From the geometry of box spaces to the geometry and measured couplings of groups

Abstract

In this paper, we prove that if two `box spaces' of two residually finite groups are coarsely equivalent, then the two groups are `uniform measured equivalent' (UME). More generally, we prove that if there is a coarse embedding of one box space into another box space, then there exists a `uniform measured equivalent embedding' (UME-embedding) of the first group into the second one. This is a reinforcement of the easier fact that a coarse equivalence (resp.\ a coarse embedding) between the box spaces gives rise to a coarse equivalence (resp.\ a coarse embedding) between the groups. We deduce new invariants that distinguish box spaces up to coarse embedding and coarse equivalence. In particular, we obtain that the expanders coming from $SL_n(\mathbb{Z})$ can not be coarsely embedded inside the expanders of $SL_m(\mathbb{Z})$, where $n>m$ and $n,m\geq 3$. Moreover, we obtain a countable class of residually groups which are mutually coarse-equivalent but any of their box spaces are not coarse-equivalent.