Towards thermodynamics of universal horizons in Einstein-{\ae}ther theory

TL;DR

This paper investigates the thermodynamic properties of universal horizons in Einstein-{\

Contribution

It demonstrates that universal horizons in Einstein-{\

Findings

Universal horizons obey a first law of black hole mechanics.

Universal horizons radiate as blackbodies at fixed temperature.

Holographic theories may extend beyond current assumptions.

Abstract

Holography grew out of black hole thermodynamics, which relies on the causal structure and general covariance of general relativity. In Einstein-{\ae}ther theory, a generally covariant theory with a dynamical timelike unit vector, every solution breaks local Lorentz invariance, thereby grossly modifying the causal structure of gravity. However, there are still absolute causal boundaries, called "universal horizons", which are not Killing horizons yet obey a first law of black hole mechanics and must have an entropy if they do not violate a generalized second law. We couple a scalar field to the timelike vector and show via the tunneling approach that the universal horizon radiates as a blackbody at a fixed temperature, even if the scalar field equations also violate local Lorentz invariance. This suggests that the class of holographic theories may be much broader than currently assumed.