Constraining cosmic polarization rotation and implications for primordial B-modes.

Cosmological Birefringence (CB) is a phenomenon, caused by parity violating modifications to electrodynamics, whereby the linear polarisation angle of light changes as photons traverse a vacuum. It is possible to use a number of different analysis techniques to constrain this effect using Cosmic Microwave Background (CMB) polarisation observations. We investigate two different methods of constraining direction dependent birefringence for present and future CMB experiments including BICEP/Keck, Simons Observatory (SO), and LiteBIRD . Specifically we compare the constraints placed on anisotropic CB from a quadratic estimator technique to those derived from estimates of the $B$-mode power-spectrum for the three different experiments. The constraints derived from estimates of the $B$-mode power spectrum are found to be comparable to those derived from quadratic estimator for BICEP/Keck and SO, but not LiteBIRD due to its larger sky coverage. These forecasted upper bounds for CB are converted to constraints on primordial magnetic fields and the coupling between photons and pseudo Nambu-Goldstone bosons. Finally we show that even with the best constraints on CB, for the respective experiments, the potentially induced $B$-mode power can act as a significant contaminant in the prospective measurement of primordial $B$-modes.