A theoretical study described in a ScienceDaily report proposes that dark energy’s effective density could have changed from negative to positive about 9.5 billion years ago. The result comes from a less explored solution within a modified theory of gravity. It is a calculated possibility, not evidence that the Universe’s history has been observed to unfold that way.
The international team was led by Prof. Özgür Akarsu of Istanbul Technical University (ITU), with postdoctoral researcher Dr. N. Merve Uzun and doctoral student Bilal Bulduk. They collaborated with Prof. Nihan Katırcı of Doğuş University and Prof. Antonio De Felice of Kyoto University’s Yukawa Institute for Theoretical Physics. Their study was published in Physical Review D in 2025, according to the report.
The researchers examined teleparallel f(T) gravity, an alternative framework that describes gravity through torsion rather than the curvature of spacetime used in general relativity. Earlier work on an exponential version of this theory had focused on solutions in which dark energy’s density stayed positive. The team explored another mathematical branch by relaxing that assumption.
The calculations produced two possible cosmic histories for a Universe with the same amount of matter today. One resembles the standard cosmological model. In the other, the model’s effective dark-energy density is negative in the distant past, reaches zero at about redshift 1.5—roughly 9.5 billion years ago—and becomes positive afterward. That sign change is a feature of the theory’s calculated history, not a direct measurement of dark energy’s density at that time.
The branch also matters because it does not require an additional adjustable parameter to produce the sign change. The report says the present matter density determines which mathematical branch applies, while the model keeps the six cosmological parameters used in the standard ΛCDM model. The theory further predicts that its departures from general relativity would be suppressed in dense environments, allowing it to remain compatible with precise Solar System gravity measurements.
The idea connects to a broader question raised by observations. The Dark Energy Spectroscopic Instrument (DESI) maps the locations of galaxies and quasars to trace cosmic expansion. Findings from DESI’s first three years, announced in March 2025, strengthened indications that dark energy might not behave like a fixed cosmological constant when combined with measurements of the cosmic microwave background, supernovae and weak gravitational lensing. But the strength of the evidence depends on which datasets are combined, and it remains below the level needed to establish a discovery.
The ITU-led team’s paper was first submitted in October 2024, before DESI announced those results. The report says the researchers have not yet tested their model against DESI’s complete dataset. DESI has completed its planned five-year survey, observing more than 47 million galaxies and quasars; the full dataset is expected to offer a more demanding test of whether the apparent signs of evolving dark energy persist.
A sign change could also help researchers investigate the Hubble tension: the continuing discrepancy between different methods of measuring the Universe’s present expansion rate. Earlier analyses of sign-switching models found that they could agree with DESI observations and potentially ease some cosmological disagreements. The new theory offers one possible mechanism for such a change, because the reversal arises from the mathematical structure of the gravity model rather than being specified separately.
An extended version that includes a cosmological constant was reported to remain consistent with cosmic microwave background observations while allowing the higher expansion rates measured locally. It also permits the possibility that cosmic acceleration weakened or stopped in the relatively recent past. Neither that slowdown nor the proposed sign reversal has been established observationally.
The next tests include measuring how cosmic structures grow, how expansion changes with redshift, and how gravity bends light. Those observations will help determine whether this less familiar branch describes the Universe or remains an interesting mathematical possibility.