CMB polarization as a tool to constrain the optical properties of the Universe

TL;DR

This paper introduces a new formalism to describe how the polarization of cosmic microwave background radiation can be affected by novel physics, allowing for improved constraints on such effects through polarization measurements.

Contribution

The paper develops a formalism for generalized Faraday effect in cosmology, linking polarization changes to anisotropic or chiral media, and provides new observational bounds on circular polarization.

Findings

Set an upper bound on CMB circular polarization power spectrum at $C_{\ell}^{VV} < 2 imes 10^{-5} \mu K^2$

Reinterpreted known cosmic birefringence results within the new formalism

Projected that future polarization surveys will improve constraints on GFE effects

Abstract

We present a novel formalism to describe the $in$ $vacuo$ conversion between polarization states of propagating radiation, also known as generalized Faraday effect (GFE), in a cosmological context. Thinking of GFE as a potential tracer of new, isotropy- and/or parity-violating physics, we apply our formalism to the cosmic microwave background (CMB) polarized anisotropy power spectra, providing a simple framework to easily compute their observed modifications. In so doing, we re-interpret previously known results, namely the $in$ $vacuo$ rotation of the linear polarization plane of CMB photons (or cosmic birefringence) but also point out that GFE could lead to the partial conversion of linear into circular polarization. We notice that GFE can be seen as an effect of light propagating in an anisotropic and/or chiral medium (a "dark crystal") and recast its parameters as the components of an effective "cosmic susceptibility tensor". For a wave number-independent susceptibility tensor, this allows us to set an observational bound on a GFE-induced CMB circularly polarized power spectrum, or $VV$, at $C_{\ell}^{VV} < 2 \times 10^{-5} \mu K^2$ (95 \% C.L.), at its peak $\ell\simeq 370$, which is some 3 orders of magnitude better than presently available direct $VV$ measurements. We argue that, unless dramatic technological improvements will arise in direct $V$-modes measurements, cosmic variance-limited linear polarization surveys expected within this decade should provide, as a byproduct, superior bounds on GFE-induced circular polarization of the CMB.