Theoretical physicists propose dark energy sign flip 9.5 billion years ago

An international team led by Prof. Özgür Akarsu at Istanbul Technical University reexamined teleparallel f(T) gravity, an alternative mathematical description of gravity based on torsion rather than spacetime curvature. The researchers tested an exponential version of the theory without imposing the usual assumption that dark energy must always have positive density. Their calculations, published in Physical Review D, identified a previously overlooked solution in which effective dark energy changes sign during cosmic history.

The model produced two possible cosmic histories from the same present-day matter content. One resembles the standard ΛCDM model, where dark energy is effectively constant. The other predicts negative effective dark energy in the distant past, reaching zero at redshift 1.5, or about 9.5 billion years ago, before becoming positive. The team reported that this behavior required no extra adjustable parameter beyond the six parameters used in the standard model, because the present matter density selects the mathematical branch.

The work sits alongside observational hints from the Dark Energy Spectroscopic Instrument, which is mapping galaxies and suggesting dark energy's influence may have changed over billions of years, though not at discovery strength. Akarsu's team did not propose a new dark energy substance; they found that a modified gravity framework can naturally accommodate a sign flip if researchers relax a convention inherited from general relativity. That makes the result a theory-side response to evolving dark energy questions.

The paper itself is a theoretical possibility, not direct evidence, and it does not establish that dark energy flipped. Still, if future observations support such behavior, a sign-changing model would reshape cosmology by making the universe's expansion history dynamic rather than fixed by a constant term. It would affect how astronomers interpret supernovae, galaxy surveys and expansion data, and could point to gravity itself evolving on large scales.