Cosmological constraints on axionic dark radiation from axion-photon conversion in the early Universe

TL;DR

This paper investigates how axion-photon conversion in primordial magnetic fields affects cosmological observations, leading to constraints on axion properties and early Universe magnetic fields.

Contribution

It provides new constraints on axion-photon coupling and primordial magnetic fields based on cosmological effects of axionic dark radiation.

Findings

String theory axions can contribute to dark radiation.

Axion-photon conversion impacts cosmological observables.

Constraints on axion-photon coupling and magnetic fields are derived.

Abstract

Axions seem ubiquitous in string theories and some of them may be produced non-thermally by heavy scalar decays, contributing to dark radiation. We study various cosmological effects of photons produced from the axionic dark radiation through axion-photon conversion in the presence of primordial magnetic fields, and derive tight constraints on the combination of the axion-photon coupling and the primordial magnetic field.