The relation between general relativity and a class of Ho\v{r}ava gravity theories

TL;DR

This paper investigates the relationship between general relativity and a class of Hořava gravity theories, showing that despite deviations in some parameters, their observational predictions for black holes and gravitational phenomena are indistinguishable from general relativity, with new insights into horizon structures.

Contribution

The study demonstrates that certain Hořava gravity theories produce black hole and gravitational wave behaviors identical to general relativity, revealing a universal horizon and confirming strong coupling of additional scalar modes.

Findings

Black-hole quasinormal modes match general relativity

Formation of a universal horizon inside the event horizon

Scalar degree of freedom remains strongly coupled at low energies

Abstract

Violations of Lorentz (and specifically boost) invariance can make gravity renormalizable in the ultraviolet, as initially noted by Ho\v{r}ava, but are increasingly constrained in the infrared. At low energies, Ho\v{r}ava gravity is characterized by three dimensionless couplings, $\alpha$, $\beta$ and $\lambda$, which vanish in the general relativistic limit. Solar system and gravitational wave experiments bound two of these couplings ($\alpha$ and $\beta$) to tiny values, but the third remains relatively unconstrained ($0\leq\lambda\lesssim 0.01-0.1$). Moreover, demanding that (slowly moving) black-hole solutions are regular away from the central singularity requires $\alpha$ and $\beta$ to vanish {\it exactly}. Although a canonical constraint analysis shows that the class of khronometric theories resulting from these constraints ($\alpha=\beta=0$ and $\lambda\neq0$) cannot be equivalent to General Relativity, even in vacuum, previous calculations of the dynamics of the solar system, binary pulsars and gravitational-wave generation show perfect agreement with General Relativity. Here, we analyze spherical collapse and compute black-hole quasinormal modes, and find again that they behave {\it exactly} as in General Relativity, as far as {\it observational} predictions are concerned. Nevertheless, we find that spherical collapse leads to the formation of a regular {\it universal} horizon, i.e. a causal boundary for signals of arbitrary propagation speeds, inside the usual event horizon for matter and tensor gravitons. Our analysis also confirms that the additional scalar degree of freedom present alongside the spin-2 graviton of General Relativity remains strongly coupled at low energies, even on curved backgrounds. These puzzling results suggest that any further bounds on Ho\v{r}ava gravity will probably come from cosmology.