Short hierarchically hyperbolic groups I: uncountably many coarse median structures

TL;DR

This paper introduces the theory of short hierarchically hyperbolic groups and demonstrates that certain groups, like mapping class groups of punctured spheres and specific right-angled Artin groups, admit uncountably many distinct coarse median structures.

Contribution

It develops the theory of short hierarchically hyperbolic groups and constructs uncountably many coarse median structures for these groups, including cases not derived from cubulations.

Findings

Uncountably many coarse median structures for specific groups.

Development of tools using quasimorphisms to construct coordinate spaces.

Clarification of hierarchical hyperbolicity for certain Artin groups.

Abstract

We prove that the mapping class group of a sphere with five punctures admits uncountably many coarsely equivariant coarse median structures. The same is shown for right-angled Artin groups whose defining graphs are connected, triangle- and square-free, and have at least three vertices. Remarkably, in the latter case, the coarse median structures we produce are not induced by cocompact cubulations. To obtain the above results, we develop the theory of short hierarchically hyperbolic groups (HHG), which also include Artin groups of large and hyperbolic type, graph manifold groups, and extensions of Veech groups. We develop tools to modify their hierarchical structure, including using quasimorphisms to construct quasilines that serve as coordinate spaces, and this is where the abundance of coarse median structures comes from. These techniques are of independent interest, and are used in a follow-up paper with Alessandro Sisto to study quotients of short HHG. In the process, we also clarify a proof of Hagen, Martin, and Sisto on hierarchical hyperbolicity of Artin groups of large and hyperbolic type.