Fixed-Topology Lorentzian Triangulations: Quantum Regge Calculus in the Lorentzian Domain

TL;DR

This paper introduces a Lorentzian signature model for quantum gravity based on fixed-topology triangulations, highlighting its differences from Euclidean models and demonstrating its analytical and computational advantages in 1+1 dimensions and beyond.

Contribution

The paper formulates a Lorentzian quantum gravity model analogous to Quantum Regge Calculus, emphasizing the removal of triangle inequalities and exploring its properties in various dimensions.

Findings

Derived scaling relations for 1+1D pure gravity

Demonstrated easier generation of large universes

Established a transfer matrix and loop amplitudes

Abstract

A key insight used in developing the theory of Causal Dynamical Triangulations (CDTs) is to use the causal (or light-cone) structure of Lorentzian manifolds to restrict the class of geometries appearing in the Quantum Gravity (QG) path integral. By exploiting this structure the models developed in CDTs differ from the analogous models developed in the Euclidean domain, models of (Euclidean) Dynamical Triangulations (DT), and the corresponding Lorentzian results are in many ways more "physical". In this paper we use this insight to formulate a Lorentzian signature model that is analogous to the Quantum Regge Calculus (QRC) approach to Euclidean Quantum Gravity. We exploit another crucial fact about the structure of Lorentzian manifolds, namely that certain simplices are not constrained by the triangle inequalities present in Euclidean signature. We show that this model is not related to QRC by a naive Wick rotation; this serves as another demonstration that the sum over Lorentzian geometries is not simply related to the sum over Euclidean geometries. By removing the triangle inequality constraints, there is more freedom to perform analytical calculations, and in addition numerical simulations are more computationally efficient. We first formulate the model in 1+1 dimensions, and derive scaling relations for the pure gravity path integral on the torus using two different measures. It appears relatively easy to generate "large" universes, both in spatial and temporal extent. In addition, loop-to-loop amplitudes are discussed, and a transfer matrix is derived. We then also discuss the model in higher dimensions.