Maxwell's field coupled nonminimally to quadratic torsion: Axion and birefringence

We consider a possible (parity conserving) interaction between the electromagnetic field F and a torsion field Tα of spacetime. For generic elementary torsion, gauge invariant coupling terms of lowest order fall into two classes that are both nonminimal and quadratic in torsion and respect Maxwell's equations. The first class ∼FT2 admits F's without the presence of charges and is excluded. The second class ∼F2T2 modifies the constitutive tensor of spacetime and can also be completely described in the framework of metricfree electrodynamics. We recognize three physical effects of the torsion: (i) An axion field that induces an optical activity into spacetime, (ii) a modification of the light cone structure that yields birefringence, and (iii) a torsion dependence of the velocity of light. We study these effects in the background of a Friedmann universe with torsion