Rotational Perturbations in Neveu-Schwarz-Neveu-Schwarz String Cosmology

Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong(December 8, 2000)First order rotational perturbations of the flat Friedmann-Robertson-Walker (FRW) metric areconsidered in the framework of four dimensional Neveu-Schwarz–Neveu-Schwarz (NS-NS) stringcosmological models coupled with dilaton and axion fields. The decay rate of rotation dependsmainly upon the dilaton field potential U. The equation for rotation imposes strong limitationsupon the functional form of U, restricting the allowed potentials to two: the trivial case U = 0 anda generalized exponential type potential. In these two models the metric rotation function can beobtained in an exact analytic form in both Einstein and string frames. In the potential-free case thedecay of rotational perturbations is governed by an arbitrary function of time while in the presenceof a potential the rotation tends rapidly to zero in both Einstein and string frames.I. INTRODUCTION