Hierarchically hyperbolic spaces I: curve complexes for cubical groups

TL;DR

This paper develops a hierarchical framework for cubical groups, introducing contact graphs as analogues of curve complexes, and proves new properties including acylindrical actions and a rank theorem for these spaces.

Contribution

It introduces hierarchically hyperbolic spaces for cubical groups, extending the theory of curve complexes and subsurface projections to a broader class of groups.

Findings

Contact graph is hyperbolic and admits acylindrical action.

Quasi-Lipschitz images from nilpotent Lie groups are close to product of hierarchy geodesics.

Proves a rank theorem generalizing previous results.

Abstract

In the context of CAT(0) cubical groups, we develop an analogue of the theory of curve complexes and subsurface projections. The role of the subsurfaces is played by a collection of convex subcomplexes called a \emph{factor system}, and the role of the curve graph is played by the \emph{contact graph}. There are a number of close parallels between the contact graph and the curve graph, including hyperbolicity, acylindricity of the action, the existence of hierarchy paths, and a Masur--Minsky-style distance formula. We then define a \emph{hierarchically hyperbolic space}; the class of such spaces includes a wide class of cubical groups (including all virtually compact special groups) as well as mapping class groups and Teichm\"{u}ller space with any of the standard metrics. We deduce a number of results about these spaces, all of which are new for cubical or mapping class groups, and most of which are new for both. We show that the quasi-Lipschitz image from a ball in a nilpotent Lie group into a hierarchically hyperbolic space lies close to a product of hierarchy geodesics. We also prove a rank theorem for hierarchically hyperbolic spaces; this generalizes results of Behrstock--Minsky, Eskin--Masur--Rafi, Hamenst\"{a}dt, and Kleiner. We finally prove that each hierarchically hyperbolic group admits an acylindrical action on a hyperbolic space. This acylindricity result is new for cubical groups, in which case the hyperbolic space admitting the action is the contact graph; in the case of the mapping class group, this provides a new proof of a theorem of Bowditch.