Bi-exact groups, strongly ergodic actions and group measure space type III factors with no central sequence (1510.07987v3)

Abstract: We investigate the asymptotic structure of (possibly type III) crossed product von Neumann algebras $M = B \rtimes \Gamma$ arising from arbitrary actions $\Gamma \curvearrowright B$ of bi-exact discrete groups (e.g. free groups) on amenable von Neumann algebras. We prove a spectral gap rigidity result for the central sequence algebra $N' \cap M^\omega$ of any nonamenable von Neumann subalgebra with normal expectation $N \subset M$. We use this result to show that for any strongly ergodic essentially free nonsingular action $\Gamma \curvearrowright (X, \mu)$ of any bi-exact countable discrete group on a standard probability space, the corresponding group measure space factor ${\rm L}^\infty(X) \rtimes \Gamma$ has no nontrivial central sequence. Using recent results of Boutonnet-Ioana-Salehi Golsefidy [BISG15], we construct, for every $0 < \lambda \leq 1$, a type III$\lambda$ strongly ergodic essentially free nonsingular action $\mathbf F\infty \curvearrowright (X_\lambda, \mu_\lambda)$ of the free group $\mathbf F_\infty$ on a standard probability space so that the corresponding group measure space type III$\lambda$ factor ${\rm L}^\infty(X\lambda, \mu_\lambda) \rtimes \mathbf F_\infty$ has no nontrivial central sequence by our main result. In particular, we obtain the first examples of group measure space type III factors with no nontrivial central sequence.