Axions and Cosmic Magnetic Fields
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.
Cite
@article{arxiv.2307.05425,
title = {Axions and Cosmic Magnetic Fields},
author = {George B. Field and Sean M. Carroll},
journal= {arXiv preprint arXiv:2307.05425},
year = {2023}
}