Circular polarization in a spherical basis

TL;DR

This paper revisits the calculation of cosmic microwave background circular polarization, emphasizing second-order effects and employing the total-angular-momentum formalism for simplified and insightful analysis.

Contribution

It introduces a new approach using total-angular-momentum formalism to compute the CMB circular polarization power spectrum, especially for photon-photon scattering.

Findings

Simplified calculation of the circular polarization angular power spectrum.

Enhanced understanding of second-order effects in CMB polarization.

Clarified the role of birefringence sources in polarization generation.

Abstract

Circular polarization of the cosmic microwave background (CMB) arises in the standard cosmological model from Faraday conversion of the linear polarization generated at the surface of last scatter by various sources of birefringence along the line of sight. If the sources of birefringence are generated at linear order in primordial density perturbations the principal axes of the index-of-refraction tensor are determined by gradients of the primordial density field. Since linear polarization at the surface of last scatter is generated at linear order in density perturbations, the circular polarization thus arises at second order in primordial perturbations. Here, we re-visit the calculation of the circular polarization using the total-angular-momentum formalism, which allows for some simplifications in the calculation of the angular power spectrum of the circular polarization---especially for the dominant photon-photon scattering contribution---and also provides some new intuition.