Rigidity and regularity for almost homogeneous spaces with Ricci curvature bounds

Abstract

We say that a metric space $X$ is $(\epsilon ,G)$-homogeneous if $G<Iso(X)$ is a discrete group of isometries with $diam(X/G)<\epsilon$.\ A sequence of $({\epsilon }_i,G_i)$-homogeneous spaces $X_i$ with ${\epsilon }_i\to 0$ is called a sequence of almost homogeneous spaces. In this paper we show that the Gromov-Hausdorff limit of a sequence of almost homogeneous RCD$(K,N)$ spaces must be a nilpotent Lie group with $Ric⩾K$. We also obtain a topological rigidity theorem for $(\epsilon ,G)$-homogeneous RCD$(K,N)$ spaces, which generalizes a recent result by Wang. Indeed, if $X$ is an $(\epsilon ,G)$-homogeneous RCD$(K,N)$ space and $G$ is an almost-crystallographic group, then $X/G$ is bi-Hölder to an infranil orbifold. Moreover, we study $(\epsilon ,G)$-homogeneous spaces in the smooth setting and prove rigidity and $\epsilon$-regularity theorems for Riemannian orbifolds with Einstein metrics and bounded Ricci curvatures respectively.

28 pages