Universal properties of the near-horizon geometry

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.