Dark energy may have started out negative — a flipped sign that could help explain the universe's stubborn expansion puzzle

Dark energy is the name for whatever is pushing the universe to expand faster and faster, and the standard story treats it as constant forever. A team led by Professor Özgür Akarsu at Istanbul Technical University explored a branch of an existing gravity theory that everyone else had set aside — one where dark energy's density starts out negative. In that version, the density crossed zero about 9.5 billion years ago and has been positive ever since, with no extra adjustable ingredients needed. Early checks suggest this history could ease the Hubble tension, a long-running disagreement between two ways of measuring how fast the universe expands today, while still passing precise tests of gravity in our own Solar System. It is a possibility, not a proof — the team says the next step is a rigorous test against the full dataset from DESI, a survey that has mapped more than 47 million galaxies and quasars.