Last scattering, relic gravitons and the circular polarization of the CMB

TL;DR

This paper investigates the origins of circular polarization in the CMB, concluding that relic gravitons and magnetized plasma do not produce V-mode polarization, with large-scale curvature fluctuations being the main source.

Contribution

It demonstrates that tensor modes and magnetized plasma do not generate V-mode polarization, highlighting the role of large-scale curvature fluctuations in the CMB.

Findings

Tensor contribution to V-mode polarization vanishes at last scattering.

Relic gravitons cannot generate V-mode polarization.

Large-scale curvature fluctuations dominate circular dichroism.

Abstract

The tensor contribution to the $V$-mode polarization induced by a magnetized plasma at last scattering vanishes exactly. Conversely a polarized background of relic gravitons cannot generate a $V$-mode polarization. The reported results suggest that, in the magnetized $\Lambda$CDM paradigm, the dominant source of circular dichroism stems from the large-scale fluctuations of the spatial curvature.