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Hierarchically hyperbolic spaces, I: Curve complexes for cubical groups

Jason Behrstock, Mark F Hagen and Alessandro Sisto

Geometry & Topology 21 (2017) 1731–1804

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 factor system, and the role of the curve graph is played by the 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 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 Teichmüller 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 and Minsky, of Eskin, Masur and Rafi, of Hamenstädt, and of 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.

hierarchically hyperbolic, cube complexes, acylindrical, Teichmüller space, curve complex
Mathematical Subject Classification 2010
Primary: 20F36, 20F55, 20F65
Received: 22 April 2015
Revised: 8 October 2016
Accepted: 10 November 2016
Published: 10 May 2017
Proposed: Walter Neumann
Seconded: Bruce Kleiner, Dmitri Burago
Jason Behrstock
The Graduate Center and Lehman College
365 5th Avenue
New York, NY 10036
United States
Barnard College
Columbia University
New York
Mark F Hagen
Department of Pure Mathematics and Mathematical Statistics
University of Cambridge
Wilberforce Road
United Kingdom
Alessandro Sisto
Departement Mathematik HG G 28
Rämistrasse 101
CH-8092 Zürich