For decades, scientists have generally assumed that dark energy, the mysterious component thought to be responsible for the Universe's accelerating expansion, has remained constant throughout cosmic history. That long-standing assumption is now facing new scrutiny. Observations from the Dark Energy Spectroscopic Instrument (DESI), which is constructing the largest three-dimensional map of the Universe, suggest that dark energy's influence may have changed over billions of years. Although the evidence is not yet strong enough to establish a discovery, it has renewed interest in a fundamental question: Could dark energy evolve rather than remain constant?
Researchers at Istanbul Technical University (ITU) have explored that possibility by taking a fresh look at an established alternative theory of gravity. In a 2025 study published in Physical Review D, an international team led by Prof. Özgür Akarsu of ITU's Department of Physics investigated a mathematical possibility that previous researchers had largely overlooked. Their analysis revealed a surprising scenario in which the effective density of dark energy was negative during the Universe's early history before eventually becoming positive.
The research was conducted within Akarsu's group at ITU, alongside postdoctoral researcher Dr. N. Merve Uzun and doctoral student Bilal Bulduk. The team also collaborated with Prof. Nihan Katırcı of Doğuş University, who previously worked as a postdoctoral researcher in Akarsu's group, and Prof. Antonio De Felice of Kyoto University's Yukawa Institute for Theoretical Physics, an internationally recognized center for theoretical physics.
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