DESI observations suggest dark energy may not have remained constant throughout cosmic history. A 2025 study published in Physical Review D by researchers at Istanbul Technical University, led by Prof. Özgür Akarsu, investigated an overlooked mathematical solution within teleparallel f(T) gravity theory. The analysis revealed a scenario where dark energy's effective density was negative in the early Universe before switching to positive at approximately redshift 1.5, corresponding to roughly 9.5 billion years ago.
Teleparallel gravity describes gravity through torsion rather than spacetime curvature as in Einstein's general relativity. Previous studies of exponential teleparallel gravity had focused on solutions assuming dark energy density should always be positive. Akarsu's team removed that restriction and examined a previously neglected branch of the theory.
The model predicts two possible cosmic histories with identical present-day matter density. One closely matches the standard cosmology model. The other shows dark energy transitioning from negative to positive density billions of years ago. The sign change arises naturally from the theory's mathematical structure without requiring additional adjustable parameters.
The Dark Energy Spectroscopic Instrument has mapped over 47 million galaxies and quasars across its five-year survey. In March 2025, DESI released findings combining its observations with cosmic microwave background, supernova, and weak gravitational lensing data. These results strengthened earlier indications that dark energy might not behave as a fixed cosmological constant, though the statistical significance remains below the threshold for a definitive discovery.
The proposed model may also address the Hubble tension, the persistent discrepancy between different expansion rate measurements. Akarsu stated: "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. The open question was what physical mechanism could produce such behavior. So we deliberately looked at a branch of a well-known theory of gravity that had been set aside because it violates the usual expectation that the dark energy density stays positive. The sign change emerged naturally from the theory itself."