  String theory can (in principle) describe gravity at all curvature scales,
and can be applied to cosmology to look back in time beyond the Planck epoch.
The duality symmetries of string theory suggest a cosmological picture in which
the imprint of a primordial, pre-big bang phase could still be accessible to
present observations. The predictive power of such a scenario relies, however,
on our ability to connect in a smooth way the pre-big bang to the present
cosmological regime. Classical radiation back reaction seems to play a key role
to this purpose, by isotropizing and turning into a final expansion any state
of anisotropic contraction possibly emerging from the pre-big bang at the
string scale.
