A crisis in cosmology could mean that hidden dimensions really do exist

Last year, cosmologists working with the Dark Energy Spectroscopic Instrument (DESI) reported hints that the mysterious dark energy thought to drive the universe’s expansion may be weakening over time. If these startling findings turn out to be 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 focused on what it means for the Standard Model of cosmology, known as lambda-CDM, our best attempt to explain the evolution of the universe.

If the results hold, we may finally have the clues required to build a better theory. Scientists are already busy trying to rethink dark energy, and possibly dark matter and gravity as well.

But if the strength of dark energy really does wane over cosmic time, the implications could run far wider and deeper. More broadly, in the sense that it can provide new impetus for proponents of alternative cosmologies that change our understanding of the fate of the universe. And deeper, because it can even tell us something profound about the deepest structure of space-time. “There are certainly very, very interesting possibilities for changing a lot of physics,” says Eric Linder, a physicist and cosmologist at the University of California, Berkeley.

According to lambda-CDM, the universe in its first moments underwent an exponential expansion in a split second. This explanation, known as inflation, appears to provide a reason for why the universe is so smooth, flat and homogeneous at its largest scales. But inflation has its critics, most prominent among them Paul Steinhardt, a physicist at Princeton University. “Inflation doesn’t work,” he says bluntly, adding that it requires implausible initial conditions, is too flexible and leads to a multiverse scenario that many find implausible.

A cyclic universe

Steinhardt has long argued for an alternative hypothesis known as the cyclic universe, in which the universe endlessly expands, contracts and bounces back. However, to make such models work, dark energy must be developed.

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

This is not to say that the DESI results provide evidence for cyclical cosmologies. We can still find systemic errors in the measurements and analysis, and it is entirely possible for dark energy to weaken without ever producing a contraction or a bounce. If hints of decaying dark energy intensify, however, it will lend credence to Steinhardt’s long-standing argument. “I tend to be very conservative and very patient,” he says. “However, what I will say is that now the battle is on.”

The same can be said for another controversial idea that has received a boost from the DESI results. Broadly speaking, string theory says that everything is ultimately made of vanishingly small strings, compressed into hidden extra dimensions, whose vibrations manifest as the various particles and forces we can see. It rose to prominence in the 1980s because it appeared to offer a path toward a theory of quantum gravity, which unites 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, Harvard University theoretical physicist Cumrun Vafa and his colleagues built on a set of proposals known as the Swampland conjectures, which aim to separate theories of particles, forces and space-time that might arise from a consistent theory of quantum gravity. Using this framework, they proposed that dark energy cannot be a cosmological constant, but must instead be a kind of field – similar to the one thought to have driven inflation – whose energy changes over time.

At the time, such a proposal conflicted with the long-held belief that dark energy remained the same over cosmic time. “People said, ‘String theory is out of the question because dark energy is a constant,'” says Vafa.

Hidden dimensions

But he and his colleagues persisted. In 2022, they proposed a model in which space-time has a large hidden extra dimension, possibly as large as a micrometer, whose size changes gradually over cosmic time. As the geometry of this dimension changes, so does the amount of energy in the universe we observe. The researchers claimed that this would manifest itself as a slowly decaying dark energy. “There is nothing exotic (here) from the perspective of string theory,” says Vafa. “The extra dimension is changing, and both dark energy and dark matter are responding to it.”

It’s easy to see why the DESI results are exciting for string theorists: Vafa and his colleagues had predicted that dark energy would gradually weaken, and now it seems that’s what we’re seeing. When Vafa and his team analyzed the DESI data combined with other cosmological data sets in 2025, they actually found that their model fit far better than lambda-CDM and about as well as the best conventional models that allow dark energy to evolve. The difference here, he says, is that their model includes a physical explanation for what we see. “This is why I’m so excited,” he says. “It’s very satisfying.”

To be clear, the DESI results do not provide concrete evidence for string theory. First, the extent to which they prefer to develop dark energy over a cosmological constant still depends on what other cosmological data sets they are combined with. Moreover, unstringed models that do not invoke hidden extra dimensions fit the existing data equally well.

But if we assume for a moment that the DESI data hold and the statistical significance grows to the level of detection, evidence of attenuation would not only remove an empirical obstacle to string theory, it would also weaken the argument that string theory offers no testable predictions. “We came up with this model many years ago,” says Vafa. “Now they’re observing it, and it looks exactly like what we expected.”

To make good on the notion that this might provide observational evidence in support of string theory, however, theorists like Vafa would have to build a sharper model that makes more precise predictions, distinct from unstringed alternatives, and show that it fits the full range of cosmological data better than other alternatives. Interestingly, the framework already suggests several testable signatures, including deviations from the standard picture of how dark matter evolves and deviations from general relativity on micrometer scales.

Some cosmologists are not convinced that the DESI results have any significance at all for fundamental physics, even if they are solid. “Dark energy operates on certain scales, and that’s what we can talk about,” says Pedro Ferreira, a cosmologist and astrophysicist at the University of Oxford. “(As for) what happens at quantum levels, I don’t think we can go there.”

But others are open to the possibility that these hints may have ripples far beyond cosmology, not least because they may give us a first glimpse into the deep quantum structure of spacetime. “What Cumrun Vafa has come 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 digging into really fundamental things, so the knock-on effects can be huge.”