Black holes and their horizons in semiclassical and modified theories of gravity

For distant observers, black holes are trapped spacetime domains bounded by apparent horizons. We review properties of the near-horizon geometry emphasizing consequences two common implicit assumptions semiclassical physics. The first is a consequence cosmic censorship conjecture, namely, that curvature scalars finite at second horizons form in asymptotic time (i.e. according to observers), property implicitly assumed conventional descriptions hole formation and evaporation. Taking these as only requirements within framework, we find spherical symmetry classes dynamic solutions admissible, both describing evaporating expanding white holes. their present implications. null energy condition violated vicinity outer horizon satisfied inner apparent/anti-trapping horizon. Apparent anti-trapping timelike surfaces intermediately singular behavior, which manifests itself negative density firewalls. These other also axially symmetric solutions. Different generalizations surface gravity spacetimes discordant do not match results. conclude discussing signatures models implications for identification observed ultra-compact objects.