Consistent Histories in Quantum Cosmology

TL;DR

This paper demonstrates that in a quantum cosmological model, decoherence analysis shows the universe inevitably encounters singularities, ruling out the possibility of a quantum bounce and challenging previous naive interpretations.

Contribution

It provides a detailed consistent histories analysis showing that quantum cosmology models are singular, contradicting earlier claims of non-singular quantum bounces.

Findings

Quantum universe is almost certainly singular.

Decoherence confirms the inevitability of singularities.

Quantum bounce is not supported in this model.

Abstract

We illustrate the crucial role played by decoherence (consistency of quantum histories) in extracting consistent quantum probabilities for alternative histories in quantum cosmology. Specifically, within a Wheeler-DeWitt quantization of a flat Friedmann-Robertson-Walker cosmological model sourced with a free massless scalar field, we calculate the probability that the univese is singular in the sense that it assumes zero volume. Classical solutions of this model are a disjoint set of expanding and contracting singular branches. A naive assessment of the behavior of quantum states which are superpositions of expanding and contracting universes may suggest that a "quantum bounce" is possible i.e. that the wave function of the universe may remain peaked on a non-singular classical solution throughout its history. However, a more careful consistent histories analysis shows that for arbitrary states in the physical Hilbert space the probability of this Wheeler-DeWitt quantum universe encountering the big bang/crunch singularity is equal to unity. A quantum Wheeler-DeWitt universe is inevitably singular, and a "quantum bounce" is thus not possible in these models.