Constraints on background torsion from birefringence of CMB polarization
  We show that a non-minimal coupling of electromagnetism with background
torsion can produce birefringence of the electromagnetic waves. This
birefringence gives rise to a B-mode polarization of the CMB. From the bounds
on B-mode from WMAP and BOOMERanG data, one can put limits on the background
torsion at $\xi_{1}T_{1}=(-3.35 \pm 2.65) \times 10^{-22} GeV^{-1}$.
