Cubical structures and the large-scale geometry of graph braid groups

Abstract

We study the large-scale geometry of graph braid groups through the cubical structure of their unordered discrete configuration spaces, focusing on quasi-isometry to right-angled Artin groups (RAAGs). We first give a complete classification of graph braid groups quasi-isometric to free groups. For the $2$-braid group on a graph $\Gamma$, we study the union $U{P}_2(\Gamma )$ of maximal product subcomplexes of the associated unordered discrete configuration space. We introduce a hierarchy recording geometric and algebraic properties of this inclusion and identify conditions under which the quasi-isometry type of its fundamental group is determined by that of the ambient braid group. For normal bunches of grapes, a class of graphs obtained by attaching cycles to trees, this fundamental group is a one-ended free factor, and the quasi-isometry classification of the braid groups reduces to that of these factors. Using the combinatorics of the underlying trees and the associated intersection complexes, we obtain a graph-theoretic sufficient condition and new obstructions for quasi-isometry to RAAGs, yielding infinite families of non-hyperbolic examples and nonexamples. We also construct infinitely many graph $2$-braid groups hyperbolic relative to a thick proper subgroup not isomorphic to any braid group with at most two particles on a subgraph of the underlying graph.

arXiv commentv4: Corrected gaps in several proofs and revised the treatment of intersection complexes; further improvements to the exposition. v3: Revised title and improved exposition; minor clarifications throughout