Cheeger constants and $L^2$-Betti numbers

We prove the existence of positive lower bounds on the Cheeger constants of manifolds of the form $X/\Gamma$ where $X$ is a contractible Riemannian manifold and $\Gamma<\Isom(X)$ is a discrete subgroup, typically with infinite co-volume. The existence depends on the $L^2$-Betti numbers of $\Gamma$, its subgroups and of a uniform lattice of $\Isom(X)$. As an application, we show the existence of a uniform positive lower bound on the Cheeger constant of any manifold of the form \H^4/\Gamma where \H^4 is real hyperbolic 4-space and \Gamma<\Isom(\H^4) is discrete and isomorphic to a subgroup of the fundamental group of a complete finite-volume hyperbolic 3-manifold. Via Patterson-Sullivan theory, this implies the existence of a uniform positive upper bound on the Hausdorff dimension of the conical limit set of such a $\Gamma$ when $\Gamma$ is geometrically finite. Another application shows the existence of a uniform positive lower bound on the zero-th eigenvalue of the Laplacian of \H^n/\Gamma over all discrete free groups \Gamma<\Isom(\H^n) whenever $n\ge 4$ is even (the bound depends on $n$). This extends results of Phillips-Sarnak and Doyle who obtained such bounds for $n\ge 3$ when $\Gamma$ is a finitely generated Schottky group.