Non-thermal Axion Dark Radiation and Constraints

TL;DR

This paper explores how ultra-relativistic axions, as dark radiation, are produced from the Peccei-Quinn field's initial displacement, constraining axion dark matter models and the universe's thermal history based on cosmic microwave background data.

Contribution

It demonstrates that displaced Peccei-Quinn fields can produce abundant axion dark radiation, imposing new constraints on axion models and reheating temperature.

Findings

Ultra-relativistic axions can be abundantly produced from displaced Peccei-Quinn fields.

Constraints on axion dark matter window with large decay constants.

Upper bounds on reheating temperature affecting early universe thermal history.

Abstract

The Peccei-Quinn mechanism presents a neat solution to the strong CP problem. As a by-product, it provides an ideal dark matter candidate, "the axion", albeit with a tiny mass. Axions therefore can act as dark radiation if excited with large momenta after the end of inflation. Nevertheless, the recent measurement of relativistic degrees of freedom from cosmic microwave background radiation strictly constrains the abundance of such extra relativistic species. We show that ultra-relativistic axions can be abundantly produced if the Peccei-Quinn field was initially displaced from the minimum of the potential. This in lieu places an interesting constraint on the axion dark matter window with large decay constant which is expected to be probed by future experiments. Moreover, an upper bound on the reheating temperature can be placed, which further constrains the thermal history of our Universe.