Physicists from Istanbul Technical University have proposed an alternative scenario for the evolution of the Universe in which dark energy may have had a negative density during the first billions of years and later changed sign and began accelerating cosmic expansion. The model is based on an already existing alternative theory of gravity and, according to the authors, may help resolve some contradictions in modern cosmology. However, for now this is a theoretical hypothesis that still needs to be tested against astronomical observations.

Another branch of gravitational theory

Researchers led by Professor Özgür Akarsu turned to teleparallel f(T) gravity, an alternative to Einstein’s general theory of relativity. In the standard picture, gravity is described through the curvature of spacetime. In the teleparallel approach, another geometric characteristic plays the central role: torsion.

Within this model, the scientists revisited a mathematical solution that had usually remained outside consideration. Previously, researchers had mainly studied the branch that assumes a positive dark energy density. Akarsu’s team decided to test what would happen if that restriction were removed.

As a result, the authors obtained a scenario in which the density of dark energy could have been negative in the early stages of cosmic history and then naturally passed through zero to positive values. According to the researchers’ calculations, the turning point may have occurred about 9.5 billion years ago. In this model, the change in dark energy’s behavior follows from the mathematical structure of the theory and does not require introducing additional parameters.

Why this is linked to new observations

Interest in such models has grown amid results from DESI, the Dark Energy Spectroscopic Instrument. It creates a three-dimensional map of the distribution of galaxies and quasars, making it possible to study the history of the Universe’s expansion.

Some analyses of DESI data allow for the possibility that dark energy may change over time rather than behave like an unchanging cosmological constant. But this is not yet a final conclusion: the interpretation of the results depends on the models used and on the combination of different sets of observations.

The new work proposes one possible theoretical scenario that can be compared with cosmological data. In particular, the authors expect that the model will help investigate the Hubble tension — the discrepancy between values of the Universe’s expansion rate obtained by different methods.

What still needs to be tested

According to Akarsu, the proposed scenario opens an alternative way to describe the evolution of the Universe within an already existing theory. At the same time, the authors claim that the branch of the model under consideration is compatible with tests of gravity within the Solar System.

The next step is to check how well the calculations agree with the full body of cosmological observations, including DESI data. If the model withstands such a test, it could become a useful tool for studying the history of the Universe’s expansion and the nature of dark energy.

For now, however, it is too early to say that physicists have proven a sign change in dark energy or solved the Hubble tension. The work presents a theoretical possibility, not a confirmed reconstruction of the cosmos’s past. Even if the scenario is confirmed, it would not necessarily require revising the age of the Universe: the consequences for cosmological parameters have yet to be determined.