Axions as Dark Matter, Dark Energy, and Dark Radiation

TL;DR

This work surveys axions and axion-like particles as a unified framework addressing dark matter, dark energy, and dark radiation. It emphasizes non-thermal production via the misalignment mechanism and topological-defect decays for DM, ultralight axions as quintessence for dynamical dark energy, and both thermal/non-thermal production channels contributing to $\Delta N_{\rm eff}$ for dark radiation. A combination of lattice QCD inputs, defect-network simulations, and extensive experimental programs (haloscopes, LSW, helioscopes, and dish antennas) underpins the phenomenology and detection prospects, while the axiverse provides a natural panorama of many ultralight states influencing cosmology. Current bounds from Planck, large-scale structure, and astrophysical observations constrain the parameter space, with next-generation CMB, galaxy surveys, and laboratory experiments set to probe the remaining windows and potentially reveal the interwoven roles of DM, DE, and DR in a single beyond-Standard-Model framework.

Abstract

Axions and axion-like particles are ubiquitous in extensions of the Standard Model and offer a unifying framework on open problems in cosmology. Depending on their mass and interactions, axions can act as dark matter, drive cosmic acceleration as dark energy, or contribute to the relativistic background as dark radiation. Motivated by the plenary talk at TAUP 2025, this proceeding reviews the phenomenology of light bosons in the early and late Universe, with a focus on the theoretical foundations, observational signatures, and experimental prospects.