Conceptual issues in quantum cosmology

Abstract

I give a review of the conceptual issues that arise in theories of quantum cosmology. I start by emphasising some features of ordinary quantum theory that also play a crucial role in understanding quantum cosmology. I then give motivations why spacetime cannot be treated classically at the most fundamental level. Two important issues in quantum cosmology -- the problem of time and the role of boundary conditions -- are discussed at some length. Finally, I discuss how classical spacetime can be recovered as an approximate notion. This involves the application of a semiclassical approximation and the process of decoherence. The latter is applied to both global degrees of freedom and primordial fluctuations in an inflationary Universe.