Resonant magnetic fields from inflation

TL;DR

This paper introduces a resonant mechanism for generating primordial magnetic fields during inflation, which produces narrow-band fields without strong coupling but still faces backreaction issues for observationally relevant strengths.

Contribution

It presents a novel resonant coupling approach that overcomes spectral and coupling problems in primordial magnetic field generation models.

Findings

Resonant mechanism produces narrow-band magnetic fields.

Backreaction remains problematic for observable field strengths.

No-go argument applies to subhorizon generated fields.

Abstract

We propose a novel scenario to generate primordial magnetic fields during inflation induced by an oscillating coupling of the electromagnetic field to the inflaton. This resonant mechanism has two key advantages over previous proposals. First of all, it generates a narrow band of magnetic fields at any required wavelength, thereby allaying the usual problem of a strongly blue spectrum and its associated backreaction. Secondly, it avoids the need for a strong coupling as the coupling is oscillating rather than growing or decaying exponentially. Despite these major advantages, we find that the backreaction is still far too large during inflation if the generated magnetic fields are required to have a strength of order 10^{-15} Gauss today on observationally interesting scales. We provide a more general no-go argument, proving that this problem will apply to any model in which the magnetic fields are generated on subhorizon scales and freeze after horizon crossing.