Random rigidity in the free group

Abstract

We prove a rigidity theorem for the geometry of the unit ball in random subspaces of the scl norm in BH1 of a free group. In a free group F of rank k, a random word w of length n (conditioned to lie in [F;F]) has scl(w)=log(2k-1)/n=6 log(n)/C o((n)/log.n) with high probability, and the unit ball in a subspace spanned by d random words of length O(n) is C0 close to a (suitably affinely scaled) octahedron. A conjectural generalization to hyperbolic groups and manifolds (discussed in the appendix) would show that the length of a random geodesic in a hyperbolic manifold can be recovered from the bounded cohomology of the fundamental group.