CMB Faraday rotation as seen through the Milky Way

Faraday rotation (FR) of CMB polarization, as measured through mode-coupling correlations of E and B modes, can be a promising probe of a stochastic primordial magnetic field (PMF). While the existence of a PMF is still hypothetical, there will certainly be a contribution to CMB FR from the magnetic field of the Milky Way. We use existing estimates of the Milky Way rotation measure (RM) to forecast its detectability with upcoming and future CMB experiments. We find that the Galactic RM will not be seen in polarization measurements by Planck, but that it will need to be accounted for by CMB experiments capable of detecting the weak lensing contribution to the B mode. We then discuss prospects for constraining the PMF in the presence of FR due to the Galaxy under various assumptions that include partial delensing and partial subtraction of the Galactic FR. We find that a realistic future suborbital experiment, covering a patch of the sky near the Galactic poles, can detect a scale-invariant PMF of 0.1 nanogauss at better than a 95% confidence level, while a dedicated space-based experiment can detect even smaller fields.