Antisymmetric-tensor and electromagnetic effects in an α′-non-perturbative four-dimensional string cosmology

Starting from an exact (in the Regge slope alpha') functional method for a bosonic stringy sigma-model, we investigate four-dimensional (4D) cosmological string solutions in graviton, dilaton and antisymmetric tensor backgrounds, compatible with world-sheet conformal invariance, and valid beyond perturbative expansions in powers of alpha'. The antisymmetric tensor field, playing the role of an axion in the 4D target space time, leads to spatial anisotropies of the emergent Robertson-Walker expanding Universe, and, upon coupling the system to the electromagnetic field, it results in nontrivial optical activity. Some estimates of the corresponding effects are made and their possible relevance to cosmology is briefly discussed. Probably the model can describe early Universe cosmology, where nontrivial dilaton effects, which necessarily characterize our solution, may be expected to be large.