Helical Magnetic Fields from Inflation

TL;DR

This paper investigates the generation of helical magnetic fields during inflation through a pseudoscalar coupling, identifying specific conditions under which astrophysically relevant fields can be produced as vacuum excitations.

Contribution

It introduces a model with a power-law coupling function during inflation, showing that only specific parameter ranges produce significant, helical magnetic fields.

Findings

Magnetic fields are maximally helical under certain parameters.

Only when the coupling parameter β equals 1 do interesting fields form.

Astrophysically relevant fields can be generated with specific coupling strengths.

Abstract

We analyze the generation of seed magnetic fields during de Sitter inflation considering a non-invariant conformal term in the electromagnetic Lagrangian of the form $-\frac14 I(\phi) F_{\mu \nu} \widetilde{F}^{\mu \nu}$, where $I(\phi)$ is a pseudoscalar function of a non-trivial background field $\phi$. In particular, we consider a toy model, that could be realized owing to the coupling between the photon and either a (tachyonic) massive pseudoscalar field and a massless pseudoscalar field non-minimally coupled to gravity, where $I$ follows a simple power-law behavior $I(k,\eta) = g/(-k\eta)^{\beta}$ during inflation, while it is negligibly small subsequently. Here, $g$ is a positive dimensionless constant, $k$ the wavenumber, $\eta$ the conformal time, and $\beta$ a real positive number. We find that only when $\beta = 1$ and $0.1 \lesssim g \lesssim 2$ astrophysically interesting fields can be produced as excitation of the vacuum, and that they are maximally helical.