  Various effects produced by vector perturbations (vortical peculiar velocity
fields) of a flat Friedmann-Robertson-Walker background are considered. In the
presence of this type of perturbations, the polarization vector rotates. A
formula giving the rotation angle is obtained and, then, it is used to prove
that this angle depends on both the observation direction and the emission
redshift. Hence, rotations are different for distinct quasars and also for the
Cosmic Microwave Background (CMB) radiation coming along different directions
(from distinct points of the last scattering surface). As a result of these
rotations, some correlations could appear in an initially random field of
quasar polarization orientations. Furthermore, the polarization correlations of
the CMB could undergo alterations. Quasars and CMB maps are both considered in
this paper. In the case of linear vector modes with very large spatial scales,
the maximum rotation angles appear to be of a few degrees for quasars (located
at redshifts z < 2.6) and a few tenths of degree for the CMB. These last
rotations produce contributions to the B-mode of the CMB polarization which are
too small to be observed with PLANCK (in the near future); however, these
contributions are large enough to be observed with the next generation of
satellites, which are being designed to detect the small B-mode produced by
primordial gravitational waves.
