Endeavours in Discrete Lorentzian Geometry; A thesis in five papers

TL;DR

This thesis explores various approaches to discretising spacetime in quantum gravity, including causal dynamical triangulations and causal set theory, revealing connections to other theories and examining their properties.

Contribution

It presents new models and methods in causal dynamical triangulations and causal set theory, including multicritical models, connections to Hořava-Lifshitz gravity, and explicit operators for simulations.

Findings

Established equivalence between 2D causal dynamical triangulations and Hořava-Lifshitz gravity.

Derived explicit formulas for the d'Alembertian operator in causal set theory.

Analyzed conditions for manifoldlikeness in causal sets.

Abstract

To solve the path integral for quantum gravity, one needs to regularise the space-times that are summed over. This regularisation usually is a discretisation, which makes it necessary to give up some paradigms or symmetries of continuum physics. Causal dynamical triangulations regularises the path integral through a simplicial discretisation that introduces a preferred time foliation. The first part of this thesis presents three articles on causal dynamical triangulations. The first article shows how to obtain a multicritical 2d model by coupling the theory to hard dimers. The second explores the connection to Ho\v{r}ava-Lifshitz gravity that is suggested by the time foliation and establishes that in 2d the theories are equivalent. The last article does not directly concern causal dynamical triangulations but Euclidian dynamical triangulations with an additional measure term, which are examined to understand whether they contain an extended phase without the need for a preferred time foliation. Causal set theory uses an explicitly Lorentz invariant discretisation, which introduces non-local effects. The second part of this thesis presents two articles in causal set theory. The first explicitly calculates closed form expressions for the d'Alembertian operator in any dimension, which can be implemented in computer simulations. The second develops a ruler to examine the manifoldlikeness of small regions in a causal set, and can be used to recover locality.