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We calculate the post-recombination contribution to the Cosmic MicrowaveBackground (CMB) spectral distortion due to general exotic energy injections,including dark matter (DM) decaying or annihilating to Standard Modelparticles. Upon subtracting the background distortion that would be presenteven without such energy injections, we find residual distortions that arestill potentially large enough to be detectable by future experiments such asPIXIE. The distortions also have a high-energy spectral feature that is aunique signature of the injection of high-energy particles. We present acalculation of the global ionization history in the presence of decaying darkmatter with sub-keV masses, and also show that previous calculations of theglobal ionization history in the presence of energy injection are notsignificantly modified by these additional spectral distortions. Our improvedtreatment of low-energy electrons allows us to extend calculations of the CMBanisotropy constraints for decaying DM down to arbitrarily low masses. We alsorecast these bounds as constraints on the coupling of axion-like particles tophotons.

Exotic energy injection in the early universe II: CMB spectral distortions and constraints on light dark matter