Gravity-induced birefringence within the framework of Poincaré gauge theory

Gauge theories of gravity provide an elegant and promising extension of general relativity. In this paper we show that the Poincaré 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 phase shift between two orthogonal polarization modes within the Poincaré framework. Since such a phase shift 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.