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arXiv · 2307.05425

Axions and Cosmic Magnetic Fields

George B. Field·Sean M. Carroll
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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.

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George B. Field, Sean M. Carroll. 2023-07-11. Axions and Cosmic Magnetic Fields. https://arxiv.org/abs/2307.05425

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