Cosmological Magnetic Fields from Ultralight Dark Matter

TL;DR

This paper introduces a mechanism where oscillating ultralight dark matter induces parametric resonance in electromagnetic fields, leading to the generation of cosmological magnetic fields after recombination, potentially explaining observed magnetic phenomena.

Contribution

It presents a novel post-recombination process involving axion-like dark matter inducing magnetic field generation through parametric resonance.

Findings

Magnetic fields exceeding observational bounds can be produced after recombination.

The mechanism operates on scales of about 1 Mpc.

The process relies on an oscillating pseudo-scalar dark matter field coupled to electromagnetism.

Abstract

We propose a mechanism for the generation of magnetic fields on cosmological scales that is operative after recombination. An essential ingredient is an instability (of parametric resonance type) of the electromagnetic field driven by an oscillating pseudo-scalar dark matter field, $\phi$, that is coupled to the electromagnetic field tensor via a $\phi F \wedge F$ term in the Lagrangian of axion-electrodynamics. We find that magnetic fields larger than the observational lower bounds can be generated soon after recombination on scales of $1 {\rm{Mpc}}$.