# No static black hole hairs in gravitational theories of gravity with broken Lorentz invariance

## TL;DR

This paper investigates whether static black hole hairs can exist in gravitational theories with broken Lorentz invariance, especially in the limit where the khronon field's speed becomes infinite, and finds they cannot.

## Contribution

It analytically demonstrates the non-existence of static hairs in these theories even when the sound horizon coincides with the universal horizon, extending previous results.

## Key findings

- Static hairs do not exist even at infinite khronon speed
- The non-existence holds for large but finite khronon speed
- Boundary conditions at the horizon are simplified in the infinite speed limit

## Abstract

In this paper, we revisit the issue of static hairs of black holes in gravitational theories with broken Lorentz invariance in the case that the speed $c_{\phi}$ of the khronon field becomes infinitely large, $c_{\phi} = \infty$, for which the sound horizon of the khronon field coincides with the universal horizon, and the boundary conditions at the sound horizon reduce to those given normally at the universal horizons. As a result, less boundary conditions are present in this extreme case in comparison with the case $c_{\phi} = $ finite. Then, it would be expected that static hairs might exist. However, we show analytically that even in this case static hairs still cannot exist, based on a decoupling limit analysis. We also consider the cases in which $c_{\phi}$ is finite but with $c_{\phi} \gg 1$, and obtain the same conclusion.