Gravity study models a possible reversal in dark energy’s density
The calculations place the possible change roughly 9.5 billion years ago, but the model still awaits testing against the complete DESI dataset.
Dark energy’s effective density could have crossed from negative into positive territory during cosmic history, according to calculations by an international team led by Istanbul Technical University researchers. Their study appeared in Physical Review D in 2025 and places the possible transition roughly 9.5 billion years ago.
Prof. Özgür Akarsu and colleagues explored a largely overlooked set of solutions within teleparallel f(T) gravity. That framework describes gravity using torsion rather than the spacetime curvature used in Einstein’s general relativity.
The researchers identified two possible evolutionary paths for a universe with identical present-day matter content. One follows a history similar to standard cosmology. In the other, effective dark energy density passes through zero and becomes positive. The team reported that this requires no additional adjustable parameter: the model keeps the six cosmological parameters of standard ΛCDM.
Separately, March 2025 findings from the Dark Energy Spectroscopic Instrument (DESI), combined with other measurements, strengthened indications that dark energy might vary over time. The evidence depends on the datasets combined and does not establish a discovery. The ITU-led team has not compared its model with DESI’s complete dataset.
An extended version of the theory, incorporating a cosmological constant, also allows cosmic acceleration to have weakened or stopped in the relatively recent past. Neither that possibility nor the proposed density reversal has been confirmed through observations.
