Cosmological axion-photon interactions with a nonminimal dark gauge sector

We study the cosmological evolution of radiation and dark radiation with nonminimal axion-photon-dark photon couplings. The Chern-Simons-type axion-gauge field interactions between the dark and visible sectors can induce energy conversion between photons and dark photons as well as polarization rotation during the propagation of radiation, leading to CMB spectral distortions and cosmic birefringence. Using the observed CMB monopole spectrum, we constrain the photon-dark photon energy conversion fraction, which is determined by the coupling constants and the evolution of the axion field. We further show that the parity-odd mixing interaction ϕFB̃ can partially relax the constraints on the standard axion-photon coupling, ϕFF̃, derived from the current observational limits of the cosmic birefringence angle.