Rotation of the CMB polarisation by foreground lensing

Abstract

We investigate the weak lensing corrections to the CMB polarization anisotropies. We concentrate on the effect of rotation and show that the rotation of polarisation is a true physical effect which has to be taken into account at second order in perturbation theory. We clarify inconsistencies on the treatment of this rotation in the recent literature. We also show that at first order in perturbation theory there is no rotation of polarisation also for vector and tensor modes.