From Explicit Diffeomorphism Breaking to Spontaneous Unimodularization

TL;DR

This paper explores modifications to General Relativity, focusing on Unimodular Gravity, demonstrating its potential for particle acceleration, well-posed initial value formulation, and a spontaneous mimicry model that prevents a cosmological constant from emerging.

Contribution

It shows that Unimodular Gravity can be consistent and physically relevant, offering new insights into energy nonconservation and cosmological constant issues.

Findings

UG can lead to particle acceleration.

UG admits a well-posed initial value formulation.

A spontaneous mimicry model prevents a cosmological constant.

Abstract

This contribution investigates modifications of General Relativity that allow or mimic energy nonconservation. We focus on Unimodular Gravity (UG), a theory that explicitly breaks diffeomorphism invariance, and show that it can lead to particle acceleration, opening the door to experimental constraints. We also argue that UG admits a well-posed initial value formulation, despite the presence of nondynamical structures. Finally, we introduce a simple model that spontaneously mimics UG, but in which a cosmological constant cannot arise as an integration constant.