  Space-filling S-branes can mediate a transition between a contracting and an
expanding universe in the Einstein frame. Following up on previous work that
uncovered such bouncing solutions in the context of weakly coupled thermal
configurations of a certain class of type II superstrings, we set up here the
formalism in which we can study the evolution of metric fluctuations across
such an S-brane. Our work shows that the specific nature of the S-brane, which
is sourced by non-trivial massless thermal string states and appears when the
universe reaches a maximal critical temperature, allows for a scale invariant
spectrum of curvature fluctuations to manifest at late times via a stringy
realization of the matter bounce scenario. The finite energy density at the
transition from contraction to expansion provides calculational control over
the propagation of the curvature perturbations through the bounce, furnishing a
working proof of concept that such a stringy universe can result in viable late
time cosmology.
