Is loop quantization in cosmology unique?

TL;DR

This paper critically examines different loop quantizations in flat isotropic cosmology, proposing criteria to identify physically viable models and concluding that only one consistent quantization exists.

Contribution

It introduces three key criteria for viable loop quantum cosmology models and demonstrates that these criteria significantly restrict the possible quantizations, establishing the uniqueness of the consistent choice.

Findings

Most inequivalent loop quantizations fail the proposed physical criteria.

The improved dynamics of LQC is the only quantization satisfying all criteria.

Other models like lattice refinement and loop-modified geometrodynamics are limited in physical viability.

Abstract

We re-examine the process of loop quantization for flat isotropic models in cosmology. In particular, we contrast different inequivalent `loop quantizations' of these simple models through their respective successes and limitations and assess whether they can lead to any viable physical description. We propose three simple requirements which any such admissible quantum model should satisfy: i) independence from any auxiliary structure, such as a fiducial interval/cell introduced to define the phase space when integrating over non-compact manifolds; ii) existence of a well defined classical limit and iii) provide a sensible "Planck scale" where quantum gravitational effects become manifest. We show that even when it may seem that one can have several possible loop quantizations, these physical requirements considerably narrow down the consistent choices. Apart for the so called improved dynamics of LQC, none of the other available inequivalent loop quantizations pass above tests, showing the limitations of lattice refinement models to approximate the homogeneous sector and loop modified quantum geometrodynamics. We conclude that amongst a large class of loop quantizations in isotropic cosmology, there is a unique consistent choice.