JWST Constraints on Primordial Magnetic Fields
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arXiv
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| Main Authors: | , , , |
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| Format: | Preprint |
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2026
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| _version_ | 1866914512571990016 |
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| author | Fairbairn, Malcolm Olea-Romacho, María Olalla Urrutia, Juan Vaskonen, Ville |
| author_facet | Fairbairn, Malcolm Olea-Romacho, María Olalla Urrutia, Juan Vaskonen, Ville |
| contents | 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 $τ$. This establishes early galaxy observables as among the most sensitive probes of PMFs in Gaussian, non-helical scenarios. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2604_24835 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | JWST Constraints on Primordial Magnetic Fields Fairbairn, Malcolm Olea-Romacho, María Olalla Urrutia, Juan Vaskonen, Ville Cosmology and Nongalactic Astrophysics High Energy Physics - Phenomenology 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 $τ$. This establishes early galaxy observables as among the most sensitive probes of PMFs in Gaussian, non-helical scenarios. |
| title | JWST Constraints on Primordial Magnetic Fields |
| topic | Cosmology and Nongalactic Astrophysics High Energy Physics - Phenomenology |
| url | https://arxiv.org/abs/2604.24835 |