Cheng's eigenvalue comparison on metric measure spaces and applications

Using the localization technique, we prove a sharp upper bound on the first Dirichlet eigenvalue of metric balls in essentially non-branching ${\mathsf{C}\mathsf{D}}^{\star }(K,N)$ spaces. This extends a celebrated result of Cheng to the non-smooth setting of metric measure spaces satisfying Ricci curvature lower bounds in a synthetic sense, via optimal transport. Rigidity and stability statements are provided for ${\mathsf{R}\mathsf{C}\mathsf{D}}^{\star }(K,N)$ spaces; the stability seems to be new even for smooth Riemannian manifolds. We then present some mathematical and physical applications: in the former, we obtain an upper bound on the $j^{th}$ Neumann eigenvalue in essentially non-branching ${\mathsf{C}\mathsf{D}}^{\star }(K,N)$ spaces and a bound on the essential spectrum in non-compact ${\mathsf{R}\mathsf{C}\mathsf{D}}^{\star }(K,N)$ spaces; in the latter, the eigenvalue bounds correspond to general upper bounds on the masses of the spin-2 Kaluza-Klein excitations around general warped compactifications of higher-dimensional theories of gravity.