Primordial magnetic fields may shift recombination timing and ease Hubble tension by 2-3 km/s/Mpc

{"Researchers modeled the Lorentz force on charged particles during recombination, showing that sub-nanogauss primordial fields create small-scale density fluctuations that speed hydrogen formation by several thousand years. This shifts the sound horizon scale and lowers the inferred expansion rate from CMB data without altering late-time physics. The work builds on 2011 calculations by the same team linking magnetism to CMB anisotropies.","The analysis synthesizes Planck 2018 polarization spectra, DESI baryon acoustic oscillation constraints, and JWST Cepheid-supernova distances. It finds that fields of 0.03–0.1 nG at recombination reduce the tension from 5σ to under 2σ while remaining consistent with blazar void limits. Unlike early dark energy models, this mechanism also predicts a scale-dependent suppression of small-scale CMB power that future Simons Observatory data can test.","A key limitation is the assumption of a uniform field strength across all modes; inhomogeneous fields or dynamo amplification after recombination could erase the signal. Stronger evidence would require a joint analysis of CMB spectral distortions and 21-cm tomography from HERA or SKA to directly constrain field amplitudes at z ≈ 1100.","Next steps include embedding these fields in full Boltzmann codes like CLASS and running forecasts for LiteBIRD and CMB-S4 to determine whether the required field strength survives current non-detection bounds from gamma-ray cascades."}