Measured group theory

Measured group theory looks at groups from a probabilistic perspective, by studying their actions on probability spaces up to orbit equivalence. It turns out that the orbit equivalence relations of these actions can remember information about the group, which has yielded classification and rigidity results, as well as new invariants such as cost, |^2-Betti numbers, and ergodic dimension. Initiated by Gromov and Zimmer, the subject has become a very fruitful area of research over the last 25 years. One reason for this is that to study the orbit equivalence relation of an action, one tries to measurably equip the orbits with various combinatorial and geometric structures (graphs, trees, CW-complexes) and investigate their characteristics (colorings, matchings, ends, |^2-Betti numbers), which often requires machinery from the corresponding subjects, and also contributes back to these subjects with new results. This two-way traffic has established new connections between Measured Group Theory and various different areas, including ergodic theory, descriptive set theory, probability theory, combinatorics, geometric group theory, Lie groups, and von Neumann algebras.