Cones over metric measure spaces and the maximal diameter theorem

The main result of this article states that the (K;N)-cone over some metric measure space satisfies the reduced Riemannian curvature-dimension condition RCD^*(KN;N+1) if and only if the underlying space satisfies RCD^*(N-1;N). The proof uses a characterization of reduced Riemannian curvature-dimension bounds by Bochner's inequality that was established for metric measure spaces by Erbar, Kuwada and Sturm and independently by Ambrosio, Mondino and Savar\'e. As corollary of our result and the Cheeger-Gigli-Gromoll splitting theorem we obtain a maximal diameter theorem for metric measure spaces that satisfy a reduced Riemannian curvature-dimension condition.