Ultraproducts, weak equivalence and sofic entropy

Abstract

In this work, we study pmp actions of countable groups on arbitrary diffuse probability spaces under the point of view of weak equivalence. We will show that any such an action is weakly equivalent to an action on a standard probability space. We also propose a metric on the space of actions modulo weak equivalence which is equivalent to the topology of Ab\'{e}rt and Elek. We will give a simpler proof of the compactness of the space, showing that convergence is characterized by ultraproducts. Using this topology, we will show that a profinite action is weakly equivalent to an ultraproduct of finite actions. Finally, combining our results with another result of Ab\'{e}rt and Elek, we will obtain a corollary about sofic entropy. We will show that for free groups and some property (T) groups, sofic entropy of profinite actions depends crucially on the chosen sofic approximation.