Birefringence, CMB polarization and magnetized B-mode

TL;DR

This paper explores how pseudo-scalar fields and magnetic fields in the early universe can generate B-mode polarization in the CMB, with implications for distinguishing different birefringent effects.

Contribution

It introduces a detailed analysis of how pseudo-scalar interactions and magnetic fields influence CMB B-mode polarization, providing a method to differentiate their contributions.

Findings

Pseudo-scalar fields can produce frequency-independent B-mode polarization.

Magnetic fields induce frequency-dependent B-mode autocorrelations via Faraday rotation.

Cross-correlations can help disentangle pseudo-scalar and magnetic birefringence effects.

Abstract

Even in the absence of a sizable tensor contribution, a B-mode polarization can be generated because of the competition between a pseudo-scalar background and pre-decoupling magnetic fields. By investigating the dispersion relations of a magnetoactive plasma supplemented by a pseudo-scalar interaction, the total B-mode polarization is shown to depend not only upon the plasma and Larmor frequencies but also on the pseudo-scalar rotation rate. If the (angular) frequency channels of a given experiment are larger than the pseudo-scalar rotation rate, the only possible source of (frequency dependent) B-mode autocorrelations must be attributed to Faraday rotation. In the opposite case the pseudo-scalar contribution dominates and the total rate becomes, in practice, frequency-independent. The B-mode cross-correlations can be used, under certain conditions, to break the degeneracy by disentangling the two birefringent contributions.