Non-perturbative 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 Weylbeta functions to be homogeneous in time, to all orders in α ′ . This construction is possible in any target space dimension, as a result of the non-trivial 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. Non-trivial optical activity is demonstrated, which constitutes a way of detecting string-inspired axion/dilaton effects in such models. Introduction. The target-space dynamics of strings, in a first-quantized framework, is based on equations of motion, involving backgrounds of the massless degrees of freedom of the string, which are generated by the vanishing of the Weyl-invariance beta functions β of the world sheet non-linear sigma model. The usual approach consists of seeking for perturbative solutions of the vanishing of these beta functions, truncated at most to second [email protected] [email protected] [email protected]