Superrigidity, measure equivalence, and weak Pinsker entropy

Abstract

We show that the class $\mathcal{B}$, of discrete groups which satisfy the conclusion of Popa’s cocycle superrigidity theorem for Bernoulli actions, is invariant under measure equivalence. We generalize this to the setting of discrete probability measure preserving (p.m.p.) groupoids, and as a consequence we deduce that any nonamenable lattice in a product of two noncompact, locally compact second countable groups must belong to $\mathcal{B}$. We also introduce a measure-conjugacy invariant called weak Pinsker entropy and show that, if $G$ is a group in the class $\mathcal{B}$, then weak Pinsker entropy is an orbit-equivalence invariant of every essentially free p.m.p. action of $G$.