Primordial Magnetic Fields May Ease Discrepancy in Cosmic Expansion Measurements
Simulations suggest that weak magnetic fields generated shortly after the Big Bang could alter early hydrogen formation, modestly shifting the cosmic microwave background and reducing the tension between different estimates of the Hubble constant.
New three-dimensional simulations of the early universe indicate that extremely weak magnetic fields, possibly created within seconds of the Big Bang, can influence the recombination era when electrons and protons formed neutral hydrogen. By accelerating recombination, these fields would change the size of temperature fluctuations observed in the cosmic microwave background, which serves as a “cosmic ruler” for inferring the Hubble constant.
When the Hubble constant is derived from the cosmic microwave background, data from the Planck satellite yield a value near 67 km s⁻¹ Mpc⁻¹. Direct measurements using Type Ia supernovae and Cepheid variables give a higher value around 73 km s⁻¹ Mpc⁻¹, a discrepancy known as the Hubble tension. The researchers found that incorporating primordial magnetic fields of about five to ten pico-Gauss today into the recombination model produces predictions that remain consistent with observed microwave background data, while slightly lowering the inferred expansion rate.
Across several data sets, the analysis shows a mild statistical preference—between 1.5 and 3 sigma—for the presence of such fields, though the result falls short of a definitive detection. If confirmed, these fields could also serve as the seeds for the magnetic structures observed in galaxies and clusters.
“Our results show that the proposal survives the most detailed test available today and provides clear targets for future observations,” the authors said, noting that upcoming measurements will be able to test whether these primordial fields indeed help resolve the Hubble tension.