Low energy effective string cosmology

We give the general analytic solutions derived from the low energy string effective action for four-dimensional Friedmann-Robertson-Walker models with a dilaton and antisymmetric tensor field, considering both long and short wavelength modes of the H field. The presence of a homogeneous H field significantly modifies the evolution of the scale factor and dilaton. In particular it places a lower bound on the allowed value of the dilaton. The scale factor also has a lower bound but our solutions remain singular as they all contain regions where the spacetime curvature diverges signalling a breakdown in the validity of the effective action. We extend our results to the simplest Bianchi type I metric 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 (wavelengths within the Hubble length) we obtain the usual four-dimensional radiation-dominated FRW model as the unique late time attractor.