The Degrees of Freedom of Area Regge Calculus: Dynamics, Non-metricity, and Broken Diffeomorphisms

TL;DR

This paper investigates the classical dynamics of area Regge calculus, revealing that it breaks diffeomorphism symmetry due to non-metric degrees of freedom, which impacts quantum gravity models like spin foams.

Contribution

It characterizes non-metric degrees of freedom in area Regge calculus and analyzes their effects on symmetries and dynamics, advancing understanding of discrete quantum gravity.

Findings

Diffeomorphism symmetry is broken in area Regge calculus for many geometries.

Non-metric degrees of freedom are characterized and linked to shape mismatches.

Area Regge action has fewer invariances than length-based Regge calculus.

Abstract

Discretization of general relativity is a promising route towards quantum gravity. Discrete geometries have a finite number of degrees of freedom and can mimic aspects of quantum geometry. However, selection of the correct discrete freedoms and description of their dynamics has remained a challenging problem. We explore classical area Regge calculus, an alternative to standard Regge calculus where instead of lengths, the areas of a simplicial discretization are fundamental. There are a number of surprises: though the equations of motion impose flatness we show that diffeomorphism symmetry is broken for a large class of area Regge geometries. This is due to degrees of freedom not available in the length calculus. In particular, an area discretization only imposes that the areas of glued simplicial faces agrees; their shapes need not be the same. We enumerate and characterize these non-metric, or `twisted', degrees of freedom and provide tools for understanding their dynamics. The non-metric degrees of freedom also lead to fewer invariances of the area Regge action---in comparison to the length action---under local changes of the triangulation (Pachner moves). This means that invariance properties can be used to classify the dynamics of spin foam models. Our results lay a promising foundation for understanding the dynamics of the non-metric degrees of freedom in loop quantum gravity and spin foams.