Cosmic Birefringence by Dark Photon

TL;DR

This paper explores how kinetic mixing between CMB photons and birefringent dark photons, possibly coupled to axion fields, can induce anisotropic birefringence, circular polarization, and spectral distortions in the CMB.

Contribution

It introduces a model where dark photon kinetic mixing causes anisotropic birefringence and polarization effects in the CMB, highlighting the role of parity violation in the dark sector.

Findings

Birefringence can be anisotropic over the sky.

Kinetic mixing influences CMB circular polarization.

Spectral distortions are linked to dark photon interactions.

Abstract

We study the kinetic mixing between the cosmic microwave background (CMB) photon and the birefringent dark photon. These birefringent dark photon may exist in parity-violating dark sector, for example, through the coupling to axion field. We show that the birefringence of the dark photon propagates to the CMB photon, but the resulting birefringence may not be isotropic over the sky, but will be anisotropic in general. Moreover, our investigation sheds light on the essential role played by kinetic mixing in the generation of two fundamental characteristics of the CMB: circular polarization and spectral distortion.