LOW-ENERGY EFFECTIVE STRING COSMOLOGY

We give the general analytic solutions derived from the low energy str ing effective action for four-dimensional Friedmann-Robertson-Walker m odels with a dilaton and antisymmetric tenser field, considering both long and short wavelength modes of the H field. The presence of a homo geneous H field significantly modifies the evolution of the scale fact or and dilaton. In particular it places a lower bound on the allowed v alue of the dilaton. The scale factor also has a lower bound but our s olutions remain singular as they all contain regions where the spaceti me curvature diverges signalling a breakdown in the validity of the ef fective action. We extend our results to the simplest Bianchi type I m etric in higher dimensions with only two scale factors. We again give the general analytic solutions for long and short wavelength modes for the H field restricted to the three-dimensional space, which produces an anisotropic expansion. In the case of H field radiation (wavelengt hs within the Hubble length) we obtain the usual four-dimensional radi ation dominated FRW model as the unique late time attractor.