Models for cosmic birefringence

Parity violation allow us to investigate some processes that took place in the early Universe, and to shed light on new physics beyond the Standard Model. We will analyze one of the effects that generate this violation: the Cosmic Birefringence (CB). This phenomenon is related to the in-vacuo rotation ofthe polarization angle (denoted as χ) of photons coming from very distant sources, and it could be explained by the presence of Dark Matter and Dark Energy fields in the early universe, and by their interaction with CMB photons through a Chern-Simons coupling). Since it is reasonable to think that these fields are affected by some fluctuations, they can generate anisotropies in the CB angle, making it space-dependent. In this work, after an overview about the CB models proposed up to now, we will extend the anisotropic birefringence effect to the case of a Chern-Simons coupling with a function of the Ricci scalar f (R), including the contribution of the δR fluctuations and computing the perturbed rotation angle δχ. We will find that δR, and so δχ, are directly sourced by the scalar metric perturbations Φ and Ψ (which are the Bardeen’s gravitational potential in the Poisson gauge), such that the birefringence angle power spectrum, given by Cχχ , is strictly related to one of the two gravitational potentials.