Cosmological bounds on sub-MeV mass axions

TL;DR

This paper extends cosmological bounds on sub-MeV axions, showing they are excluded up to 300 keV due to their impact on early universe processes like BBN and CMB, complementing other constraints.

Contribution

It provides the first comprehensive cosmological exclusion range for axions from 0.7 eV to 300 keV, based on multiple cosmological observations.

Findings

Axion decays alter the baryon-to-photon ratio at BBN.

Decays cause neutrino dilution relative to photons.

Spectral CMB distortions constrain axion lifetime.

Abstract

Axions with mass greater than 0.7 eV are excluded by cosmological precision data because they provide too much hot dark matter. While for masses above 20 eV the axion lifetime drops below the age of the universe, we show that the cosmological exclusion range can be extended from 0.7 eV till 300 keV, primarily by the cosmic deuterium abundance: axion decays would strongly modify the baryon-to-photon ratio at BBN relative to the one at CMB decoupling. Additional arguments include neutrino dilution relative to photons by axion decays and spectral CMB distortions. Our new cosmological constraints complement stellar-evolution limits and laboratory bounds.