von Neumann's problem and extensions of non-amenable equivalence relations

TL;DR

This paper extends Gaboriau and Lyons' results to show that non-amenable ergodic equivalence relations contain free group actions in their Bernoulli extensions, and demonstrates the diversity of actions for non-amenable groups.

Contribution

It generalizes the von Neumann problem to equivalence relations and establishes the existence of many non-equivalent free actions for non-amenable groups.

Findings

Bernoulli extensions contain free group actions

Non-amenable groups have uncountably many non-von Neumann equivalent actions

Results extend previous work on non-amenable equivalence relations

Abstract

The goals of this paper are twofold. First, we generalize the result of Gaboriau and Lyons [GL07] to the setting of von Neumann's problem for equivalence relations, proving that for any non-amenable ergodic probability measure preserving (pmp) equivalence relation $\mathcal{R}$, the Bernoulli extension over a non-atomic base space $(K, \kappa)$ contains the orbit equivalence relation of a free ergodic pmp action of $\mathbb{F}_2$. Moreover, we provide conditions which imply that this holds for any non-trivial probability space $K$. Second, we use this result to prove that any non-amenable unimodular locally compact second countable group admits uncountably many free ergodic pmp actions which are pairwise not von Neumann equivalent (hence, pairwise not orbit equivalent).