Nonperturbative time-dependent string backgrounds and axion-induced optical activity

Conformal invariance for bosonic strings in time-dependent backgrounds of graviton, dilaton, and Kalb-Ramond field is obtained by imposing Weyl-beta functions to be homogeneous in time, to all orders in alpha'. This construction is possible in any target-space dimension, as a result of the nontrivial background configurations. The electromagnetic effects of the antisymmetric-tensor field, when coupled to an Abelian (electromagnetic) gauge field, are discussed in the framework of a four-dimensional Minkowski universe, for concreteness. Nontrivial optical activity is demonstrated, which constitutes a way of detecting string-inspired axion/dilaton effects in such models.