Polymer Quantum Cosmology: Lifting quantization ambiguities using a $SL(2,\mathbb{R})$ conformal symmetry

TL;DR

This paper introduces a new loop quantum cosmology model that preserves an $ ext{SL}(2, ext{R})$ symmetry, effectively constraining quantization ambiguities and resulting in a regular bouncing universe with a duality between short and long distances.

Contribution

It leverages the classical $ ext{SL}(2, ext{R})$ symmetry to fix quantization ambiguities in loop quantum cosmology, leading to a symmetry-preserving regularization and a novel duality in cosmological dynamics.

Findings

The new model maintains the classical $ ext{SL}(2, ext{R})$ symmetry after quantization.

The regularization results in a non-singular bouncing cosmology.

A duality between short and long distance regimes is observed, akin to T-duality.

Abstract

In this letter, we stress that the simplest cosmological model consisting in a massless scalar field minimally coupled to homogeneous and isotropic gravity has an in-built $\SL(2,\mathbb{R})$ symmetry. Protecting this symmetry naturally provides an efficient way to constrain the quantization of this cosmological system whatever the quantization scheme and allows in particular to fix the quantization ambiguities arising in the canonical quantization program. Applying this method to the loop quantization of the FLRW cosmology leads to a new loop quantum cosmology model which preserves the $\SL(2,\mathbb{R})$ symmetry of the classical system. This new polymer regularization consistent with the conformal symmetry can be derived as a non-linear canonical transformation of the classical FLRW phase space, which maps the classical singular dynamics into a regular effective bouncing dynamics. This improved regularization preserves the scaling properties of the volume and Hamiltonian constraint. 3d scale transformations, generated by the dilatation operator, are realized as unitary transformations despite the minimal length scale hardcoded in the theory. Finally, we point out that the resulting cosmological dynamics exhibits an interesting duality between short and long distances, reminiscent of the T-duality in string theory, with the near-singularity regime dual to the semi-classical regime at large volume. The technical details of the construction of this model are presented in a longer companion paper \cite{BenAchour:2019ywl}.