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

Francesco D'Eramo|arXiv (Cornell University)|Feb 11, 2026

Dark Matter and Cosmic Phenomena0 citations

TL;DR

The paper classifies axion cosmology by lifetime into four regimes—dark radiation, dark matter, 21 cm energy injection, and an unstable portal—and quantifies how early-Universe production probes axion couplings across a wide mass range.

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.

Motivation & Objective

• Motivate studying axions through early-Universe cosmology by leveraging thermal production and decays.
• Classify axion parameter space by lifetime and mass to map cosmological imprints.
• Quantify dark radiation contributions via Delta N_eff using full phase-space Boltzmann analysis.
• Assess freeze-in production and its impact on small-scale structure as dark matter.
• Investigate axion-induced 21 cm signatures from decays and assess portal scenarios with hidden sectors.

Proposed method

• Use a Lagrangian framework with axion couplings to SM fields and derive lifetime scaling tau ~ (f_a/C_X)^2 m_a^-3 for a->XX decays.
• Adopt a Boltzmann equation in momentum space to track full axion phase-space distribution for dark radiation production.
• Apply phase-space methods to compute Delta N_eff and compare with current and projected cosmological bounds (Fig. 2).
• Model freeze-in production via derivative couplings to fermions and map to warm dark matter bounds via the dark matter root-mean-square velocity and the σ_q parameter (Eq. (3) and Fig. 3).
• Analyze 21 cm signatures by comparing exotic energy injection from axion decays to maximal ΛCDM 21 cm amplitudes (Fig. 4).
• Explore axion as a portal in a regime where decays occur before BBN, focusing on interactions that connect SM to hidden sectors (Sec. 6).

Experimental results

Research questions

• RQ1 How do axion couplings to SM fields generate observable dark radiation and what are the resulting ΔN_eff constraints?
• RQ2 Under what conditions can thermally produced or freeze-in axions constitute or affect dark matter?
• RQ3 Can axion decays inject enough energy to modify the global 21 cm signal in a detectable way?
• RQ4 Can axions serve as portals to hidden sectors in the regime of short lifetimes, and what are the phenomenological implications?

Key findings

• Thermally produced axions can contribute to dark radiation with ΔN_eff constraints that are already strong from CMB and are forecast to improve with future data (Fig. 2).
• For axion couplings to SM fermions, a full phase-space analysis yields robust predictions for ΔN_eff and current/projected cosmological bounds on f_a/C_X (Fig. 2).
• Freeze-in axions can be mapped to warm dark matter bounds, yielding approximate mass constraints in the 20–50 keV range depending on production details (Eq. (3) and Fig. 3).
• Axion decays with intermediate lifetimes can leave imprints in the global 21 cm signal, with projected 21 cm sensitivity probing unexplored regions of parameter space (Fig. 4).
• In scenarios where axions decay before BBN, they function as portals to hidden sectors rather than dark matter or dark radiation (Sec. 6).
• The lifetime dependence on mass and coupling provides a unifying picture: short-lived decays before BBN, intermediate lifetimes affecting cosmology, and effectively stable axions as dark matter/dark radiation (Sec. 2).