Gravity-induced birefringence within the framework of Poincare gauge   theory
  Gauge theories of gravity provide an elegant and promising extension of general relativity. In this paper we show that the Poincar\'e gauge theory exhibits gravity-induced birefringence under the assumption of a specific gauge invariant nonminimal coupling between torsion and Maxwell's field. Furthermore we give for the first time an explicit expression for the induced phaseshift between two orthogonal polarization modes within the Poincar\'e framework. Since such a phaseshift can lead to a depolarization of light emitted from an extended source this effect is, in principle, observable. We use white dwarf polarimetric data to constrain the essential coupling constant responsible for this effect. 
Oliver Preuss, Sami K. Solanki, M. P. Haugan, Stefan Jordan