Primordial magnetic fields from pseudo-Goldstone bosons

TL;DR

This work investigates whether coupling electromagnetism to a pseudo-Goldstone boson (PGB) during inflation can generate a primordial magnetic field that seeds galactic dynamos. By formulating the coupled evolution of the EM field and the PGB, the authors estimate magnetic-field growth and find that substantial amplification on ~1 Mpc scales would require super-horizon growth, which the analysis does not support. Growth from the PGB coupling occurs primarily for sub-horizon modes and, once backreaction is considered, the field decays with the usual Maxwell dynamics, yielding only a weak, non-coherent field. Consequently, the proposed PGB-mediated mechanism during inflation appears unlikely to produce a sufficiently strong large-scale primordial magnetic field.

Abstract

The existence of large-scale magnetic fields in galaxies is well established, but there is no accepted mechanism for generating a primordial field which could grow into what is observed today. We discuss a model which attempts to account for the necessary primordial field by invoking a pseudo-Goldstone boson coupled to electromagnetism. The evolution of this boson during inflation generates a magnetic field; however, it seems difficult on rather general grounds to obtain fields of sufficient strength on astrophysically interesting scales.