DESI Data Suggests Dark Energy May Be Weakening Over Time

Last year cosmologists working on the Dark Energy Spectroscopic Instrument reported hints that the strength of dark energy may be weakening over time. The instrument measures the universe's expansion history by mapping the positions of millions of galaxies.

If the findings hold, dark energy cannot be described as a fixed cosmological constant representing the energy of empty space. The standard model of cosmology, known as lambda-CDM, assumes dark energy remains constant. The new measurements have prompted researchers to reconsider aspects of the model that describes the evolution of the universe.

Teams are now working to develop alternative explanations that could also affect understanding of dark matter and gravity.

Cosmological Models Physicist Paul Steinhardt at Princeton

University has long advocated for a cyclic universe model in which the cosmos expands, contracts and rebounds repeatedly. Such models require dark energy to evolve rather than remain fixed. Steinhardt said the DESI data showing slowing acceleration matches the first stage of the behavior his model predicts.

"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 stated. " The findings have also drawn attention from researchers exploring string theory.

Cumrun Vafa at Harvard University and colleagues proposed years earlier that dark energy should slowly weaken if it arises from a field influenced by the changing size of a hidden extra dimension roughly the scale of a micrometre.

2022 Vafa's team published a model in which the geometry of an extra dimension varies over cosmic time, causing the observed dark energy density to decrease. When the group compared their model to DESI data combined with other cosmological datasets in 2025, they found it matched the observations as well as leading conventional evolving-dark-energy models.

"We came up with this model years ago," Vafa said. " He noted that the framework also predicts subtle changes in dark matter evolution and small deviations from general relativity at micrometre scales. The DESI results do not constitute evidence for string theory or extra dimensions.

Conventional models without hidden dimensions fit the data equally well. The statistical significance of the preference for evolving dark energy also varies depending on which additional datasets are included in the analysis. " Further observations and refined models will be required to determine whether the apparent weakening of dark energy persists and what underlying mechanism may be responsible.