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.
An Overlooked Possibility in Einstein’s Theory of Gravity
The discovery came from examining a part of an existing theory that had received relatively little attention. Rather than proposing an entirely new explanation for dark energy, the researchers revisited a framework known as teleparallel f(T) gravity.
Einstein’s general theory of relativity explains gravity through the curvature of spacetime. Teleparallel gravity takes a different mathematical approach, describing gravity through a property called torsion. Scientists have spent years studying modified versions of this framework to investigate whether gravity itself might account for the Universe’s accelerating expansion.
Earlier investigations of an exponential form of teleparallel gravity had concentrated on one particular set of mathematical solutions. This approach reflected a familiar assumption inherited from general relativity: dark energy should always have a positive density.
Akarsu and his colleagues decided to investigate what would happen if that restriction were removed. By considering a previously neglected branch of the theory, they uncovered a substantially different picture of how the Universe might have evolved.
Dark Energy May Have Switched From Negative to Positive
The team’s calculations showed that two distinct cosmic histories could emerge even when the Universe contains the same amount of matter today.
One mathematical solution closely resembles the standard model of cosmology. The other predicts that effective dark energy had a negative density in the distant past. According to this second possibility, the density reached zero at approximately redshift 1.5, corresponding to roughly 9.5 billion years ago, before becoming positive and remaining that way.
What makes the result particularly interesting is that the researchers did not need to introduce an additional adjustable parameter to produce this behavior. Instead, the determining factor is related to the Universe’s present matter density, which selects the mathematical branch. As a result, the model retains the same six cosmological parameters used in the standard ΛCDM model.
The theory also avoids an important potential problem. Although it predicts behavior that differs from Einstein’s gravity across enormous cosmic distances, its departures from general relativity become naturally suppressed in dense environments. This allows the alternative solution to remain compatible with the highly accurate measurements of gravity made throughout the Solar System.
A Possible Explanation for the Hubble Tension
The new findings build on years of research into whether dark energy could have changed sign during the Universe’s history.
Akarsu and his collaborators previously developed a framework called the sign-switching ΛCDM (ΛsCDM) model. In this scenario, the cosmological constant changes from negative to positive several billion years after the Big Bang.
Previous analyses found that a change in the sign of dark energy’s density could agree with DESI observations while potentially reducing several unresolved disagreements in cosmology. One of the most significant is the Hubble tension, the persistent discrepancy between different methods of measuring the Universe’s present expansion rate.
However, an important question remained unanswered. What underlying physical process could cause dark energy to undergo such a reversal?
The latest research offers a possible mechanism. Instead of requiring scientists to prescribe a sign change as part of the model, the reversal arises naturally from the mathematical structure of the modified gravity theory.
Akarsu explained why investigating an overlooked solution proved so valuable.
“For several years we have been studying a scenario in which dark energy switches sign, and it has held up remarkably well against data such as DESI’s. The open question was what physical mechanism could produce such behavior. So we deliberately looked at a branch of a well-known theory of gravity that had been set aside because it violates the usual expectation that the dark energy density stays positive. The sign change emerged naturally from the theory itself.”
DESI Observations Raise New Questions About Dark Energy
The theoretical findings have become especially relevant as astronomers gather increasingly detailed measurements of cosmic expansion.
DESI tracks the locations of millions of galaxies and quasars, allowing researchers to reconstruct how the Universe has expanded across billions of years. In March 2025, the DESI collaboration released findings based on its first three years of observations.
When combined with measurements of the cosmic microwave background, supernovae, and weak gravitational lensing, the results strengthened earlier indications that dark energy might not behave like a fixed cosmological constant.
These findings do not yet establish that dark energy changes over time. The statistical significance depends on which observational datasets are combined, and the evidence remains below the threshold required to claim a definitive discovery. Nevertheless, the possibility has motivated scientists to investigate alternative descriptions of cosmic expansion.
DESI has now completed its planned five-year survey, observing more than 47 million galaxies and quasars. The complete dataset will provide an even more demanding test of whether the apparent changes in dark energy persist as measurements become more precise.
The timing is also noteworthy. The ITU-led team’s research was initially submitted in October 2024, several months before DESI announced its March 2025 findings. The researchers have not yet tested their model against the complete DESI dataset, making that comparison an important next step.
Could the Universe’s Accelerating Expansion Be Slowing?
The significance of the proposed model extends beyond its prediction that dark energy’s density may have changed sign.
Because the alternative solution emerges from a specific theory of gravity, it makes predictions about both cosmic expansion and how gravity behaves across the Universe. These predictions give astronomers ways to investigate whether the proposed cosmic history is consistent with observations.
Measurements of how galaxies and larger structures form and grow will be particularly important. Researchers can also examine how gravity bends light and how the Universe’s expansion has changed over time. Among the most useful observational tests are structure growth, redshift-space distortions, and weak gravitational lensing.
Although comprehensive testing remains necessary, the initial results offer reasons for further investigation.
In an extended version of the theory that includes a cosmological constant, the researchers found that the model could remain consistent with observations of the cosmic microwave background while allowing the higher present-day expansion rates obtained from local measurements. This makes it a potentially useful approach to understanding the Hubble tension.
The extended model also permits another intriguing possibility: the acceleration of the Universe’s expansion could have weakened or even stopped in the relatively recent cosmic past. Recent studies examining DESI observations have raised similar questions about whether cosmic acceleration has remained steady.
Importantly, neither the proposed dark energy reversal nor a recent slowdown in cosmic acceleration has been established as an observational fact. Both remain possibilities that must be tested against increasingly precise data.
Revisiting Gravity Could Reveal a Different Cosmic History
The research highlights how familiar scientific theories can contain possibilities that were missed because earlier investigations relied on restrictive assumptions.
In this case, allowing effective dark energy to have a negative density exposed an alternative evolutionary history within an already established mathematical framework. The researchers suggest that examining similarly overlooked solutions in other modified gravity theories could lead to additional insights.
Ultimately, the strength of the new explanation will depend on how well it reproduces the Universe astronomers actually observe. The full DESI dataset and other cosmological measurements will help determine whether this unusual version of cosmic history deserves a place alongside established explanations.
“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,” said Prof. Özgür Akarsu.