# Bubble networks : framed discrete geometry for quantum gravity Article - 2019

Laurent Freidel, Etera R. Livine

Laurent Freidel, Etera R. Livine, « Bubble networks : framed discrete geometry for quantum gravity  », Gen.Rel.Grav., 2019, p. 9

Abstract

In the context of canonical quantum gravity in 3 $+$ 1 dimensions, we introduce a new notion of bubble network that represents discrete 3d space geometries. These are natural extensions of twisted geometries, which represent the geometrical data underlying loop quantum geometry and are defined as networks of $\mathrm SU(2)$ holonomies. In addition to the $\mathrm SU(2)$ representations encoding the geometrical flux, the bubble network links carry a compatible $\mathrm SL(2,\mathbb R)$ representation encoding the discretized frame field which composes the flux. In contrast with twisted geometries, this extra data allows to reconstruct the frame compatible with the flux unambiguously. At the classical level this data represents a network of 3d geometrical cells glued together. The $\mathrm SL(2,\mathbb R)$ data contains information about the discretized 2d metrics of the interfaces between 3d cells and $\mathrm SL(2,\mathbb R)$ local transformations are understood as the group of area-preserving diffeomorphisms. We further show that the natural gluing condition with respect to this extended group structure ensures that the intrinsic 2d geometry of a boundary surface is the same from the viewpoint of the two cells sharing it. At the quantum level this gluing corresponds to a maximal entanglement along the network edges. We emphasize that the nature of this extension of twisted geometries is compatible with the general analysis of gauge theories that predicts edge mode degrees of freedom at the interface of subsystems.