OE and W* superrigidity results for actions by surface braid groups

Abstract

We show that several important normal subgroups $\Gamma$ of the mapping class group of a surface satisfy the following property: any free, ergodic, probability measure preserving action $\Gamma \curvearrowright X$ is stably OE-superrigid. These include the central quotients of most surface braid groups and most Torelli groups and Johnson kernels. In addition, we show that all these groups satisfy the measure equivalence rigidity and we describe all their lattice-embeddings. Using these results in combination with previous results from [CIK13] we deduce that any free, ergodic, probability measure preserving action of almost any surface braid group is stably W*-superrigid, i.e., it can be completely reconstructed from its von Neumann algebra.

44 pages, 2 figures

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• On the structure and arithmeticity of lattice envelopes
• A Matsumoto-Mostow result for Zimmer's cocycles of hyperbolic lattices
• Approximate equivalence of actions
• Cocycle superrigidity for coinduced actions
• Measure equivalence rigidity of $\mathrm{Out}(F_N)$