Inflationary Axion Cosmology Beyond Our Horizon

TL;DR

This paper explores how axion dark matter models with large decay constants can be constrained by cosmic microwave background temperature variations caused by initial axion field perturbations, linking inflationary history to observable phenomena.

Contribution

It introduces a novel method to bound the total amount of inflation using temperature variations in the CMB induced by axion strings at extremely large distances.

Findings

Axion strings with decay constant f=10^17 GeV are potentially observable at 6 x 10^16 light-years.

Temperature variations in the CMB are highly sensitive to initial axion field perturbations.

The study establishes a new lower bound on the total inflation based on axion string observability.

Abstract

In theories of axion dark matter with large axion decay constant, temperature variations in the CMB are extremely sensitive to perturbations in the initial axion field, allowing one to place a lower bound on the total amount of inflation. The most stringent bound comes from axion strings, which for axion decay constant f=10^17 GeV would currently be observable at a distance of 6 x 10^16 light-years, nearly ten million times as far away as our horizon.