Cross ratios and cubulations of hyperbolic groups

TL;DR

This paper establishes a deep connection between cubulations of hyperbolic groups and invariant cross ratios on their boundaries, proving length-spectrum rigidity and exploring boundary relationships in cube complexes.

Contribution

It introduces a natural injection from the space of cubulations into the space of invariant cross ratios and proves length-spectrum rigidity for hyperplane-essential cubulations.

Findings

Cubulations inject into cross ratios on the boundary.

Hyperplane-essential cubulations are length-spectrum rigid.

Describes boundary relationships in cube complexes.

Abstract

Many geometric structures associated to surface groups can be encoded in terms of invariant cross ratios on their circle at infinity; examples include points of Teichm\"uller space, Hitchin representations and geodesic currents. We add to this picture by studying cubulations of arbitrary Gromov hyperbolic groups $G$. Under weak assumptions, we show that the space of cubulations of $G$ naturally injects into the space of $G$-invariant cross ratios on the Gromov boundary $\partial_{\infty}G$. A consequence of our results is that essential, hyperplane-essential cubulations of hyperbolic groups are length-spectrum rigid, i.e. they are fully determined by their length function. This is the optimal length-spectrum rigidity result for cubulations of hyperbolic groups, as we demonstrate with some examples. In the hyperbolic setting, this constitutes a strong improvement on our previous work in arXiv:1903.02447. Along the way, we describe the relationship between the Roller boundary of a ${\rm CAT(0)}$ cube complex, its Gromov boundary and - in the non-hyperbolic case - the contracting boundary of Charney and Sultan. All our results hold for cube complexes with variable edge lengths.