We establish new results and introduce new methods in the theory of measurable orbit equivalence, using bounded cohomology of group representations. Our rigidity statements hold for a wide (uncountable) class of groups arising from negative curvature geometry. Amongst our applications are (a) measurable Mostow-type rigidity theorems for products of negatively curved groups; (b) prime factorization results for measure equivalence; (c) superrigidity for orbit equivalence; (d) the first examples of continua of type II1 equivalence relations with trivial outer automorphism group that are mutually not stably isomorphic.

This paper introduces new results and methods in the theory of measurable orbit equivalence, using bounded cohomology of group representations, and introduces new applications for negatively curved groups, measure equivalence, and superrigidity.

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Orbit equivalence rigidity and bounded cohomology

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