Dark energy may have been negative in the early universe, new study suggests
Researchers at Istanbul Technical University say an overlooked branch of a gravity theory allows dark energy to have flipped sign about 9.5 billion years ago.
Scientists have long assumed dark energy, the force thought to drive the universe's accelerating expansion, has stayed constant over cosmic history. New observations from the Dark Energy Spectroscopic Instrument, which is building the largest 3D map of the universe, suggest its influence may have changed over billions of years, though the evidence is not yet strong enough to count as a discovery.
A 2025 study in Physical Review D, led by Prof. Özgür Akarsu of Istanbul Technical University with colleagues including Prof. Antonio De Felice of Kyoto University, looked at a branch of teleparallel f(T) gravity that had received little attention. The analysis found that dark energy's effective density could have been negative in the early universe before becoming positive.
The team found that two different cosmic histories could emerge even when the universe contains the same amount of matter today. The work is theoretical and remains an open question about whether dark energy evolves rather than staying constant.
Key points
- Researchers at Istanbul Technical University revisited an overlooked branch of teleparallel gravity.
- The model shows dark energy's density could have been negative about 9.5 billion years ago.
- DESI observations hint at change over time but are not yet conclusive.
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