A crisis in cosmology may mean that hidden dimensions do exist

Last year, cosmologists working on the Dark Energy Spectroscopic Instrument (DESI) reported indications that the mysterious dark energy believed 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 after all—the fixed term in our equations that represents the energy of empty space. When this bombshell hit, most of the buzz focused on what it meant 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 may finally have the clues needed to develop a better theory. Scientists are already busy trying to rethink dark energy and possibly dark matter and gravity.

But if the power of dark energy does indeed decrease over cosmic time, the consequences could be much wider and more profound. Broader in the sense that it could 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 spacetime. “There are certainly very, very interesting possibilities to change a lot of physics,” he says Eric Linderphysicist and cosmologist at the University of California, Berkeley.

According to lambda-CDM, the universe went through a spell of exponential expansion in its first moments in a fraction of a second. This explanation, known as inflation, appears to provide the reason why the universe is so smooth, flat, and homogeneous at its largest scales. But inflation has its critics, the most prominent of which Paul Steinhardtphysicist 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 consider unlikely.

Cyclic universe

Steinhardt has long argued for an alternative hypothesis known as a cyclic universe, in which the universe endlessly expands, contracts, and returns. 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 it to contract, leading to a rebound and a new cycle,” says Steinhardt. At least the first part—that the acceleration of the expansion is slowing—is exactly what we seem to see in the DESI data.

This does not mean that the DESI results provide evidence for cyclic cosmologies. We can still find systematic errors in measurement and analysis, and it is entirely possible that dark energy is weakening without ever contracting or rebounding. However, if signs of declining dark energy solidify, it will lend credence to Steinhardt’s long-held argument. “I tend to be very conservative and very patient,” he says. “However, I would say the game is on.

The same could be said for another controversial idea that took a hit from the DESI results. Broadly speaking, string theory says that everything is ultimately made of vanishing little strings, compacted into hidden extra dimensions whose vibrations manifest as various particles and forces that we recognize. It rose to prominence in the 1980s because it appeared to offer a path to a theory of quantum gravity, merging quantum theory and general relativity into what some call a theory of everything.

However, string theorists have long struggled with constructing models of the universe with a small positive cosmological constant. In a series of papers published in 2018 and 2019, theoretical physicist Cumrun Vafa at 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 consistent theory of quantum gravity from those that cannot. Using this framework, they proposed dark energy cannot be a cosmological constant but instead it must be some kind of field—similar to the one thought to have driven inflation—whose energy varies with time.

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

Hidden dimensions

But he and his colleagues persisted. In 2022, they proposed a model in which spacetime has a large hidden extra dimensionperhaps as large as a micrometer, its size gradually changing over cosmic time. As the geometry of this dimension changes, so does the amount of energy in the universe that we observe. Scientists have argued that this will manifest as dark energy that slowly fades. “There is nothing exotic about it. [here] from a string theory perspective,” says Vafa. “The next dimension is changing, and dark energy and dark matter are responding to that.”

It’s easy to see why the DESI results are of interest to string theorists: Vafa and his colleagues predicted that dark energy should gradually weaken, and now it appears that’s what we’re seeing. When Vafa and his team analyzed the DESI data in combination with other cosmological datasets in 2025, they found that the model fits much better than lambda-CDM and about the same as the best conventional models that allow for the development of dark energy. The difference, he says, is that their model includes a physical explanation for what we see. “That’s why I’m so excited,” he says. “It’s very satisfying.

To be clear, the DESI results do not offer concrete evidence for string theory. For starters, the degree to which they prefer evolving dark energy over the cosmological constant still depends on which other cosmological datasets they are combined with. What’s more, unbound models that do not invoke hidden additional dimensions fit the existing data just as well.

But if we assume for a moment that the DESI data hold and statistical significance rises to the level of discovery, evidence of weakening would not only remove the empirical obstacle to string theory, but also weaken the argument that string theory offers no testable predictions. “We came up with this model years ago,” says Vafa. “Now they are observing it and it looks exactly as we expected.”

However, to fulfill the notion that this could provide observational evidence to support string theory, theorists like Vafa would have to create a sharper model that would make more accurate predictions, distinct from non-string alternatives, and show that it fits the full range of cosmological data better than other possibilities. Interestingly, the framework already suggests other testable signatures, including deviations from the standard picture of dark matter evolution and deviations from general relativity on micrometric scales.

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

But others are open to the possibility that these hints could have ripples far beyond cosmology, not least because they could give us our first glimpse into the deep quantum structure of spacetime. “What Cumrun Vafa has come up with is the most interesting thing I’ve ever seen,” he says Mike Turnercosmologist at the University of Chicago in Illinois. “This is where cosmology and particle physics come together. We’re dealing with really fundamental things, so the knock-on effects can be huge.”

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