General Relativistic Contribution to Polarization of the Cosmic   Microwave Background Anisotropy
  We solve the wave equation for the electromagnetic field tensors associated with the CMBR photons in a universe with scalar metric perturbations. We show that the coupling of the electromagnetic fields with the curvature associated with the scalar perturbations gives rise to an optical rotation of the microwave background photons. The magnitude of the gravitationally generated V-Stokes parameter anisotropy $\Delta_V$, is however very small compared to the linear polarisation caused by Thomson scattering. 
Subhendra Mohanty and A.R. Prasanna