The Fock Space of Loopy Spin Networks for Quantum Gravity

TL;DR

This paper introduces loopy and tagged spin networks to better model curvature and torsion in quantum gravity, creating a Fock space framework that enables graph-changing dynamics on a fixed background with local degrees of freedom.

Contribution

It develops a new Fock space of loopy spin networks that generalizes standard spin networks, allowing for local excitations and a graph-changing dynamics framework in loop quantum gravity.

Findings

Constructed a Fock space for loopy spin networks.

Implemented holonomy constraints for flat connection projection.

Analyzed non-trivial flat solutions via higher derivatives of delta distributions.

Abstract

In the context of the coarse-graining of loop quantum gravity, we introduce loopy and tagged spin networks, which generalize the standard spin network states to account explicitly for non-trivial curvature and torsion. Both structures relax the closure constraints imposed at the spin network vertices. While tagged spin networks merely carry an extra spin at every vertex encoding the overall closure defect, loopy spin networks allow for an arbitrary number of loops attached to each vertex. These little loops can be interpreted as local excitations of the quantum gravitational field and we discuss the statistics to endow them with. The resulting Fock space of loopy spin networks realizes new truncation of loop quantum gravity, allowing to formulate its graph-changing dynamics on a fixed background graph plus local degrees of freedom attached to the graph nodes. This provides a framework for re-introducing a non-trivial background quantum geometry around which we would study the effective dynamics of perturbations. We study how to implement the dynamics of topological BF theory in this framework. We realize the projection on flat connections through holonomy constraints and we pay special attention to their often overlooked non-trivial flat solutions defined by higher derivatives of the $\delta$-distribution.