Generation of Circular Polarization of the Cosmic Microwave Background

TL;DR

This paper investigates how a specific extension of the standard model, involving a Chern-Simons coupling, can generate circular polarization in the cosmic microwave background, which is otherwise expected to be zero.

Contribution

It introduces a gauge-invariant model extension coupling photons to an external vector field and computes the resulting polarization transport equations, showing the effect on CMB polarization.

Findings

Circular polarization generated is negligible compared to linear polarization rotation.

The model provides a framework to probe fundamental physics via CMB polarization measurements.

Limits on circular polarization could test photon interactions and Lorentz invariance violations.

Abstract

The standard cosmological model, which includes only Compton scattering photon interactions at energy scales near recombination, results in zero primordial circular polarization of the cosmic microwave background. In this paper we consider a particular renormalizable and gauge-invariant standard model extension coupling photons to an external vector field via a Chern-Simons term, which arises as a radiative correction if gravitational torsion couples to fermions. We compute the transport equations for polarized photons from a Boltzmann-like equation, showing that such a coupling will source circular polarization of the microwave background. For the particular coupling considered here, the circular polarization effect is always negligible compared to the rotation of the linear polarization orientation, also derived using the same formalism. We note the possibility that limits on microwave background circular polarization may probe other photon interactions and related fundamental effects such as violations of Lorentz invariance.