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Can galactic magnetic fields diffuse into the voids?

Oindrila Ghosh1,*, Axel Brandenburg2,3,4,5, Chiara Caprini6,7, Andrii Neronov8,9, and Franco Vazza10

  • *Contact author: oindrila.ghosh@fysik.su.se
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Phys. Rev. D 113, 023523 – Published 20 January, 2026

DOI: https://doi.org/10.1103/4114-mgsp

Abstract

Cosmic voids are magnetized at the level of at least 10−17  G on Mpc scales, as implied by blazar observations. We show that an electrically conducting plasma is present in the voids, and that, because of the plasma, diffusion into the voids of galactic fields generated by a mean-field dynamo is far too slow to explain the present-day void magnetization. Indeed, we show that even in the presence of turbulence in the voids, dynamo-generated galactic fields diffuse out to a galactocentric radius of only 200–400 kpc. Therefore, it is challenging to meet the required volume filling-factor of the void magnetic field. We conclude that a primordial origin remains the most natural explanation to the space-filling weak fields in voids.

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Phys. Rev. D 113, 023523– Published 20 January, 2026
  • Received 17 November 2025
  • Accepted 8 December 2025
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