The Hubble tension is one of cosmology’s biggest puzzles: measurements of the universe’s expansion rate disagree with calculations based on the early universe. Now physicists say ancient magnetic fields could bridge the gap.

An international team ran the most detailed three-dimensional simulations yet of the primordial plasma at Simon Fraser University’s supercomputer. They varied the level of magnetic force in the hot mix of electrons, neutrons and photons.

They found that stronger magnetism made electrons and neutrons clump together, forming hydrogen earlier. This earlier recombination shrank the so-called sound horizon, changing the calculated Hubble constant to match observations.

The best-matching field strength was 5 to 10 pico-Gauss, about a trillionth of a fridge magnet. The data show a mild preference for such fields, at 1.5 to 3 standard deviations.

Tom Abel of SLAC said the model with a magnetic field that matches observations has the beautiful quality that it also matches the Hubble constant today. The authors stress this is a hint, not a discovery.