Boundary conditions in quantum string cosmology

We discuss in detail how to consistently impose boundary conditions in quantum string cosmology. Since a classical time parameter is absent in quantum gravity, such conditions must be imposed with respect to intrinsic variables. Constructing wave packets for minisuperspace models from different tree-level string effective actions, we explain in particular the meaning of a transition between “pre-big-bang” and “post-big-bang” branches. This leads to a scenario different from previous considerations.