Effective equations of motion for constrained quantum systems: A study of the Bianchi I loop quantum cosmology

TL;DR

This paper develops a new mathematical framework to derive effective equations of motion for constrained quantum systems with an internal clock, applied to Bianchi I loop quantum cosmology, confirming the occurrence of multiple big bounces and analyzing quantum back-reaction effects.

Contribution

It introduces a novel method for deriving effective equations in constrained quantum systems and applies it to Bianchi I loop quantum cosmology, revealing multiple bounces and quantum back-reaction effects.

Findings

Big bang singularity is replaced by up to three big bounces.

Quantum back-reaction modifies the critical density at the bounce.

Effective equations confirm the bouncing scenario in Bianchi I model.

Abstract

A new mathematical framework is formulated to derive the effective equations of motion for the constrained quantum system which possesses an internal clock. In the realm close to classical behavior, the quantum evolution is approximated by a finite system of coupled but ordinary differential equations adhered to the weakly imposed Hamiltonian constraint. For the simplified version of loop quantum cosmology in the Bianchi I model with a free massless scalar filed, the resulting effective equations of motion affirm the bouncing scenario predicted by the previous studies: The big bang singularity is resolved and replaced by the big bounces, which take place up to three times, once in each diagonal direction, whenever the directional density approaches the critical value in the regime of Planckian density. It is also revealed that back-reaction arises from the quantum corrections and modifies the precise value of the directional density at the bouncing epoch. Additionally, as an example of symmetry reduction, we study isotropy emerging from the anisotropic Bianchi I model in the context of effective equations of motion.