  If the dark sector of the Universe consists of ultra-light scalars, their
coupling to photon via a Chern-Simons term would induce a rotation of the
polarization plane of the cosmic microwave background (CMB). This rotation
would convert E -mode polarization into B -mode polarization, resulting in new
CMB BB correlation and parity-violating TB and EB cross correlations. We review
the subject giving details about the derivation of the rotational effects and
summarizing the possible signals in current and future CMB B-mode experiments.
