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arXiv · 2604.24835

JWST Constraints on Primordial Magnetic Fields

Malcolm Fairbairn·María Olalla Olea-Romacho·Juan Urrutia·Ville Vaskonen
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Abstract

Primordial magnetic fields (PMFs) enhance small-scale structure formation through the Lorentz force acting on baryons, boosting the abundance of low-mass halos and their hosted galaxies. We show that the reionisation history calibrated with the UV luminosity function (UVLF) provides stringent bounds: strong PMFs induce a characteristic double reionisation at $z \approx 24$ that is incompatible with CMB measurements of the optical depth, yielding $\sqrt{\left\langle B^2 \right\rangle} < 0.27\,{\rm nG}$ and $< 0.18\,{\rm nG}$ for $n_B = -2$ and $n_B = 2$ respectively at $95\%\,{\rm CL}$ using Planck priors on $\tau$. This establishes early galaxy observables as among the most sensitive probes of PMFs in Gaussian, non-helical scenarios.

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Malcolm Fairbairn, María Olalla Olea-Romacho, Juan Urrutia, Ville Vaskonen. 2026-04-27. JWST Constraints on Primordial Magnetic Fields. https://arxiv.org/abs/2604.24835

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