Llarull’s theorem states that any Riemannian metric on the n-sphere which has scalar curvature greater than or equal to \(n(n-1)\) , and whose distance function is bounded below by the unit sphere’s, is isometric to the unit sphere. Gromov later posed the Spherical Stability Problem, which probes the flexibility of this fact. We give a resolution to this problem in dimension 3. Informally, the main result asserts that a sequence of Riemannian 3-spheres whose distance functions are bounded below by the unit sphere’s with uniformly bounded Cheeger isoperimetric constant and scalar curvatures tending to 6 must approach the round 3-sphere in the volume preserving Sormani–Wenger Intrinsic Flat sense. The argument is based on a proof of Llarull’s Theorem due to Hirsch–Kazaras–Khuri–Zhang using spacetime harmonic functions.
