Bounds on the minimum orbital period of general static spherical black holes: a sharp upper bound and a conditional lower bound

Abstract

We investigate bounds on the minimum orbital period around four-dimensional general static spherical black holes. For all static spherically symmetric black holes with an event horizon, asymptotic flatness and non-negative energy density outside the horizon, we prove $T_{\min}\leq 6\sqrt{3}\,πM$. The bound is sharp and is saturated by the Schwarzschild solution. The lower bound requires an additional condition on the exterior matter: under the tangential pressure condition $p_τ\geq(κ/2)ρ$ with $κ>1$, imposed for all $r\geq r_{g}$, where $r_{g}$ is the largest radius at which $T(r)$ attains its global minimum, we prove $T_{\min}\geq c(κ)M$ with $c(κ)=6\sqrt{3}π(κ-1)\sqrt{κ+2}/κ^{3/2}$. In summary, both bounds apply to the same geometric class and differ only in what they demand of the matter: non-negative energy density suffices for the upper bound, while the lower bound is established under the additional tangential pressure condition.