Large-scale polarization of the microwave background and foreground

The DASI discovery of cosmic microwave background (CMB) polarization has opened a new chapter in cosmology. Most of the useful information about inflationary gravitational waves and reionization is on large angular scales where galactic foreground contamination is the worst, so a key challenge is to model, quantify, and remove polarized foregrounds. We use the POLAR experiment, COBE/DMR and radio surveys to provide the strongest limits to date on the TE cross-power spectrum of the CMB on large angular scales and to quantify the polarized synchrotron radiation, which is likely to be the most challenging polarized contaminant for the WMAP satellite. We find that the synchrotron E and B contributions are equal to within 10% from 408–820 MHz with a hint of E domination at higher frequencies. We quantify Faraday rotation and depolarization effects in the two-dimensional $(\mathcal{l},\nu )$ plane and show that they cause the synchrotron polarization percentage to drop both towards lower frequencies and towards lower multipoles.