Generation of Circular Polarization of CMB via Polarized Compton Scattering

TL;DR

This paper explores how polarized Compton scattering, influenced by external magnetic fields and new physics, can generate circular polarization in the CMB, which could be detectable if certain conditions are met.

Contribution

It introduces a mechanism for CMB circular polarization generation via polarized Compton scattering affected by magnetic fields and new physics interactions.

Findings

Circular polarization power spectrum is proportional to temperature anisotropy spectrum.

A polarized electron fraction of less than 10^-4 is needed for consistency with observations.

Damping effects in polarized Compton scattering can produce measurable circular polarization.

Abstract

The standard scenario of cosmology predicts a measurable amount for linear polarization of the Cosmic Microwave Background radiation (CMB) via Thomson scattering, while through this scenario, the generation of circular polarization is excluded. On the another hand, the circular polarization of CMB has not been excluded in observational evidence. The generation of CMB photons circular polarization via their Compton scattering with polarized cosmic electrons is considered in this paper. Our motivation for considering polarized Compton scattering comes from the effects of the external magnetic field in large scale, the chiral magnetic instability and new physics interactions of the cosmic electrons. It is shown that damping term of polarized Compton scattering in the presence of scalar perturbation can generate circular polarization in CMB radiation, so that the power spectrum of circular polarization of CMB $C_l^{V(S)}$ is proportional to the power spectrum of temperature anisotropy of CMB $C_l^{I(S)}$ and also $\delta^2$ which is a fraction of polarized electron number density to the total one with net Left- or Right-handed polarizations. We have discussed that at least we need $\delta<10^{-4}$ to find consistency with a reported upper limit of CMB circular polarization.