The upper bound of event horizon formation time in generalized Oppenheimer–Snyder collapse

We prove that, in the framework of the Oppenheimer-Snyder collapse, the Schwarzschild exterior maximizes the event horizon formation time Δ T eh = 19 6 m among all asymptotically flat, static, spherically-symmetric black holes with the same ADM mass m that satisfy the weak energy condition. This bound extends the typical black hole inequalities–such as the Penrose inequality, which constrains spatial geometry–to temporal setting.