Dark energy may have flipped from negative to positive 9.5 billion years ago, challenging decades of cosmological assumptions that the force driving the universe’s accelerating expansion has remained constant.
For decades, researchers assumed that dark energy possessed a constant, positive density across cosmic history. Observations from the Dark Energy Spectroscopic Instrument, which is constructing the largest three-dimensional map of the universe, now suggest that this influence may have evolved over billions of years.
While the evidence does not yet meet the statistical threshold to establish a definitive discovery, it has revived debates about whether cosmic acceleration can change character. Researchers at Istanbul Technical University explored that possibility by analyzing an alternative theory of gravity published in a 2025 study in Physical Review D.
Teleparallel Gravity Solutions
Einstein’s general theory of relativity links gravity to the curvature of spacetime, whereas teleparallel gravity describes the force through a mathematical property called torsion. An international team led by Prof. Özgür Akarsu investigated a previously neglected branch of teleparallel f(T) gravity.
The team included postdoctoral researcher Dr. N. Merve Uzun, doctoral student Bilal Bulduk, Prof. Nihan Katırcı of Doğuş University, and Prof. Antonio De Felice of Kyoto University’s Yukawa Institute for Theoretical Physics. Their calculations revealed that effective dark energy could maintain a negative density in the early universe before shifting to a positive value.
The model predicts that this density reached zero at approximately redshift 1.5, corresponding to roughly 9.5 billion years ago, before remaining positive thereafter. This mathematical behavior emerges naturally from the framework without requiring an additional adjustable parameter, retaining the standard six parameters of the ΛCDM model.
Addressing the Hubble Tension
The alternative evolutionary history builds on the sign-switching ΛCDM model developed by Akarsu and his collaborators. That previous framework sought to resolve the Hubble tension, which is the persistent discrepancy between different methods of measuring the present expansion rate of the universe.
The new research provides a potential underlying physical mechanism for that sign change, deriving it from the structure of the modified gravity theory rather than prescribing it artificially. The model also avoids conflict with Solar System measurements because its departures from general relativity are naturally suppressed in dense environments.
“For several years we have been studying a scenario in which dark energy switches sign, and it has held up remarkably well against data such as DESI’s,” noted Prof. Özgür Akarsu.
“The open question was what physical mechanism could produce such behavior. Thus, our team purposefully examined a specific branch of a well-known gravitational theory that had previously been dismissed because it breaks the conventional assumption that dark energy density remains positive. The sign change emerged naturally from the theory itself,” Akarsu said.
Observational Testing Against DESI Data
The theoretical findings align with recent empirical observations as astronomers map millions of galaxies and quasars. In March 2025, the DESI collaboration released findings from its first three years of operations, which strengthened indications that dark energy might not operate as a fixed cosmological constant.
DESI has now completed its planned five-year survey, recording data from more than 47 million galaxies and quasars. The ITU-led team submitted its initial research in October 2024, several months prior to the March 2025 DESI announcement, and has not yet tested its mathematical model against the complete DESI dataset.
Comprehensive testing via structure growth, redshift-space distortions, and weak gravitational lensing will determine whether the alternative model holds up against empirical data. Neither the proposed dark energy reversal nor a recent cosmic slowdown has been established as an observational fact.
“We identified an alternative path for the Universe within an existing theory. The next step is to test it rigorously against the full observational data. Initial signs look promising, and we remain particularly curious about potential implications for the Hubble tension,” explained Akarsu.
