Fourth order spatial derivative gravity

TL;DR

This paper investigates a modified gravity theory with fourth order spatial derivatives, analyzing its propagator, unitarity, and potential implications for renormalizability, revealing new particles and potential issues with extra terms.

Contribution

It introduces a detailed analysis of a fourth order spatial derivative gravity model, highlighting the effects on propagator structure, unitarity, and potential renormalizability.

Findings

Identifies an extra pole corresponding to a nonrelativistic massless spin-two particle.

Proves unitarity at tree-level with only the new pole showing dynamics.

Derives Newton's potential plus a Darwin-like correction from the modified propagator.

Abstract

In this work we study a modified theory of gravity that contains up to fourth order spatial derivatives as a model for the Horava-Lifshitz gravity. The propagator is evaluated and, as a result, it is obtained one extra pole corresponding to a spin two nonrelativistic massless particle, an extra term which jeopardizes renormalizability, besides the unexpected general relativity unmodified propagator. Then, unitarity is proved at the tree-level, where the general relativity pole has shown to have no dynamics, remaining only the two degrees of freedom of the new pole. Next, the nonrelativistic effective potential is determined from a scattering process of two identical massive gravitationally interacting bosons. In this limit, Newton's potential is obtained, together with a Darwin-like term that comes from the extra non-pole term in the propagator. Regarding renormalizability, this extra term may be harmful, by power counting, but it can be eliminated by adjusting the free parameters of the model. This adjustment is in accord with the detailed balance condition suggested in the literature and shows that the way in which extra spatial derivative terms are added is of fundamental importance.