Axions and Cosmic Magnetic Fields

TL;DR

This paper proposes a mechanism where axion dark matter induces exponential growth of cosmic magnetic fields through a modified magnetohydrodynamics process, potentially explaining observed astrophysical magnetic structures.

Contribution

It introduces a novel approach using a modified Ohm's Law to demonstrate how axions can generate large-scale magnetic fields in the universe.

Findings

Exponential magnetic field growth driven by axion coupling

Magnetic fields reach 10-100 kpc scales after inverse cascade

Modified Ohm's Law enables field amplification despite negligible resistivity

Abstract

We argue that if axions are the dark matter, their coupling to electromagnetism results in exponential growth of a helical magnetic field when the axion field first rolls down its potential. After an inverse cascade, the relevant length scales to day are of order 10-100 kpc, of astrophysical interest. Our mechanism for allowing the field to grow relies on a nuance of MHD. Faraday's Law says that an electric field is needed to create a magnetic field. Previous authors relied on conventional Ohm's law to calculate E, but the resistivity is negligible and therefore they assume E is as well. We use a modified Ohm's Law that includes the effects of self-induction in limiting the current driven by a given E, which allows a magnetic field to grow.