In the past year, cosmologists analyzing data from the Dark Energy Spectroscopic Instrument (DESI) have reported tentative indications suggesting that the enigmatic dark energy, which is believed to be driving the universe’s expansion, might be diminishing in strength. If these noteworthy findings are confirmed, it would imply that dark energy is not a simple cosmological constant—a fixed value in our theoretical models representing the energy of empty space—as previously assumed.
This potential revelation has significantly impacted discussions surrounding the standard model of cosmology, known as lambda-CDM. This model represents our most sophisticated attempt to explain the universe’s evolutionary trajectory. Should these results solidify, they could provide crucial impetus for developing a more refined theoretical framework. Researchers are actively engaged in re-evaluating our understanding of dark energy, and potentially dark matter and gravity as well.
However, the implications of a decreasing dark energy strength over cosmic time could extend far beyond the realm of established cosmology. Such a discovery might invigorate alternative cosmological theories that propose different ultimate fates for the universe. Furthermore, it could offer profound insights into the fundamental fabric of spacetime itself. As physicist and cosmologist Eric Linder of the University of California, Berkeley, noted, “There certainly are very, very interesting possibilities for changing a lot of physics.”
Challenging the Standard Model: Inflation and Its Critics
The lambda-CDM model posits that in its earliest moments, the universe experienced a brief period of extremely rapid, exponential expansion, termed inflation. This theoretical event is thought to account for the universe’s observed smoothness, flatness, and homogeneity on its largest scales. Yet, inflation is not without its detractors, notably physicist Paul Steinhardt of Princeton University. Steinhardt contends that “Inflation doesn’t work,” citing its reliance on improbable initial conditions, its inherent flexibility, and its tendency to predict a multiverse scenario, which many find implausible.
The Cyclic Universe Hypothesis
Steinhardt has long advocated for an alternative cosmological model known as the cyclic universe. This hypothesis proposes that the universe undergoes endless cycles of expansion, contraction, and subsequent ‘bounce’ into a new phase. For such models to be viable, however, dark energy must be dynamic and evolve over time.
“It must be some kind of decaying dark energy that stops accelerating the expansion of the universe, starts decelerating it and then eventually causes contraction, leading to a bounce and a new cycle,” Steinhardt explained. The initial observation from DESI—that the acceleration of the universe’s expansion appears to be slowing—aligns with the first part of this requirement.
DESI Data and Alternative Cosmologies
It is important to distinguish that the DESI results do not definitively confirm cyclic cosmologies. The possibility of systematic errors in the measurements or analysis remains, and it is entirely plausible that dark energy could weaken without triggering a contraction or a bounce. Nevertheless, if the hints of decaying dark energy are substantiated, they would lend significant support to Steinhardt’s long-held arguments. He adopts a cautious stance, stating, “I tend to be very conservative and very patient. What I would say, however, is that now the game is afoot.”
The implications of the DESI findings extend to another controversial concept that has gained renewed interest. String theory, broadly speaking, suggests that fundamental reality is composed of infinitesimally small strings whose vibrations give rise to the particles and forces we observe. It gained prominence in the 1980s as a potential pathway to a theory of quantum gravity, aiming to unify quantum mechanics and general relativity into a “theory of everything.”
String Theory and the Dynamic Nature of Dark Energy
A significant challenge for string theorists has been constructing models that accommodate a small, positive cosmological constant. In a series of papers published in 2018 and 2019, theoretical physicist Cumrun Vafa of Harvard University and his collaborators advanced the Swampland conjectures. These conjectures aim to differentiate between theories that can arise from a consistent theory of quantum gravity and those that cannot. Their framework suggested that dark energy could not be a cosmological constant but rather a dynamic field, akin to the one implicated in cosmic inflation, whose energy content varies over time.
At the time of this proposal, it contradicted the prevailing view that dark energy remained constant throughout cosmic history. “People were saying: ‘String theory is ruled out because dark energy is a constant,’” Vafa recalled.
Hidden Dimensions and Evolving Dark Energy
Undeterred, Vafa and his colleagues continued their research. In 2022, they introduced a model incorporating a large, hidden extra dimension in spacetime, potentially as expansive as a micrometer. The size of this dimension is proposed to change gradually over cosmic time. As the geometry of this hidden dimension evolves, the observable energy content of the universe would also change, manifesting as a weakening dark energy.
“There’s nothing exotic [here] from the perspective of string theory,” Vafa stated. “The extra dimension is changing, and both dark energy and dark matter are responding to it.”
The DESI results hold particular intrigue for string theorists because Vafa and his team had predicted a gradual weakening of dark energy, which now appears to be observed. In 2025, their analysis of combined DESI and other cosmological datasets indicated that their model provided a better fit than lambda-CDM and was comparable to leading conventional models that allow for evolving dark energy. Vafa highlighted that their model offers a physical explanation for the observed phenomenon, expressing his excitement: “This is why I’m so excited. It’s very satisfying.”
Caveats and Future Directions
It is crucial to note that the DESI results do not constitute definitive proof of string theory. The degree to which they favor evolving dark energy over a cosmological constant is partly dependent on the specific complementary cosmological datasets used. Moreover, non-stringy models that do not posit hidden extra dimensions can also adequately fit the current data.
However, assuming the DESI data holds firm and its statistical significance increases to discovery levels, evidence of weakening dark energy would not only remove an empirical hurdle for string theory but also bolster the argument that string theory can make testable predictions. “We came up with this model years ago,” Vafa remarked. “Now they’re observing it, and it looks exactly like what we expected.”
To fully establish this as observational evidence supporting string theory, theorists like Vafa must refine their models to generate more precise, distinct predictions compared to non-stringy alternatives. They would also need to demonstrate that these refined models fit the complete spectrum of cosmological data more effectively. Intriguingly, the current theoretical framework already suggests additional observable signatures, including deviations in the standard understanding of dark matter evolution and potential departures from general relativity at micrometer scales.
Skepticism and Broader Implications
Some cosmologists remain unconvinced that the DESI results, even if confirmed, have direct relevance to fundamental physics. Pedro Ferreira, a cosmologist and astrophysicist at the University of Oxford, stated, “Dark energy operates on certain scales, and that is what we can talk about. [When it comes to] what happens at quantum levels, I don’t think we can go there.”
Conversely, others are receptive to the possibility that these observations could have far-reaching consequences beyond cosmology, potentially offering a first glimpse into the deep quantum structure of spacetime. “What Cumrun Vafa has come up with, it’s the most interesting thing I’ve seen,” commented Mike Turner, a cosmologist at the University of Chicago. “This is where cosmology and particle physics come together. We’re digging at really fundamental things, so the knock-on effects can be tremendous.”