Time in quantum cosmology

TL;DR

This paper investigates how quantum corrections and internal time choices affect covariance and the definition of proper-time evolution in quantum cosmology, revealing challenges in maintaining reparameterization invariance.

Contribution

It introduces an effective constraints framework that enables consistent proper-time evolution and explores how quantum and factor ordering corrections impact covariance.

Findings

Proper-time evolution can be consistently defined with effective constraints.

Quantum and factor ordering corrections can break covariance.

Internal time choices influence the inclusion of quantum fluctuations.

Abstract

A cosmological model with two global internal times shows that time reparameterization invariance, and therefore covariance, is not guaranteed by deparameterization. In particular, it is impossible to derive proper-time effective equations from a single deparameterized model if quantum corrections from fluctuations and higher moments are included. The framework of effective constraints shows how proper-time evolution can consistently be defined in quantum cosmological systems, such that it is time reparameterization invariant when compared with other choices of coordinate time. At the same time, it allows transformations of moment corrections in different deparameterizations of the same model, indicating partial time reparameterization of internal-time evolution. However, in addition to corrections from moments such as quantum fluctuations, also factor ordering corrections may appear. The latter generically break covariance in internal-time formulations. Fluctuation effects in quantum cosmology are therefore problematic, in particular if derivations are made with a single choice of internal time or a fixed physical Hilbert space.