Spectral dependence of CMB polarization and parity

The polarization of the cosmic microwave background radiation (CMBR) can serve as a probe for nonstandard parity violating interactions. Many such interactions are predicted in particle physics models arising from theories with extra dimensions such as superstring theory. These interactions produce an optical activity that depends on the space-time nature of the parity violating field. In particular, it is possible to obtain a frequency-dependent differential rotation of the polarization axis. The form of the frequency dependence is sensitive to the time evolution of the parity-odd background field. Thus, one may be able to study a broad class of parity violating operators through polarization measurements of the CMBR.