Bubble networks: framed discrete geometry for quantum gravity

  • Laurent Freidel
  • Etera R. LivineEmail author
Research Article


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.


Loop quantum gravity Twisted geometry Spin network Discrete geometry Discrete gravity 


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© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Perimeter Institute for Theoretical PhysicsWaterlooCanada
  2. 2.Laboratoire de Physique, ENS de Lyon, CNRS, UCBLUniversité de LyonLyonFrance

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