Conceptual issues in quantum cosmology

TL;DR

This paper reviews fundamental conceptual issues in quantum cosmology, including the problem of time, boundary conditions, and how classical spacetime emerges from quantum descriptions through semiclassical approximations and decoherence.

Contribution

It provides a comprehensive discussion of conceptual challenges and the mechanisms for recovering classical spacetime in quantum cosmology.

Findings

Analysis of the problem of time in quantum cosmology

Discussion of boundary conditions in quantum universe models

Explanation of how decoherence leads to classical spacetime emergence

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.