The consistent histories approach to loop quantum cosmology

TL;DR

This paper applies the consistent histories approach to loop quantum cosmology, showing how it enables making physical predictions without observers, and demonstrating the resolution of singularities via quantum bounces in cosmological models.

Contribution

It extends the consistent histories formulation to loop quantum cosmology, providing a framework for predictions in closed quantum systems without relying on measurements.

Findings

Loop quantum models predict a bounce from large volume to large volume.

Conventional Wheeler-DeWitt models are invariably singular.

The approach allows for assigning probabilities without observers.

Abstract

We review the application of the consistent (or decoherent) histories formulation of quantum theory to canonical loop quantum cosmology. Conventional quantum theory relies crucially on "measurements" to convert unrealized quantum potentialities into physical outcomes that can be assigned probabilities. In the early universe and other physical contexts in which there are no observers or measuring apparatus (or indeed, in any closed quantum system), what criteria determine which alternative outcomes may be realized and what their probabilities are? In the consistent histories formulation it is the vanishing of interference between the branch wave functions describing alternative histories -- as determined by the system's decoherence functional -- that determines which alternatives may be assigned probabilities. We describe the consistent histories formulation and how it may be applied to canonical loop quantum cosmology, describing in detail the application to homogeneous and isotropic cosmological models with scalar matter. We show how the theory may be used to make definite physical predictions in the absence of "observers". As an application, we demonstrate how the theory predicts that loop quantum models "bounce" from large volume to large volume, while conventional "Wheeler-DeWitt"-quantized universes are invariably singular. We also briefly indicate the relation to other work.