ScienceNot yet confirmed elsewhere1 publisher2 min readPublished
Overlooked branch of f(T) gravity makes dark energy negative until about 9.5 billion years ago
Istanbul Technical University physicists found a branch of f(T) gravity where effective dark energy was negative until about 9.5 billion years ago. It offers a candidate mechanism for the sign-switching dark energy that earlier analyses found consistent with DESI data.
The Scientist · Science desk
Key figures: on the overlooked f(T) branch, dark energy density crossed zero at around redshift 1.5; the same amount of matter today could lead to two cosmic evolutions; the model has six cosmological parameters, the same number as standard Lambda-CDM.
| Figure | Value | Note | Claim |
|---|---|---|---|
| Redshift where dark energy density crossed zero, around | 1.5 redshift | 7 | |
| Paths, same matter today | 2 evolutions | 6 | |
| Parameters, like Lambda-CDM | 6 parameters | 8 |
What happened
- The paper appeared in Physical Review D in 2025, led by Özgür Akarsu of ITU, and deliberately explored a region of the theory that earlier studies had set aside.
- The theory is teleparallel f(T) gravity, which builds on torsion where general relativity uses spacetime curvature.
- From the same amount of matter today, the equations allow two cosmic evolutions, and one of them aligns closely with the standard cosmological model.
- The value that selects the branch depends on the current matter density, so the model keeps the six cosmological parameters of standard Lambda-CDM.
- The paper shows the branch can stay consistent with precise solar system tests because its deviations from Einstein's theory are suppressed in dense environments.
Why it matters
- constraint The DESI link is indirect. Consistency with DESI was shown for the earlier sign-switching model, so the f(T) branch is a candidate mechanism until someone fits it to the data.
- capability With the same six parameters as Lambda-CDM, a direct comparison between the two would not depend on the new model having more freedom to tune, which is a clean test in our view.
- decision Anyone fitting the exponential f(T) model to cosmological data now has two branches to test for the same present-day matter density, where earlier studies followed one.
The branch stayed unexplored because of an assumption. Earlier studies of the exponential version of the theory followed a single mathematical path, guided by the expectation, inherited from general relativity, that dark energy density must always be positive [5]. Akarsu and colleagues relaxed that restriction and examined the branch that had been left aside [5].
The observational side belongs to DESI, which is building the largest three-dimensional map of the universe. Its data suggest that dark energy's effect might have evolved [1]. According to the phys.org account, that is not conclusive enough to declare a discovery [2].
The match with DESI data belongs to an earlier scenario from the same group, the sign-switching Lambda-CDM model, in which the cosmological constant flips sign, going from negative to positive, a few billion years after the big bang [11]. Later work on that scenario found the sign-changing density consistent with DESI data and able to ease several long-standing tensions, including the Hubble tension [12]. What had been missing was a physical mechanism that would make the switch happen naturally, and many groups in the field are now looking for one [13].
The f(T) branch has the same pattern, negative early and positive later [10]. The thing this doesn't tell you is whether its expansion history is close enough to the sign-switching model's, in detail, to inherit that consistency with DESI.
What to watch
- A direct fit of the f(T) sign-switching branch to DESI data, scored against Lambda-CDM at equal parameter count.
- Whether DESI's evolving dark energy hints firm up enough to be called a discovery, since the whole line of work rests on them.
- Whether the f(T) branch eases the Hubble tension the way analyses of the earlier sign-switching model did.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence40
- Adoption
- Insufficient
- Hype gap+10
- Incentives35
- Confidence50
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
DESI, the Dark Energy Spectroscopic Instrument, is creating the largest three-dimensional map of the universe, and its data suggest that dark energy's effect might have evolved over time.
- [2]
The DESI hints are not conclusive enough to declare a discovery.
- [3]
In a 2025 paper in Physical Review D, an international team led by professor Özgür Akarsu of Istanbul Technical University's Department of Physics examined an alternative theory of gravity, deliberately exploring a region of the theory that earlier studies had set aside.
- [4]
The theory is teleparallel f(T) gravity, which uses torsion as its foundation, unlike Einstein's general relativity, which describes gravity through spacetime curvature.
- [5]
In the exponential version of the theory, past cosmological studies followed a single mathematical path, guided by the expectation inherited from general relativity that dark energy density must always be positive; Akarsu and colleagues relaxed that assumption and examined the branch that had been left aside.
- [6]
The same amount of matter today could lead to two very different cosmic evolutions, and one path aligns closely with the standard cosmological model.
- [7]
On the overlooked branch, dark energy 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.
- [8]
The value that determines which branch appears depends on the current matter density of the universe, so the model has the same number of cosmological parameters as the standard six-parameter Lambda-CDM model.
- [9]
The paper shows this branch can remain consistent with precise solar system tests of gravity because, in dense environments, its deviations from Einstein's theory are naturally suppressed.
- [10]
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.
- [11]
Akarsu and collaborators developed the sign-switching Lambda-CDM scenario, in which the cosmological constant switches sign from negative to positive a few billion years after the big bang.
- [12]
Analyses of the sign-switching scenario have shown a sign-changing dark energy density to be consistent with DESI data and capable of easing several long-standing cosmology tensions, including the Hubble tension.
- [13]
What has been missing is a physical mechanism that produces sign-switching behavior naturally, and the search for one is active across the field.
Sources
1 independent publisher whose own reporting we read for this story.
- phys.orgAlternative gravity theory offers possible explanation for dark energy changing over time
1 article · October 12, 2026
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