Perturbative instabilities in Horava gravity

TL;DR

This paper analyzes scalar and tensor perturbations in Horava gravity, revealing ghost-like instabilities and other issues that challenge the theory's consistency and viability across different energy scales.

Contribution

It provides a comprehensive perturbation analysis of Horava gravity, highlighting fundamental instabilities and difficulties in achieving a consistent UV-IR flow.

Findings

Scalar perturbations exhibit ghost-like instabilities in renormalizable parameter ranges.

Tensor sector shows potential superluminal propagation and fine-tuning issues.

Challenges in connecting UV behavior to IR limits in Horava gravity.

Abstract

We investigate the scalar and tensor perturbations in Horava gravity, with and without detailed balance, around a flat background. Once both types of perturbations are taken into account, it is revealed that the theory is plagued by ghost-like scalar instabilities in the range of parameters which would render it power-counting renormalizable, that cannot be overcome by simple tricks such as analytic continuation. Implementing a consistent flow between the UV and IR limits seems thus more challenging than initially presumed, regardless of whether the theory approaches General Relativity at low energies or not. Even in the phenomenologically viable parameter space, the tensor sector leads to additional potential problems, such as fine-tunings and super-luminal propagation.