Surface gravities for non-Killing horizons

TL;DR

This paper reviews various definitions of surface gravity for non-Killing horizons, emphasizing their differences in modified gravity and analogue spacetimes, where degeneracies present in standard cases are lifted.

Contribution

It provides a concise overview of the challenges and distinctions in defining surface gravity for non-Killing horizons in alternative gravity theories and analogue models.

Findings

Different characterizations of surface gravity are non-degenerate for non-Killing horizons.

Standard definitions degenerate for stationary horizons in general relativity.

The paper highlights issues in defining surface gravity in modified gravity and analogue spacetimes.

Abstract

There are many logically and computationally distinct characterizations of the surface gravity of a horizon, just as there are many logically rather distinct notions of horizon. Fortunately, in standard general relativity, for stationary horizons, most of these characterizations are degenerate. However, in modified gravity, or in analogue spacetimes, horizons may be non-Killing or even non-null, and hence these degeneracies can be lifted. We present a brief overview of the key issues, specifically focusing on horizons in analogue spacetimes and universal horizons in modified gravity.