---
title: "Dark energy may have turned from negative to positive 9.5 billion years ago, a gravity model finds"
url: https://badiyaa.com/dark-energy-sign-switch-negative-positive/
published: 2026-11-13T21:00:00+00:00
modified: 2026-11-13T21:00:00+00:00
section: Science
publisher: Badiyaa
---

# Dark energy may have turned from negative to positive 9.5 billion years ago, a gravity model finds

Dark energy may have been negative in the early universe and turned positive about 9.5 billion years ago, a teleparallel f(T) gravity model suggests.

Dark energy, the mysterious push behind the universe’s speeding-up expansion, may have started out negative and only crossed over to positive about 9.5 billion years ago, according to a cosmology model revisited by a team that includes researchers at Istanbul Technical University. In one version of the model, cosmic acceleration even switches off again roughly 1 billion years ago.

The work, led by Özgür Akarsu with Bilal Bulduk, Antonio De Felice, Nihan Katırcı and N. Merve Uzun, appeared in *Physical Review D* and is free to read as a [preprint of the f(T) gravity paper](https://arxiv.org/abs/2410.23068). The university has now released a figure that lays out the idea in a single chart.

## A branch nobody had followed

The team works with a family of gravity theories called teleparallel f(T) gravity, which tweak how Einstein’s theory describes gravity on the largest scales. Most earlier work on one popular exponential version of f(T) assumed the effective dark energy density must always be positive. That assumption picks out one branch of solutions, set by a parameter the authors call beta. With beta positive, the universe ends up only modestly different from the standard picture.

Akarsu’s group followed the other branch, with beta negative. There, the model’s dark energy is negative in the early universe, passes smoothly through zero at a redshift of about 1.5, and has been positive since. Redshift 1.5 translates to roughly 9.5 billion years ago on the standard cosmic clock, the figure the university uses in its chart caption. The authors say this branch can still pass tests of gravity in our own neighborhood through a screening effect similar to the chameleon mechanism.

## Why a sign flip matters

In the standard model of cosmology, dark energy is a fixed quantity, Einstein’s cosmological constant. It makes up about 68 percent of everything in the present-day universe, with dark matter at 27 percent and ordinary matter at 5 percent. Its existence was first inferred in 1998 from distant supernovae that looked dimmer than expected, work that earned the 2011 Nobel Prize in Physics.

Here is the scale of the claim in plain numbers. The universe is about 13.8 billion years old, so a crossover 9.5 billion years ago would put the switch at roughly 4.3 billion years after the Big Bang, long before our solar system formed. For most of cosmic history since then, the model’s dark energy has been pushing outward, as observed.

The idea also connects to recent data. In March 2025 the Dark Energy Spectroscopic Instrument team reported hints that dark energy is slowly weakening. When Akarsu’s team added a cosmological constant to the negative-beta branch and tuned it to the locally measured expansion rate, its dark energy peaked and then declined, and acceleration halted about 1 billion years ago. The authors write that this possibility of slowed or stopped acceleration lines up with recent analyses such as those from DESI.

Badiyaa covered the telescope built to probe this question in [our look back at Euclid’s first images of the dark universe](https://badiyaa.com/euclid-telescope-first-images-dark-universe/), and another cosmic puzzle in [the final parsec problem for merging black holes](https://badiyaa.com/supermassive-black-holes-final-parsec-problem-pulsars/).

## Limits

This is a theoretical model fitted to existing data, not a new measurement. The authors note that the sign-changing version on its own boosts the present expansion rate more than expected, which is why they extended it with a cosmological constant. The DESI hints of evolving dark energy range from 2.8 to 4.2 sigma, short of the 5-sigma level physicists usually want before calling a result firm. The team ends its abstract by urging others to revisit existing f(T) and f(Q) gravity models in light of the overlooked branch.

## Further reading

- [Akarsu et al., Unexplored regions in teleparallel f(T) gravity: Sign-changing dark energy density (arXiv)](https://arxiv.org/abs/2410.23068)

- [Wikipedia: Dark energy](https://en.wikipedia.org/wiki/Dark_energy)

*Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team, Public domain via Wikimedia Commons*

*This article was produced with AI assistance and reviewed by our Editorial Desk. [How we work](https://badiyaa.com/editorial-policy/).*