Measure equivalence embeddings of free groups and free group factors

Abstract

We give a simple and explicit proof that the free group ${\mathbb{F}}_2$ admits a measure equivalence embedding into any nonamenable locally compact second countable (lcsc) group $G$. We use this to prove that every nonamenable lcsc group $G$ admits strongly ergodic actions of any possible Krieger type and admits nonamenable, weakly mixing actions with any prescribed flow of weights. We also introduce concepts of measure equivalence and measure equivalence embeddings for $I{I}_1$ factors. We prove that a $I{I}_1$ factor $M$ is nonamenable if and only if the free group factor $L({\mathbb{F}}_2)$ admits a measure equivalence embedding into $M$. We prove stability of property (T) and the Haagerup property under measure equivalence of $I{I}_1$ factors.

v3: minor changes, final version, to appear in Annales Scientifiques de l'Ecole Normale Superieure. v2: minor changes, including the extra theorem 4.3