Massles multi gravity: Covariant, stable, two-derivative interacting massless multi-graviton theories

TL;DR

This paper introduces a covariant, stable, two-derivative multi-gravity theory that overcomes previous inconsistencies, ensuring smooth limits to general relativity and maintaining positive energy conditions.

Contribution

It constructs a new class of interacting massless multi-graviton theories that are stable, covariant, and have no higher derivatives, challenging prior no-go results.

Findings

Hamiltonian is bounded from below.

Smooth zero interaction limit to GR.

Maintains dominant energy condition.

Abstract

We present a general model of interacting metric fields with the sum of massless non-interacting spin 2 fields as the linear limit. In the non-interacting limit the model is reduced to a sum of general relativity actions, with the usual Einstein-Hilbert kinetic term for each metric field. Until now the accepted view was that such theories are inconsistent since it has been proven that ghost terms and/or discontinuity in the number of degrees of freedom at the zero interaction limit, are unavoidable. We do not refute these results, but instead prove by construction that they do not necessarily lead to inconsistencies. In particular, our suggested theories are: 1. Energetically stable, i.e. their Hamiltonian is bounded from below and the multi-Minkowski metric configuration is the unique ground state. 2. Continuous in the zero interaction limit so that general relativity solutions are restored smoothly. 3. Include no higher than two derivative Lagrangian terms. In addition, the dominant energy condition is maintained with respect to all metric fields for all field configurations.