  Polarized foregrounds are going to be a serious challenge for detecting CMB
cosmological B-modes. Both diffuse Galactic emission and extragalactic sources
contribute significantly to the power spectrum on large angular scales. At low
frequencies, Galactic synchrotron emission will dominate with fractional
polarization $\sim 20-40%$ at high latitudes while radio sources can contribute
significantly even on large ($\sim 1^{\circ}$) angular scales. Nevertheless,
simulations suggest that a detection at the level of $r=0.001$ might be
achievable if the foregrounds are not too complex.
