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

Abstract

The goals of this paper are twofold. First, we generalize the result of Gaboriau and Lyons [17] 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).