Inapproximability of actions and Kazhdan's property (T)

Abstract

Let G be a countable group with Kazhdan property (T) and let G be a finitely generated group. We prove that if a sofic approximation of G x D has the property that its spanning subgraphs consisting of the G-labeled edges form an expander sequence, then D is locally embeddable into finite groups (LEF). Consequently, if D is not LEF, then every almost free p.m.p. action of G x D whose restriction to G is ergodic fails to weakly contain any sequence of finite labeled graphs; in particular, such an action is not a local-global limit of finite graphs. We also prove that, when D is amenable, the existence of an expander sofic approximation of G x D forces D to be LEF. The main technical input is a self-improvement theorem for almost automorphisms of expander sofic approximations of property (T) groups. It implies that the Hamming-distance clusters of sufficiently accurate almost automorphisms carry a natural finite group structure.