Class. Quantum Grav. 30 (2013) 125006 arXiv:1208.0354 AEI-2012-078 Laplacians on discrete and quantum geometries Gianluca Calcagni , Daniele Oriti and Johannes Thürigen ‡ Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Am Mühlenberg 1, D-14476 Potsdam, Germany E-mail:
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[email protected] Abstract. We extend discrete calculus for arbitrary (p-form) fields on embedded lattices to abstract discrete geometries based on combinatorial complexes. We then provide a general definition of discrete Laplacian using both the primal cellular complex and its combinatorial dual. The precise implementation of geometric volume factors is not unique and, comparing the definition with a circumcentric and a barycentric dual, we argue that the latter is, in general, more appropriate because it induces a Laplacian with more desirable properties. We give the expression of the discrete Laplacian in several different sets of geometric variables, suitable for computations in different quantum gravity formalisms. Furthermore, we investigate the possibility of transforming from position to momentum space for scalar fields, thus setting the stage for the calculation of heat kernel and spectral dimension in discrete quantum geometries. PACS numbers: 02.10.Ox, 02.40.Sf, 04.60.-m, 04.60.Nc, 04.60.Pp arXiv:1208.0354v2 [hep-th] 17 May 2013 Present address: Instituto de Estructura de la Materia — CSIC, calle Serrano 121, E-28006 Madrid, ‡ Spain. Laplacians on discrete and quantum geometries 2 1. Introduction In a variety of current approaches to quantum gravity, including loop quantum gravity (LQG) [1, 2] and spin-foam models [3, 4, 5], group field theory [6, 7], simplicial quantum gravity, be it quantum Regge calculus [8] or (causal) dynamical triangulations [9], the basic building blocks of geometry and spacetime are discrete in nature.