Bounded-degree graphs of non-negative Ollivier-Ricci curvature have subexponential growth and diffusive random walk

Abstract

We study the geometric properties of graphs with non-negative Ollivier-Ricci curvature, a discrete analogue of non-negative Ricci curvature in Riemannian geometry. We prove that for each $d<\infty$ there exists a constant $C_d$ such that if $G=(V,E)$ is a finite graph with non-negative Ollivier-Ricci curvature and with degrees bounded by $d$ then the average log-volume growth and random walk displacement satisfy \[ \frac{1}{|V|} \sum_{x\in V} \log \#B(x,r) \leq \exp\left[C_d \sqrt{\log r}\right] = r^{o(1)} \] and \[ \frac{1}{|V|} \sum_{x\in V} \mathbf{E}_x [d(X_0,X_n)^2] \leq n \exp\left[C_d \sqrt{\log n}\right] = n^{1+o(1)} \] for every $n,r\geq 2$. This significantly strengthens a result of Salez (GAFA 2022), who proved that the average displacement of the random walk is $o(n)$ and deduced that non-negatively curved graphs of bounded degree cannot be expanders. Our results also apply to infinite transitive graphs and, more generally, to bounded-degree unimodular random rooted graphs of non-negative Ollivier-Ricci curvature.