  In spite of its great phenomenological success, current models of scalar
field-driven inflation suffer from important unresolved conceptual issues. New
fundamental physics will be required to address these questions. String theory
is a candidate for a unified quantum theory of all four forces of nature. As
will be shown, string theory may lead to a cosmological background quite
different from an inflationary cosmology, and may admit a new stringy mechanism
for the origin of a roughly scale-invariant spectrum of cosmological
fluctuations.
