Time Orientability and Particle Production from Universal Horizons

TL;DR

This paper investigates particle production at universal horizons in Einstein--Aether and Horava gravity, revealing that horizon foliation orientation reversal is essential for unitarity and leads to Hawking radiation.

Contribution

It demonstrates the necessity of foliation orientation reversal for unitarity and explicitly computes Hawking temperature for universal horizons in various spacetimes.

Findings

Universal horizons emit Hawking radiation.

Foliation orientation reversal is required for unitarity.

Explicit temperature calculations for known solutions.

Abstract

We discuss particle production in spacetimes endowed with a universal horizon in Einstein--Aether and Horava gravity. We argue that continuity and differentiability of the lapse function require the orientation of the foliation in the interior of the horizon to be reversed with respect to the exterior one. Unless this is allowed, interaction of gravitating scalar fields with the universal horizon leads to unitarity violations in the quantum theory. This property is responsible for particle production by the universal horizon, as we show by computing explicitly its Hawking temperature for all stationary and spherically symmetric spacetimes. We particularize our result to known analytic solutions, including those compatible with observational constraints.