Imprints of Axion Superradiance in the CMB

TL;DR

This paper explores how axion clouds around black holes, through superradiance and resonant decay, could produce observable low-energy photon signatures that impact the intergalactic medium and cosmic microwave background.

Contribution

It introduces the phenomenological consequences of axion superradiance-induced photon production and its potential observable effects on the universe's ionization and spectral features.

Findings

Resonant decay of axion clouds can generate large fluxes of low-energy photons.

Produced photons are absorbed over short cosmological distances, creating shockwaves.

Potential observable imprints include spectral distortions and inhomogeneous optical depth features.

Abstract

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.