mechanics is built on the idea that there’s a quantum system, and then some big, giant experimentalist comes along and measures that system,” Green said. In de Sitter space, where there’s no line between the quantum system and the observer, this machinery falls apart.

Through the Looking Glass

The problems with de Sitter space get worse. Much of physicists’ intuition stops being helpful in an expanding universe. Energy, for instance, is not conserved. “The expansion is literally pumping energy, changing the universe,” said João Penedones of the Swiss Federal Institute of Technology Lausanne.

Even the concept of a particle is different. We typically think of a particle as an object that has some location and moves through space. “In de Sitter, there is no such thing,” said Manuel Loparco, a former graduate student of Penedones who is now at the University of Turin. The constant influx of energy in de Sitter space will eventually cause a particle to spread out or decay.

In a paper first posted to the scientific preprint site arxiv.org in May 2025, Penedones and Loparco tried asking a simple question: What does a photon, or a particle of light, look like in exponentially expanding space? The answer, which arose from careful mathematics, shocked them. Photons, which are massless, could be made out of massive particles in de Sitter space.

The finding has strange implications. For example, if photons don’t have any mass, they should be stable, because particles can only decay into lighter things. But massive photons in de Sitter space could spontaneously decay into matter — which could then decay back into light again. “We’re still trying to understand the physical implications of that,” Penedones said.

These are the types of calculations that physicists are working to make sense of in de Sitter space. Their goal is to sort the hard technical problems from the hard conceptual problems, asking “What can we calculate; what can’t we calculate?” Green said. The hope, he said, is that this work will leave scientists “in a much better position to solve some of these other bigger problems, because we won’t be confusing them with the smaller, more tractable problems.”

Penedones still finds value in the challenge of trying to understand different versions of the universe — de Sitter, anti-de Sitter, flat — if only to better understand quantum theory. “De Sitter tells you that your intuition that you develop in flat space is not true in all spaces,” he said. “That’s why it’s useful to do some things in de Sitter: to lose your prejudice.”

Pushing Boundaries

To try to make sense of the quantum mechanics of de Sitter space, some physicists have turned to black holes. Black holes are ultra-dense objects from which light cannot escape. While you cannot physically probe a black hole, physicists have studied their insides theoretically. And over the last few years, they’ve made a lot of progress.

Advancements in understanding black holes build on holography, the idea that the two-dimensional surface of the black hole somehow captures everything about the three-dimensional space inside it. Physicists treat the volume of the black hole as illusory, like a hologram.

Black holes have been useful settings for studying quantum gravity, since the extreme gravity of a black hole acts strongly even on the quantum scale. But in recent years, physicists have noted that black holes are also surprisingly similar to de Sitter space.

Around a black hole, the region where light can no longer overcome the black hole’s gravitational pull forms what’s called a horizon. In de Sitter space, a kind of horizon forms around an observer because space is expanding too quickly for light to reach them from beyond a certain distance. If our universe continues to expand forever as physicists predict, then it will be as if we are trapped in a black hole; everything beyond our de Sitter horizon will always remain out of reach.

“We think of black hole cosmology as being sort of a warm-up problem for understanding quantum effects and cosmology,” said Tom Hartman, a physicist at Stanford University. “So anytime we make progress on black holes, we go back and ask: Can we apply this to de Sitter?”

So far, when Hartman and others have tried to apply their advancements in understanding black holes to de Sitter space, they can’t seem to make sense of the results. A black hole has a single horizon, whereas de Sitter space has many, centered on different observers. Without a single boundary to anchor physicists’ calculations, a de Sitter universe seems incapable of holding anything quantum at all. “There’s something empty about it,” Hartman said. “Like if you try to formulate a quantum theory of de Sitter space, there’s some sense in which it wants to not have any states in it.”

This is a clear contrast to the world we observe, which is both full of quantum particles and increasingly de Sitter–looking. “The most likely answer is that we’re not interpreting that calculation correctly,” Green said.

Still, physicists are hopeful that holography will one day apply more generally to de Sitter space, and that it will answer some of our biggest questions about quantum gravity. “That’s always been kind of believed, but I think in the last few years, it’s become more convincing,” Hartman said.

What other surprises de Sitter holds are yet to be seen, but the landscape seems to be ripe for insights, Loparco said. “It’s really low-hanging fruit that we’re picking.”