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

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.

💡 Research Summary

This paper surveys how axion‑like particles (ALPs) can be probed through the early Universe, emphasizing that the high temperatures and dense plasma of the first seconds inevitably produce axions via Standard Model (SM) interactions. The authors organize axion phenomenology into four classes ordered by lifetime: (i) stable or very long‑lived axions that contribute to dark radiation, (ii) stable or long‑lived axions that become non‑relativistic and constitute dark matter, (iii) metastable axions whose decays inject energy into the primordial plasma and leave imprints on the global 21 cm signal, and (iv) very short‑lived axions that act as portals to hidden sectors.

Starting from the generic effective Lagrangian
( \mathcal{L}{\rm int}= \frac{a}{8\pi f_a} C_V V{\mu\nu}\tilde V^{\mu\nu}+ \frac{\partial_\mu a}{f_a} C_\psi \bar\psi\gamma^\mu\gamma^5\psi),
the paper shows that the decay width into a pair of photons scales as (\Gamma_{a\to\gamma\gamma}\propto (C_\gamma/f_a)^2 m_a^3). Consequently the lifetime (\tau_a) spans many orders of magnitude depending on the axion mass (m_a) and the effective coupling (f_a/C_X).

Assuming radiation domination up to at least the TeV scale, the authors argue that axions are unavoidably produced through thermal scatterings and decays of SM particles. Production can proceed via (a) freeze‑out, when the axion–SM interaction is strong enough to bring axions into thermal equilibrium before they decouple, or (b) freeze‑in, when the interaction is too weak for equilibrium and axions are slowly populated. Accurate predictions require solving the Boltzmann equation in momentum space, which the authors do using the formalism of Ref.