Bulk and Boundary Unitary Gravity in 3D: MMG$_2$

TL;DR

This paper introduces MMG$_2$, a 3D massive gravity theory that resolves the bulk-boundary unitarity conflict, supports holography, and features two massive modes, with implications for causality and dual conformal field theories.

Contribution

The paper constructs MMG$_2$, a novel 3D massive gravity theory extending TMG with two helicity modes, maintaining unitarity without requiring the chiral limit.

Findings

MMG$_2$ is free of bulk-boundary unitarity conflict.

The theory admits all TMG solutions with a redefined parameter.

Flat-space causality is preserved in MMG$_2$.

Abstract

We construct a massive spin-2 theory in 2+1 dimensions that is immune to the bulk-boundary unitarity conflict in anti-de Sitter space and hence amenable to holography. The theory is an extension of Topologically Massive Gravity, just like the recently found Minimal Massive Gravity (MMG), but it has two massive helicity modes instead of a single one. The theory admits all the solutions of TMG with a redefined topological parameter. We calculate the Shapiro time-delay and show that flat-space (local) causality is not violated. We show that there is an interesting relation between the theory we present here (which we call MMG$_2$), MMG and the earlier New Massive Gravity (NMG): Namely, field equations of these theories are non-trivially related. We study the bulk excitations and boundary charges of the conformal field theory that could be dual to gravity. We also find the chiral gravity limit for which one of the massive modes becomes massless. The virtue of the model is that one does not have to go to the chiral limit to achieve unitarity in the bulk and on the boundary and the log-terms that appear in the chiral limit and cause instability do not exist in the generic theory.