Lower Bounds on Ricci Curvature and the Almost Rigidity of Warped Products

The basic rigidity theorems for manifolds of nonnegative or positive Ricci curvature are the implies metric cone theorem, the maximal diameter theorem, Cg, and the splitting theorem, CG. Each asserts that if a certain geometric quantity (volume or diameter) is as large as possible relative to the pertinent lower bound on Ricci curvature, then the metric on the manifold in question is a warped product metric of a particular type. In this paper we provide quantitative generalizations of the above mentioned results. Among the applications are the splitting theorem for GromovHausdorff limit spaces X, where Mn -* X, Ricmn ? -i see FY. Other applications include the assertion that for complete manifolds, M', with Ricmf > 0 and Euclidean volume growth, all tangent cones at infinity are metric cones; compare BKN, CT, P1. Via resealing arguments, there are also strong consequences for the local structure of manifolds whose Ricci curvature satisfies a fixed lower bound and for their Gromov-Hausdorff limits. Some of these are announced in CCol; for a more detailed discussion see CCo2, CCo3, CCo4. Our work further develops and significantly extends techniques which were introduced in Col, Co2 and significantly extended in Co3, in order to prove certain stability conjectures of Anderson-Cheeger, Gromov and Perelman. The results of Col-Co3 were announced in Co4. We briefly review some of those results. Let dGH denote the Gromov-Hausdorff distance between metric spaces; see GLP. Let S' denote the unit sphere and recall that S' is the unique complete