Unimodular Loop Quantum Cosmology

TL;DR

This paper explores the quantization of unimodular gravity within loop quantum cosmology, demonstrating a quantum bounce replacing the classical singularity and analyzing the implications of the unimodular clock as an emergent time variable.

Contribution

It introduces a novel approach to quantizing unimodular gravity in loop quantum cosmology using the unimodular clock, reproducing key features of previous models and establishing a connection to spin foam models.

Findings

Quantum bounce replaces classical singularity.

Small expectation value for the cosmological constant under semi-classicality.

Unique vertex expansion in spin foam models due to unimodular time.

Abstract

Unimodular gravity is based on a modification of the usual Einstein-Hilbert action that allows one to recover general relativity with a dynamical cosmological constant. It also has the interesting property of providing, as the momentum conjugate to the cosmological constant, an emergent clock variable. In this paper we investigate the cosmological reduction of unimodular gravity, and its quantization within the framework of flat homogeneous and isotropic loop quantum cosmology. It is shown that the unimodular clock can be used to construct the physical state space, and that the fundamental features of the previous models featuring scalar field clocks are reproduced. In particular, the classical singularity is replaced by a quantum bounce, which takes place in the same condition as obtained previously. We also find that requirement of semi-classicality demands the expectation value of the cosmological constant to be small (in Planck units). The relation to spin foam models is also studied, and we show that the use of the unimodular time variable leads to a unique vertex expansion.