Gravity model allows negative dark energy density in the early universe

An international team led by Özgür Akarsu of Istanbul Technical University proposed a teleparallel f(T) gravity model in which effective dark energy density was negative for the first few billion years and flipped sign about 9.5 billion years ago. The six-parameter model remains consistent with the cosmic microwave background and DESI data and may ease the Hubble tension.

• Dark energy density switches sign at a redshift of about 1.5
• The model has six cosmological parameters, matching standard ΛCDM
• Deviations from general relativity are suppressed in dense environments, passing Solar System tests
• The study was published in Physical Review D in 2025