Universal properties of the near-horizon geometry

TL;DR

This paper investigates the universal near-horizon geometry of spherically symmetric black holes, revealing only two solution types and suggesting that observed black holes may be horizonless ultra-compact objects due to quantum effects.

Contribution

It identifies universal properties of near-horizon geometries and extends the analysis to modified gravity theories, proposing a new perspective on black hole horizons.

Findings

Only two types of near-horizon solutions are admissible.

A special form of the second solution type is consistent.

Astrophysical black holes may be horizonless ultra-compact objects.

Abstract

We derive universal properties of the near-horizon geometry of spherically symmetric black holes that follow from the observability of a regular apparent horizon. Only two types of solutions are admissible. After reviewing their properties we show that only a special form of the solutions of the second type is consistent. We describe how these results extend to modified theories of gravity, including Einstein--Cartan theories. Then we describe the unique black hole formation scenario that necessarily involves both types of solutions. The generalized surface gravity is infinite at the apparent horizon. This feature and comparison of the required energy and timescales with the known semiclassical results suggest that the observed astrophysical black holes are horizonless ultra-compact objects, and the presence of a horizon is associated with currently unknown physics.