A ug 2 00 7 Asymptotic isoperimetry on groups and uniform embeddings into Banach spaces

We characterize the possible asymptotic behaviors of the compression associated to a uniform embedding into some Lp-space, with 1 < p < ∞, for a large class of groups including connected Lie groups with exponential growth and word-hyperbolic finitely generated groups. In particular, the Hilbert compression rate of these groups is equal to 1. This also provides new and optimal estimates for the compression of a uniform embedding of the infinite 3-regular tree into some Lp-space. The main part of the paper is devoted to the explicit construction of affine isometric actions of amenable connected Lie groups on Lp-spaces whose compressions are asymptotically optimal. These constructions are based on an asymptotic lower bound of the Lp-isoperimetric profile inside balls. We compute the asymptotic of this profile for all amenable connected Lie groups and for all 1 ≤ p < ∞, providing new geometric invariants of these groups. We also relate the Hilbert compression rate with other asymptotic quantities such as volume growth and probability of return of random walks. For instance, we use estimates on random walks to prove that B(Z ≀ Z) ≥ 2/3, which improves the previously known lower bound 1/2.