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

Understand

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".