The Cosmic Axion Background

TL;DR

This paper explores the potential to detect a relativistic cosmic axion background produced in the early universe using existing and future axion detection experiments, offering new insights into early universe physics.

Contribution

It demonstrates that current axion experiments can be repurposed to detect relativistic axions, expanding the scope of axion searches beyond dark matter.

Findings

Current experiments can detect axion energy densities below cosmological limits.

Relativistic axion backgrounds have characteristic spectra linked to their production mechanisms.

Repurposing dark matter searches enhances sensitivity to early universe phenomena.

Abstract

Existing searches for cosmic axions relics have relied heavily on the axion being non-relativistic and making up dark matter. However, light axions can be copiously produced in the early Universe and remain relativistic today, thereby constituting a Cosmic $\textit{axion}$ Background (C$a$B). As prototypical examples of axion sources, we consider thermal production, dark-matter decay, parametric resonance, and topological defect decay. Each of these has a characteristic frequency spectrum that can be searched for in axion direct detection experiments. We focus on the axion-photon coupling and study the sensitivity of current and future versions of ADMX, HAYSTAC, DMRadio, and ABRACADABRA to a C$a$B, finding that the data collected in search of dark matter can be repurposed to detect axion energy densities well below limits set by measurements of the energy budget of the Universe. In this way, direct detection of relativistic relics offers a powerful new opportunity to learn about the early Universe and, potentially, discover the axion.