Sharp Holder continuity of tangent cones for spaces with a lower Ricci curvature bound and applications

We prove a new kind of estimate that holds on any manifold with a lower Ricci bound. It relates the geometry of two small balls with the same radius, potentially far apart, but centered in the interior of a common minimizing geodesic. It reveals new, previously unknown, properties that all generalized spaces with a lower Ricci curvature bound must have and it has a number of applications. This new kind of estimate asserts that the geometry of small balls along any minimizing geodesic changes in a Hölder continuous way with a constant depending on the lower bound for the Ricci curvature, the dimension of the manifold, and the distance to the end points of the geodesic. We give examples that show that the Hölder exponent, along with essentially all the other consequences that we show follow from this estimate, are sharp. The unified theme for all of these applications is convexity. Among the applications is that the regular set is convex for any non-collapsed limit of Einstein metrics. In the general case of potentially collapsed limits of manifolds with just a lower Ricci curvature bound we show that the regular set is weakly convex and a.e. convex, that is almost every pair of points can be connected by a minimizing geodesic whose interior is contained in the regular set. We also show two conjectures of Cheeger-Colding. One of these asserts that the isometry group of any, even collapsed, limit of manifolds with a uniform lower Ricci curvature bound is a Lie group; the key point for this is to rule out small subgroups. The other asserts that the dimension of any limit space is the same everywhere. Finally, we show a Reifenberg type property holds for collapsed limits and discuss why this indicates further regularity of manifolds and spaces with Ricci curvature bounds.