Unimodular approaches to the cosmological constant problem

TL;DR

Unimodular gravity modifies general relativity by treating the cosmological constant as a global degree of freedom, offering a semi-classical solution to the old cosmological constant problem through global constraints on spacetime volume.

Contribution

This paper reviews unimodular gravity's approach to the cosmological constant problem, emphasizing the importance of initial data and quantum aspects, and explores extensions that preserve its resolution.

Findings

Semi-classical analysis supports quantum-level viability.

Global spacetime volume constraints are crucial.

Relation to vacuum energy sequester is established.

Abstract

We review selected aspects of unimodular gravity and we discuss its viability as a solution of the old cosmological constant problem. In unimodular gravity the cosmological constant is promoted to a global degree of freedom. We highlight the importance of correctly setting up its initial data in order to achieve a resolution of the cosmological constant problem on a semi-classical level. We review recent path integral analysis of quantum aspects of unimodular gravity to note that the semi-classical findings carry over to the quantum level as well. We point out that a resolution of the problem inherently relies on a global constraint on the space-time four-volume. This makes the theory closely related to the vacuum energy sequester, which operates in a similar way. We discuss possible avenues of extending unimodular gravity that preserve the resolution of the cosmological constant problem.