Discrete uniformizing metrics on distributional limits of sphere packings

Abstract

Suppose that $\{G_n\}$ is a sequence of finite graphs such that each $G_n$ is the tangency graph of a sphere packing in $\mathbb{R}^d$. Let $ρ_n$ be a uniformly random vertex of $G_n$ and suppose that $(G,ρ)$ is the distributional limit of $\{(G_n,ρ_n)\}$ in the sense of Benjamini and Schramm. Then the conformal growth exponent of $(G,ρ)$ is at most $d$. In other words, there exists a unimodular "unit volume" weighting of the graph metric on $(G,ρ)$ such that the volume growth of balls in the weighted path metric is bounded by a polynomial of degree $d$. This generalizes to limits of graphs that can be "coarsely" packed in an Ahlfors $d$-regular metric measure space. Using our previous work, this implies that, under moment conditions on the degree of the root $ρ$,the almost sure spectral dimension of $G$ is at most $d$. This fact was known previously only for graphs packed in $\mathbb{R}^2$ (planar graphs), and the case of $d > 2$ eluded approaches based on extremal length. In the process of bounding the spectral dimension, we establish that the spectral measure of $(G,ρ)$ is dominated by a variant of the $d$-dimensional Weyl law.