Non-local massive gravity

TL;DR

This paper introduces a covariant, non-local massive gravity theory that unifies non-polynomial and super-renormalizable gravity models, ensuring unitarity and providing a mechanism for late-time cosmic acceleration without dark energy.

Contribution

It proposes a novel covariant action for non-local massive gravity that is both unitary and free of vDVZ discontinuity, and explains late-time acceleration through scalar growth.

Findings

Massless limit smoothly recovered without vDVZ discontinuity

Theory remains unitary at tree level around Minkowski space

Late-time cosmic acceleration achieved without dark energy

Abstract

We present a general covariant action for massive gravity merging together a class of "non-polynomial" and super-renormalizable or finite theories of gravity with the non-local theory of gravity recently proposed by Jaccard, Maggiore and Mitsou (Phys. Rev. D 88 (2013) 044033). Our diffeomorphism invariant action gives rise to the equations of motion appearing in non-local massive massive gravity plus quadratic curvature terms. Not only the massive graviton propagator reduces smoothly to the massless one without a vDVZ discontinuity, but also our finite theory of gravity is unitary at tree level around the Minkowski background. We also show that, as long as the graviton mass $m$ is much smaller the today's Hubble parameter $H_0$, a late-time cosmic acceleration can be realized without a dark energy component due to the growth of a scalar degree of freedom. In the presence of the cosmological constant $\Lambda$, the dominance of the non-local mass term leads to a kind of "degravitation" for $\Lambda$ at the late cosmological epoch.