CPT conserving cosmological birefringence
  We demonstrate that the cosmological birefringence can arise from CPT conserving effect, originated from the CPT-even dimension-six Chern-Simons-like term. We show that a sizable rotation polarization angle in the data of the cosmic microwave background radiation polarization can be induced. 
C. Q. Geng, S. H. Ho and J. N. Ng