This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For many years, the simplest view of dark energy has been that it remains unchanged. Throughout cosmic history, the mysterious ingredient believed to drive the universe's accelerated expansion is thought to have behaved in the same way at every epoch.
However, an increasing number of observations are challenging this idea. Data from the Dark Energy Spectroscopic Instrument (DESI), which is creating the largest three-dimensional map of the universe, suggest that dark energy's effect might have evolved over time. While not conclusive enough to declare a discovery, this has intensified one of cosmology's key questions: What if dark energy isn't constant?
It is also the question behind recent theoretical research led by scientists at Istanbul Technical University (ITU). In a 2025 paper in Physical Review D, an international team led by professor Özgür Akarsu of ITU's Department of Physics examined an alternative theory of gravity, deliberately exploring a region of the theory that earlier studies had set aside. What they found was striking: The theory naturally produced an effective form of dark energy whose density was negative during the universe's first few billion years before turning positive later on.
The work was carried out in Akarsu's research group at ITU with postdoctoral researcher Dr. Merve Uzun and doctoral student Bilal Bulduk, in collaboration with professor Nihan Katırcı of Doğuş University, a former postdoctoral researcher in the group, and professor Antonio De Felice of the Yukawa Institute for Theoretical Physics at Kyoto University, one of the world's leading centers for theoretical physics. Imagine a map with many roads, but most people have traveled the same one for years.
The other roads are already on the map; they just haven't been explored as much. A similar situation occurred with the theory studied by the ITU-led team. They didn't invent a new form of dark energy but revisited an existing alternative theory of gravity called teleparallel f(T) gravity.
Unlike Einstein's general relativity, which describes gravity through spacetime curvature, teleparallel gravity uses torsion as its foundation. Researchers have experimented with extensions of this theory for years to see whether modified gravity might explain the universe's accelerating expansion. In an exponential version of this theory, past cosmological studies had followed a single mathematical path, guided by the usual expectation, inherited from general relativity, that the density of dark energy must always be positive.
Extract — continue reading at the source.
