Universal properties of the near-horizon geometry

We derive universal properties of the near-horizon geometry of spherically symmetric black holes. These properties follow solely from the existence of an apparent horizon and its regularity. Only two types of solutions are possible, and both appear at different stages of the black hole formation. If semiclassical gravity is valid, then accretion after horizon formation inevitably leads to a firewall that violates the quantum energy inequalities. Consequently, physical black holes can only evaporate once a horizon has formed. We describe how these results extend to modified theories of gravity, including Einstein-Cartan theories. Comparison of the required energy and time scales with the known semiclassical results suggests that the observed astrophysical black holes are horizonless ultra-compact objects, and the presence of a horizon is associated with currently unknown physics.