Alternative gravity theory offers possible explanation for dark energy changing over time

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. N. 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.

An old map of gravity reveals an overlooked route

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. Akarsu and colleagues deliberately relaxed that assumption, explored the broader landscape and examined the branch that had been left aside.

The differences were significant. The same amount of matter today could lead to two very different cosmic evolutions. One path aligns closely with the standard cosmological model, while the overlooked branch presents a different behavior of dark energy: Its density was negative in the past, crossed zero at around redshift 1.5, roughly 9.5 billion years ago, and has been positive ever since.

Importantly, this unusual behavior wasn’t created simply by adding a free parameter. The value that determines which branch appears depends on the current matter density of the universe. Consequently, this model has the same number of cosmological parameters as the standard ΛCDM model with six parameters.

Despite its unusual behavior on cosmic scales, the paper shows that this branch can remain consistent with the precise tests of gravity carried out in the solar system, because in dense environments its deviations from Einstein’s theory are naturally suppressed.

The result did not come out of nowhere. In recent years, Akarsu and collaborators have developed a scenario known as the sign-switching ΛCDM (ΛsCDM) model, in which the cosmological constant switches sign from negative to positive a few billion years after the big bang.

Analyses of that scenario have shown a sign-changing dark energy density to be consistent with DESI data and capable of easing several long-standing tensions in cosmology, including the disagreement over the present-day expansion rate known as the Hubble tension.

What has been missing is a physical mechanism that produces such behavior naturally, and the search for one is active across the field. The new analysis provides one concrete example: Here the sign change is not inserted by hand but emerges from the structure of the theory itself.

Akarsu described this result as a reminder that even established theories can hide unexplored possibilities.

“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.”

Then the observations became more interesting

The timing of that theoretical exploration could hardly have been better. DESI measures the positions of millions of galaxies and quasars, enabling scientists to trace the universe’s expansion over billions of years. In March 2025, the DESI collaboration announced results from its first three years of data. When combined with observations of the cosmic microwave background, supernovae and weak lensing, these findings strengthened earlier hints that dark energy might change over time instead of acting as a simple cosmological constant.

The strength of this evidence varies with the data sets used and has not yet reached the level for a definitive discovery. Nonetheless, the potential significance drives scientists to explore various explanations. Having completed its planned five-year survey, DESI has observed more than 47 million galaxies and quasars. Its full data set will offer an even more rigorous test of whether dark energy’s apparent evolution continues.

This progress allows theories once considered unconventional to be tested against precise observational data. The ITU-led study was first submitted in October 2024, months before DESI’s March 2025 results; the model has not yet been tested against the full DESI data set, and doing so is the team’s next step.

An overlooked possibility, and a new question for the universe

This study’s significance goes beyond uncovering an unusual mathematical solution. By exploring a largely overlooked aspect of a well-known gravity theory, the researchers proposed an alternative cosmic history, turning that idea into a hypothesis that can be tested through observations.

The model forecasts not only the universe’s expansion but also possible changes in gravity at cosmic scales. Data on galaxy growth, light bending and the universe’s expansion over time can help determine if this alternative history is plausible. Key tests include structure growth, redshift-space distortions and weak gravitational lensing.

That rigorous test against the full observational data still lies ahead, but the preliminary analysis in the paper is encouraging. In its extended form, which includes a cosmological constant, the model remains consistent with the cosmic microwave background while accommodating the higher values of the present-day expansion rate measured locally, which makes it a promising candidate for addressing the Hubble tension.

The same extension also allows cosmic acceleration to slow down, or even come to a halt, in the very recent past, a possibility that recent analyses of DESI data have raised.

Overall, the research emphasizes the value of revisiting established theories. Relaxing common assumptions, such as insisting that effective dark energy density stay positive, can reveal new possibilities within familiar frameworks. The authors suggest other modified gravity models might also benefit from this reevaluation.

“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 Akarsu.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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