Constraints on String Cosmology
  String theory contains sources like orientifold planes that support higher
derivative interactions. These interactions make possible static flux
compactifications which are forbidden in supergravity. They can also lead to
violations of the strong energy condition (SEC) which is needed for an
accelerating universe. We examine how large a violation is possible in the
context of the heterotic string compactified to four dimensions. We find that
de Sitter solutions are still not possible but that classically forbidden
anti-de Sitter solutions are possible.
