Title: Energy Injection into CMB from Axion-Gauge Field Dynamics

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Abstract: Measurements of the cosmic microwave background (CMB) radiation in the last half century have immensely advanced our knowledge and understanding of the history of our universe. The precision of the CMB observations has enabled us to show the blackbody nature of the spectrum of this remnant radiation with a great accuracy. However, any form of energy injected into CMB at redshifts between 10^3 and 10^6 could cause deviation from the perfect blackbody spectrum. Such a signature serves as a powerful probe of physical processes in the universe during that period before the last scattering of photons. In this context, we investigate the mechanism of production of massless spin-1 particles through their coupling to axion-like fields. The (possibly abundant) presence of axions is generically expected in UV-complete theory, and they interact with gauge fields in a unique way due to their symmetries. We consider the consequence of this interaction and show that coherent oscillation of axions leads to copious production of gauge quanta in the early universe. The produced energy can disturb the blackbody nature of the CMB spectrum before recombination, resulting in spectral distortions. We evaluate the signals of this effect, in view of forthcoming missions such as PIXIE that are expected to improve their spectral sensitivity by as much as 3 orders of magnitude.