Recent observations from the Dark Energy Spectroscopic Instrument have prompted renewed examination of assumptions about dark energy, the force believed to drive the Universe’s accelerating expansion. For many decades, scientists treated dark energy as a constant throughout cosmic history. New data, however, suggests that dark energy’s properties may have changed significantly over billions of years, challenging this long-standing view.
An international research team led by Prof. Özgür Akarsu of Istanbul Technical University published findings in Physical Review D exploring whether dark energy could evolve rather than remain static. The researchers investigated teleparallel f(T) gravity, an alternative mathematical framework for describing gravity that uses torsion rather than spacetime curvature. By examining a previously overlooked branch of this established theory, the team discovered a scenario in which dark energy’s effective density was negative in the Universe’s early history, reached zero at approximately 9.5 billion years ago, and subsequently became positive.
The model emerges naturally from the mathematical structure of the modified gravity theory without requiring additional adjustable parameters beyond those used in the standard cosmological model. Notably, while the theory predicts different behavior across cosmic distances, its departures from Einstein’s general relativity become suppressed in dense environments such as the Solar System, maintaining compatibility with precise gravitational measurements.
These findings may help address the Hubble tension, a persistent disagreement between different methods of measuring the Universe’s current expansion rate. Earlier research by Akarsu’s group had shown that sign-switching dark energy could align with observational data while potentially resolving several outstanding cosmological discrepancies. The new work provides a potential physical mechanism for how such a reversal could occur naturally rather than requiring it to be imposed externally as an assumption.
While DESI observations have hinted that dark energy may not behave as a fixed constant, the evidence remains below the threshold needed to declare a definitive discovery. The collaboration has now completed its five-year survey of over 47 million galaxies and quasars, providing astronomers with increasingly detailed measurements to test competing theoretical models of cosmic expansion.
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