Breaking Diffeomorphism Invariance and Tests for the Emergence of Gravity

TL;DR

This paper explores potential small violations of diffeomorphism invariance in gravity, proposing a phenomenological framework that can be tested through pulsar timing, leading to very stringent bounds on such violations.

Contribution

It introduces a method to incorporate and test for small diffeomorphism violations in gravity using the PPN formalism, providing new constraints on emergent gravity theories.

Findings

Pulsar timing tests constrain violations to about 10^{-20} of gravitational strength.

The proposed phenomenology links small symmetry violations to observable effects in gravitational experiments.

Strong bounds challenge the idea of gravity as an emergent phenomenon with broken diffeomorphism invariance.

Abstract

If general relativity is an emergent phenomenon, there may be small violations of diffeomorphism invariance. We propose a phenomenology of perturbatively small violations of general relativity by the inclusion of terms which break general covariance. These can be tested by matching to the Parametrized Post Newtonian (PPN) formalism. The most sensitive tests involve pulsar timing and provide an extremely strong bound, with a dimensionless constraint of order 10^{-20} relative to gravitational strength.