Eur. Phys. J. C (2015) 75: 561
https://doi.org/10.1140/epjc/s10052-015-3767-0

Regular Article - Theoretical Physics

A note on classical and quantum unimodular gravity

School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK

^* e-mail: isaltas@fc.ul.pt

Received: 21 September 2015
Accepted: 29 October 2015
Published online: 27 November 2015

Abstract

We discuss unimodular gravity at a classical level, and in terms of its extension into the UV through an appropriate path integral representation. Classically, unimodular gravity is locally a gauge fixed version of general relativity (GR), and as such it yields identical dynamics and physical predictions. We clarify this and explain why there is no sense in which it can “bring a new perspective” to the cosmological constant problem. The quantum equivalence between unimodular gravity and GR is more of a subtle question, but we present an argument that suggests one can always maintain the equivalence up to arbitrarily high momenta. As a corollary to this, we argue, whenever inequivalence is seen at the quantum level, that just means we have defined two different quantum theories that happen to share a classical limit. We also present a number of alternative formulations for a covariant unimodular action, some of which have not appeared, to our knowledge, in the literature before.