Polarization of the Cosmic Microwave Background Radiation

The CMB radiation is completely characterized by its temperature anisotropy and polarization in each direction in the sky. Today, studies of the temperature anisotropies in the CMB have provided strong constraints on many of the cosmological parameters. However, for precision cosmology we still need to consider the polarization information of the CMB to break some of the degenerecies that exist between some combinations of parameters. Furthermore, the additional information from CMB polarization is very helpful for studies of cosmological physics on super-horizon scales, where our ability of extracting cosmological information is limited by cosmic variance. Moreover, polarization data can help to distinguish the ingredients which go to make up the temperature power spectrum and therefore the cosmological model. We know that different modes of temperature anisotropies (scalar, vector and tensor) give distinguishable polarization patterns. On the other hand, polarization provides us with a direct probe of the last scattering surface, as opposed to the temperature anisotropies that are affected by various physical effects that occur between the last scattering surface and present. Therefore polarization can help distinguish the sources of anisotropy. However, The CMB polarization is extremely difficult to measure, because it is small and because the polarized foregrounds are poorly known. Nevertheless, there are a number of upcoming experiments designed to make such measurements.