Abstract:

String theory purports to be the correct theory of quantum gravity, and as such it is expected to provide a viable quantum cosmology. But stable time-dependent backgrounds with well-defined quantum-mechanical observables remain elusive.

We first address quantum cosmology by discussing holography in G?del universes, with an eye toward de Sitter space. Holography may someday provide a good definition of quantum cosmology in spacetimes without simple asymptotic behavior.

Supersymmetry is the best candidate for understanding stability and naturalness in quantum cosmology. But if it exists, supersymmetry is broken at low energies. We study in technical detail anomaly-mediated supersymmetry breaking, demonstrating its phenomenologically attractive insensitivty to the details of high-energy physics.

The final part of this dissertation presents an alternative to inflationary cosmology which is embedded in heterotic M-theory. This modification of the ekpyrotic scenario offers better calculability than the original, the ekpyrotic phase transition occuring when a membrane tunnels into our visible universe from a computed potential well in the extra dimension.