CMB and Lyman-α constraints on dark matter decays to photons

Abstract Dark matter energy injection in the early universe modifies both ionization history and temperature of intergalactic medium. In this work, we improve CMB bounds on sub-keV dark extend previous from Lyman-α observations to same mass range, resulting new competitive constraints axion-like particles (ALPs) decaying into two photons. The limits depend underlying reionization history, here accounted self-consistently by our modified version publicly available DarkHistory CLASS codes. Future measurements such as ones CMB-S4 experiment may play a crucial, leading role search for type light candidates.