We announce some new results regarding the classification problem for separable von Neumann algebras. Our results are obtained by applying the notion of Borel reducibility and Hjorth's theory of turbulence to the isomorphism relation for separable von Neumann algebras.

We show that for every sofic chunk E there is a bijective homomorphism \, where \ is a chunk of the group of computable permutations of \ so that the approximating morphisms of E can be viewed as restrictions of permutations of \ to finite subsets of \. Using this we study some relevant effectivity conditions associated with sofic chunks and their profiles.

The Connes Embedding Problem asks whether every separable II1 factor embeds into an ultrapower of the hyperfinite II1 factor. We show that the CEP is equivalent to the statement that every type II1 tracial von Neumann algebra has a computable universal theory.

We prove a strong dichotomy for the number of ultrapowers of a given model of cardinality ≤ 2ℵ0 associated with nonprincipal ultrafilters on ℕ. They are either all isomorphic, or else there are 22ℵ0 many nonisomorphic ultrapowers. We prove the analogous result for metric structures, including C*-algebras and II1 factors, as well as their relative commutants and include several applications. We also show that the CAF001-algebra [Formula: see text] always has nonisomorphic relative commutants in its ultrapowers associated with nonprincipal ultrafilters (...) on ℕ. (shrink)

The main result of the paper states that there is a finitely presented group _G_ with decidable word problem where detection of finite subsets of _G_ which generate amenable subgroups is not decidable.