CDT---an Entropic Theory of Quantum Gravity

TL;DR

This paper explores a lattice-based approach to quantum gravity called causal dynamical triangulations (CDT), demonstrating how classical spacetime like de Sitter emerges from quantum laws through entropic effects and nonperturbative measures.

Contribution

It introduces a nonperturbative lattice formulation of quantum gravity that explains the emergence of classical spacetime and connects to the asymptotic safety scenario.

Findings

Emergence of de Sitter spacetime as an infrared limit

Entropic effects encode the emergence of classicality

A new scaling relation related to asymptotic safety

Abstract

In these lectures we describe how a theory of quantum gravity may be constructed in terms of a lattice formulation based on so-called causal dynamical triangulations (CDT). We discuss how the continuum limit can be obtained and how to define and measure diffeomorphism-invariant correlators. In four dimensions, which has our main interest, the lattice theory has an infrared limit which can be identified with de Sitter spacetime. We explain why this infrared property of the quantum spacetime is nontrivial and due to "entropic" effects encoded in the nonperturbative path integral measure. This makes the appearance of the de Sitter universe an example of true emergence of classicality from microscopic quantum laws. We also discuss nontrivial aspects of the UV behaviour, and show how to investigate quantum fluctuations around the emergent background geometry. Finally, we consider the connection to the asymptotic safety scenario, and derive from it a new, conjectured scaling relation in CDT quantum gravity.