Researchers at Istanbul Technical University have uncovered an alternative gravity model suggesting dark energy’s density was negative early in cosmic history before turning positive. Published in Physical Review D, the theoretical framework offers a fresh explanation for observations hinting that dark energy evolves over time.
For many years, the simplest view of dark energy has been that it remains unchanged. Scientists have long assumed that the elusive force responsible for pushing the universe apart has acted identically across all eras of time. However, an increasing number of observations are challenging this idea. Measurements gathered by the Dark Energy Spectroscopic Instrument (DESI) in its quest to build the cosmos’s most extensive three-dimensional atlas imply that the influence of dark energy may actually change as time progresses.
Even though this falls short of official proof, it has amplified a central debate in cosmology: What if dark energy isn’t constant? That uncertainty inspired theoretical work led by scientists at Istanbul Technical University.
Exploring Unexplored Regions in Teleparallel Gravity
In a 2025 paper published in Physical Review D under the DOI 10.1103/1xd4-k91h, an international team led by professor Özgür Akarsu from the Department of Physics at Istanbul Technical University investigated an alternative theory of gravity called teleparallel f(T) gravity. Specifically, the researchers explored a region of the theory that previous studies had chosen to set aside.
The investigation revealed that the framework naturally generated an effective form of dark energy with a negative density during the first few billion years of the universe, which later shifted to a positive density. The work was carried out in Akarsu’s research group at Istanbul Technical University with postdoctoral researcher Dr. N. Merve Uzun and doctoral student Bilal Bulduk, in collaboration with professor Nihan Katırcı of Doğuş University and professor Antonio De Felice of the Yukawa Institute for Theoretical Physics at Kyoto University. This unusual behavior did not rely on simply introducing a free parameter. Instead, the specific branch that emerges is determined by the current matter density of the universe.
Testing Against Observational Data and the Hubble Tension
When these theoretical findings are paired with observations of the cosmic microwave background, supernovae, and weak lensing, they reinforce earlier clues that dark energy varies over time rather than behaving as a simple cosmological constant. Although the strength of this evidence fluctuates depending on the data sets used and has not reached the threshold for a definitive discovery, the potential implications are pushing researchers to investigate multiple possibilities.
The study led by Istanbul Technical University was initially submitted in October 2024, months ahead of DESI’s March 2025 results. Testing the alternative model against that full data set remains the team’s next objective.
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. The preliminary picture is encouraging, and we are particularly interested in what it may tell us about the Hubble tension.
Özgür Akarsu, professor in the Department of Physics at Istanbul Technical University
Beyond providing an unusual mathematical solution, the study outlines an alternative cosmic history that yields testable hypotheses regarding galaxy growth, light bending, and cosmic expansion over time.
