Are magnetic fields in cosmic voids primordial?

TL;DR

This paper proposes a classical electrodynamics mechanism where galactic dipole fields could generate magnetic fields in cosmic voids, potentially eliminating the need for primordial fields to explain observations of TeV blazars.

Contribution

It introduces a novel, classical physics-based model for void magnetic fields, challenging the necessity of primordial origins.

Findings

Galactic dipole fields can produce void magnetic fields with a white-noise spectrum.

The model explains the absence of GeV halos around TeV blazars.

White-noise spectral shape can test the presence of these fields in voids.

Abstract

The nature of magnetic fields in the voids of the large-scale structure of the Universe has been a multifaceted open puzzle for decades. On one hand, their origin is not clear with most of the magnetogenesis models using physics beyond the standard model in the early Universe, and on the other hand, their existence and potential role in explaining the spectra of TeV blazars have been intensely debated in the past decade. Here, we propose a mechanism, within classical electrodynamics, that could fill the voids with late-Universe fields and, under certain conditions, dispel the need for primordial fields altogether to explain the void fields. Specifically, we use the dipole component of the galactic fields to generate space-filling magnetic fields in voids with white-noise spectrum and sufficient amplitude to explain the lack of GeV halos around TeV blazars observed by Fermi-LAT. A definitive test for such fields in the voids will be the white-noise spectral shape, which will constrain possible plasma processes in the voids to the ones that allow for the propagation of these dipole fields into the voids.