2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/203930The surface gravity of any Killing horizon, in any spacetime dimension, can be interpreted as a local, two-dimensional expansion rate seen by freely falling observers when they cross the horizon. Any two-dimensional congruence of geodesics invariant under the Killing flow can be used to define this expansion, provided that the observers have unit Killing energy.7 pages; v2: 8 pages, derivation of (8) in section 3 streamlined, added new expression for the 2d expansion, minor editingGeneral Relativity and Quantum CosmologyHigh Energy Physics - Theorytext