Polarization of the Microwave Background in Defect Models

We compute the polarization power spectra for global strings, monopoles, textures, and nontopological textures, and compare them to inflationary models. We find that topological defect models predict a significant ( $\sim 1$ μK) contribution to magnetic type polarization on degree angular scales, which is produced by the large vector component of the defect source. We also investigate the effect of decoherence on polarization. It leads to a smoothing of acoustic oscillations both in temperature and polarization power spectra and strongly suppresses the cross correlation between temperature and polarization relative to inflationary models. Both effects should be testable with the next generation of CMB satellites.