Light to Heavy, Brief to Eternal: An Axion for Every Occasion (in the Early Universe)

TL;DR

This paper reviews how the early universe can be used to explore various axion models, covering different lifetimes and roles, and highlights the potential of cosmological data to probe axion properties beyond terrestrial experiments.

Contribution

It categorizes axion scenarios based on lifetime and explores their cosmological implications, emphasizing the early universe's role in axion phenomenology.

Findings

Early universe cosmology can probe axions across a broad mass and lifetime spectrum.

Different axion scenarios leave distinct observable signatures in cosmological data.

Cosmological observations can complement laboratory searches for axions.

Abstract

The early universe grants access to energy scales far beyond those achievable in terrestrial experiments and allows unstable Standard Model particles to play an active dynamical role. In this contribution, we focus on recent studies aimed at quantifying the potential of the early universe to probe the properties and interactions of axions. The discussion is organized around four classes of axion scenarios, ordered from long to short lifetimes: (i) stable or long-lived axions contributing to dark radiation; (ii) stable or long-lived axions produced out-of-equilibrium and constituting dark matter; (iii) metastable axions whose decays inject energy into the primordial plasma and leave observable signatures in the global 21 cm signal; and (iv) very short-lived axions that act only as portals to additional degrees of freedom. Together, these scenarios highlight the interplay between axion phenomenology and early universe cosmology and demonstrate the potential of cosmological data to probe axions over a broad range of masses and lifetimes.