Back Reaction and Graceful Exit in String Inflationary Cosmology

Classical string cosmology consists of two branches related to each other by scale-factor duality: a superinflation branch and a Friedmann-Robertson-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-dimensional compactification model it is shown that quantum back reaction retards curvature and string coupling growth and connects the superinflation branch to the FRW branch without encountering a singularity. This may offer an attractive solution to the graceful exit problem in string inflationary cosmology.