Primordial magnetism in CMB B modes

Cosmic microwave background (CMB) polarization B modes induced by Faraday rotation (FR) can provide a distinctive signature of primordial magnetic fields because of their characteristic frequency dependence and because they are only weakly damped on small scales. FR also leads to mode-coupling correlations between the E-and B-type polarizations and between the temperature and the B mode. These additional correlations can further help distinguish magnetic fields from other sources of B modes. We review the FR-induced CMB signatures and present the constraints on primordial magnetism that can be expected from upcoming CMB experiments. Our results suggest that FR of CMB will be a promising probe of primordial magnetic fields.