BACK-REACTION AND GRACEFUL EXIT IN STRING INFLATIONARY COSMOLOGY

Classical string cosmology consists of two branches related to each ot her by scale-factor duality: a superinflation branch and a Friedmann-R obertson-Walker (FRW) branch Curvature and string coupling singularity separates the two branches, hence posing ''graceful exit problem'' to superinflationary string cosmology. In an exactly soluble two-dimensi onal compactification model it is shown that quantum back reaction ret ards curvature and string coupling growth and connects the superinflat ion branch to the FRW branch without encountering a singularity. This may offer an attractive solution to the graceful exit problem in strin g inflationary cosmology.