Quantum geometry with a nondegenerate vacuum: a toy model

TL;DR

This paper develops a polymer quantization of a scalar field in a loop quantum gravity-inspired framework with a nondegenerate vacuum, resulting in a quantum spacetime with discrete and continuous geometric features.

Contribution

It introduces a new kinematical representation with smooth embedding geometries and constructs a nonseparable physical state space in this context.

Findings

Quantum spacetime composed of discrete strips

Length operator spectrum combines continuous and discrete contributions

Conformal symmetry is broken to Poincare translations

Abstract

Motivated by a recent proposal (by Koslowski-Sahlmann) of a kinematical representation in Loop Quantum Gravity (LQG) with a nondegenerate vacuum metric, we construct a polymer quantization of the parametrised massless scalar field theory on a Minkowskian cylinder. The dif- feomorphism covariant kinematics is based on states which carry a continuous label corresponding to smooth embedding geometries, in addition to the discrete embedding and matter labels. The physical state space, obtained through group averaging procedure, is nonseparable. A physical state in this theory can be interpreted as a quantum spacetime, which is composed of discrete strips and supercedes the classical continuum. We find that the conformal group is broken in the quantum theory, and consists of all Poincare translations. These features are remarkably different compared to the case without a smooth embedding. Finally, we analyse the length operator whose spectrum is shown to be a sum of contributions from the continuous and discrete embedding geometries, being in perfect analogy with the spectra of geometrical operators in LQG with a nondegenerate vacuum geometry.