Discrete Gravity Models and Loop Quantum Gravity: a Short Review

Symmetry Integrability and Geometry Methods and Applications (Impact Factor: 1.3). 04/2012; DOI: 10.3842/SIGMA.2012.052
Source: arXiv

ABSTRACT We review the relation between Loop Quantum Gravity on a fixed graph and
discrete models of gravity. We compare Regge and twisted geometries, and
discuss discrete actions based on twisted geometries and on the discretization
of the Plebanski action. We discuss the role of discrete geometries in the spin
foam formalism, with particular attention to the definition of the simplicity

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    ABSTRACT: We investigate the generalization of loop gravity's twisted geometries to a q-deformed gauge group. In the standard undeformed case, loop gravity is a formulation of general relativity as a diffeomorphism-invariant SU(2) gauge theory. Its classical states are graphs provided with algebraic data. In particular closure constraints at every node of the graph ensure their interpretation as twisted geometries. Dual to each node, one has a polyhedron embedded in flat space R^3. One then glues them allowing for both curvature and torsion. It was recently conjectured that q-deforming the gauge group SU(2) would allow to account for a non-vanishing cosmological constant Lambda, and in particular that deforming the loop gravity phase space with real parameter q>0 would lead to a generalization of twisted geometries to a hyperbolic curvature. Following this insight, we look for generalization of the closure constraints to the hyperbolic case. In particular, we introduce two new closure constraints for hyperbolic tetrahedra. One is compact and expressed in terms of normal rotations (group elements in SU(2) associated to the triangles) and the second is non-compact and expressed in terms of triangular matrices (group elements in SB(2,C)). We show that these closure constraints both define a unique dual tetrahedron (up to global translations on the three-dimensional one-sheet hyperboloid) and are thus ultimately equivalent.


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