GRAVITON BACKGROUND AT EARLY TIMES IN STRING COSMOLOGY

We discuss some peculiar properties of the stochastic graviton backgro und predicted by string cosmology. At Planckian times, for the values of the parameters of the model which are more interesting for the dete ction in gravitational wave experiments, the number density of gravito ns is parametrically large compared to Planck density, and is peaked a t small energies. The large parameter is related to the duration of th e string phase and is a characteristic of string cosmology. When the g ravitons reenter the horizon their typical interaction time is paramet rically larger than the expansion time and therefore the shape of the graviton spectrum is not distorted by graviton-graviton rescattering a nd can carry information on the pre-big-bang phase suggested by string cosmology.