A crisis in cosmology may mean hidden dimensions really exist

Last year, cosmologists working on the Dark Energy Spectroscopic Instrument (DESI) reported hints that the mysterious dark energy, thought to be driving the expansion of the universe, may be weakening over time. If these surprising findings prove correct, then dark energy cannot be a cosmological constant – a fixed term in our equations that represents the energy of empty space – after all. When this bombshell hit, most of the buzz focused on what this means for the Standard Model of cosmology, known as lambda-CDM, our best attempt to explain the evolution of the universe.

If the results are confirmed, we might finally have the clues we need to develop a better theory. Researchers are already working to rethink dark energy, and perhaps also dark matter and gravity.

But if the strength of dark energy actually diminishes over cosmic time, the implications could be much broader and deeper. Broader, in the sense that it could give new impetus to proponents of alternative cosmologies that change our understanding of the fate of the universe. And deeper, because it might even tell us something profound about the deepest structure of space-time. “There are certainly very, very exciting possibilities for changing a lot of things in physics,” says Eric Linder, a physicist and cosmologist at the University of California, Berkeley.

According to lambda-CDM, in its earliest moments, the universe underwent a fraction-of-a-second period of exponential expansion. Known as inflation, this explanation appears to explain why the universe is so smooth, flat and homogeneous at its largest scales. But inflation has its critics, chief among them Paul Steinhardt, a physicist at Princeton University. “Inflation doesn’t work,” he says bluntly, adding that it requires improbable initial conditions, is too flexible and leads to a multiverse scenario that many find implausible.

A cyclical universe

Steinhardt has long advocated an alternative hypothesis known as the cyclical universe, in which the universe expands, contracts, and bounces endlessly. However, for such models to work, dark energy must evolve.

“It must be some kind of decaying dark energy that stops accelerating the expansion of the universe, starts slowing it down, and eventually causes a contraction, leading to a rebound and a new cycle,” says Steinhardt. At least the first part of this finding – that the acceleration in expansion is slowing – is precisely what we seem to see with the DESI data.

This is not to say that DESI results provide evidence for cyclical cosmologies. We can still find systemic errors in measurements and analyses, and it is entirely possible that dark energy will weaken without ever producing a contraction or rebound. However, if the evidence of decaying dark energy were confirmed, it would lend credence to Steinhardt’s long-standing argument. “I tend to be very conservative and very patient,” he says. “What I would say, though, is that now the game is on.”

The same could be said for another controversial idea that received a boost thanks to DESI results. Generally speaking, string theory says that everything is ultimately made up of extremely small strings, compacted into hidden extra dimensions, whose vibrations manifest as various particles and forces that we discern. It rose to prominence in the 1980s because it seemed to offer a path to a theory of quantum gravity, reconciling quantum theory and general relativity into what some call a theory of everything.

But string theorists have long struggled to construct models of the universe with a small positive cosmological constant. In a series of papers published in 2018 and 2019, theoretical physicist Cumrun Vafa of Harvard University and his colleagues built on a set of propositions known as the Swampland conjectures, which aim to distinguish theories of particles, forces, and spacetime that can arise from a coherent theory of quantum gravity from those that cannot. Using this framework, they suggested that dark energy cannot be a cosmological constant but rather must be some kind of field – similar to that which would have caused inflation – whose energy changes over time.

At the time, such a proposal contradicted the long-held belief that dark energy remained the same over cosmic time. “People were saying, ‘String theory is ruled out because dark energy is a constant,'” Vafa says.

Hidden Odds

But he and his colleagues persisted. In 2022, they proposed a model in which spacetime has a large hidden extra dimension, perhaps as large as a micrometer, whose size gradually changes over cosmic time. As the geometry of this dimension changes, so does the amount of energy we observe in the universe. The researchers argued that this would manifest as dark energy that would slowly weaken. “There is nothing exotic [here] from a string theory perspective,” Vafa explains. “The extra dimension changes, and dark energy and dark matter react to it.”

It’s easy to see why the DESI results intrigue string theorists: Vafa and his colleagues predicted that dark energy should gradually weaken, and now we’re seeing that. Indeed, when Vafa and his team analyzed DESI data combined with other cosmological datasets in 2025, they found that their model fit much better than lambda-CDM and about as well as the best conventional models allowing dark energy to evolve. The difference here, he says, is that their model includes a physical explanation for what we see. “That’s why I’m so excited,” he said. “It’s very satisfying.”

To be clear, the DESI results offer no concrete evidence for string theory. For starters, the extent to which they prefer dark energy evolution to a cosmological constant always depends on what other cosmological data sets they are combined with. Additionally, non-stringy models that do not invoke additional hidden dimensions fit existing data just as well.

But if we assume for a moment that the DESI data hold up and that the statistical significance increases to the level of discovery, evidence of weakening would not only remove an empirical obstacle to string theory, but also weaken the argument that string theory does not offer testable predictions. “We came up with this model years ago,” says Vafa. “Now they’re observing it, and it looks exactly like we expected.”

However, to realize the idea that this could provide observational evidence in support of string theory, theorists like Vafa would need to build a more precise model that makes more accurate predictions, distinct from non-stringy alternatives, and show that it fits the cosmological data set better than other options. Intriguingly, the framework already hints at additional testable signatures, including deviations from the standard picture of dark matter evolution and deviations from general relativity at the micrometer scale.

Some cosmologists are not convinced that the DESI results have any bearing on fundamental physics, even if they are confirmed. “Dark energy works on certain scales, and that’s what we can talk about,” says Pedro Ferreira, a cosmologist and astrophysicist at the University of Oxford. “[When it comes to] what’s happening at quantum levels, I don’t think we can go there.

But others are open to the possibility that these clues could have implications far beyond cosmology, not least because they could give us a first glimpse into the deep quantum structure of space-time. “What Cumrun Vafa came up with is the most interesting thing I’ve seen,” says Mike Turner, a cosmologist at the University of Chicago in Illinois. “This is where cosmology and particle physics come together. We’re exploring really fundamental things, so the ripple effects can be huge.”