Imprints of Axion Superradiance in the CMB
  Light axions ($m_a \lesssim 10^{-10}$ eV) can form dense clouds around
rapidly rotating astrophysical black holes via a mechanism known as rotational
superradiance. The coupling between axions and photons induces a parametric
resonance, arising from the stimulated decay of the axion cloud, which can
rapidly convert regions of large axion number densities into an enormous flux
of low-energy photons. In this work we consider the phenomenological
implications of a superradiant axion cloud undergoing resonant decay. We show
that the low energy photons produced from such events will be absorbed over
cosmologically short distances, potentially inducing massive shockwaves that
heat and ionize the IGM over Mpc scales. These shockwaves may leave observable
imprints in the form of anisotropic spectral distortions or inhomogeneous
features in the optical depth.
