# Pseudo-Cartesian coordinates in a model of Causal Dynamical Triangulations

## TL;DR

This paper introduces a method to define pseudo-Cartesian coordinates in Causal Dynamical Triangulations with three-torus topology, enabling a semi-classical description of spatial geometry in quantum gravity simulations.

## Contribution

It proposes a novel approach to define spatial coordinates and observables in background-independent quantum gravity models with toroidal topology.

## Key findings

- Defined new observables related to winding numbers
- Enabled semi-classical spatial descriptions in simulations
- Extended effective models to include genuine spatial coordinates

## Abstract

Causal Dynamical Triangulations is a non-perturbative quantum gravity model, defined with a lattice cut-off. The model can be viewed as defined with a proper time but with no reference to any three-dimensional spatial background geometry. It has four phases, depending on the parameters (the coupling constants) of the model. The particularly interesting behavior is observed in the so-called de Sitter phase, where the spatial three-volume distribution as a function of proper time has a semi-classical behavior which can be obtained from an effective mini-superspace action. In the case of the three-sphere spatial topology, it has been difficult to extend the effective semi-classical description in terms of proper time and spatial three-volume to include genuine spatial coordinates, partially because of the background independence inherent in the model. However, if the spatial topology is that of a three-torus, it is possible to define a number of new observables that might serve as spatial coordinates as well as new observables related to the winding numbers of the three-dimensional torus. The present paper outlines how to define the observables, and how they can be used in numerical simulations of the model.