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 de-lensing and
partial subtraction of the galactic FR. We find that a realistic future
sub-orbital experiment, covering a patch of the sky near the galactic poles,
can detect a scale-invariant PMF of 0.1 nano-Gauss at better than 95%
confidence level, while a dedicated space-based experiment can detect even
smaller fields.
