Incomparable actions of free groups

Abstract

Suppose that $X$ is a Polish space, $E$ is a countable Borel equivalence relation on $X$, and $\mu$ is an $E$-invariant Borel probability measure on $X$. We consider the circumstances under which for every countable non-abelian free group $\Gamma$, there is a Borel sequence $(\cdot_r)_{r \in \mathbb{R}}$ of free actions of $\Gamma$ on $X$, generating subequivalence relations $E_r$ of $E$ with respect to which $\mu$ is ergodic, with the further property that $(E_r)_{r \in \mathbb{R}}$ is an increasing sequence of relations which are pairwise incomparable under $\mu$-reducibility. In particular, we show that if $E$ satisfies a natural separability condition, then this is the case as long as there exists a free Borel action of a countable non-abelian free group on $X$, generating a subequivalence relation of $E$ with respect to which $\mu$ is ergodic.