Bounds on the minimum orbital period of four-dimensional black holes: From singular to non-singular spacetimes

It was conjectured that the lower bound $T_{\min}\geq 4πM $ may be a universal property of four-dimensional black holes, where $M$ is the black hole mass and $T_{\min}$ is the minimum orbital period around black holes. Recently, we established an upper bound $T_{\min}\le 6\sqrt{3}πM$ for Schwarzschi