  We propose a new cosmological scenario which resolves the conventional
initial singularity problem. The space-time geometry has an unconventional
time-like singularity on a lower dimensional hypersurface, with localized
energy density. The natural interpretation of this singularity in string theory
is that of negative tension branes, for example the orientifolds of type II
string theory. Space-time ends at the orientifolds, and it is divided in three
regions: a contracting region with a future cosmological horizon; an
intermediate region which ends at the orientifols; and an expanding region
separated from the intermediate region by a past cosmological horizon. We study
the geometry near the singularity of the proposed cosmological scenario in a
specific string model. Using D-brane probes we confirm the interpretation of
the brane singularity as an orientifold. The boundary conditions on the
orientifolds and the past/future transition amplitudes are well defined.
Assuming the trivial vacuum in the past, we derive a thermal spectrum in the
future.
