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

We investigate the asymptotic structure of (possibly type III) crossed product von Neumann algebras $M = B \rtimes Γ$ arising from arbitrary actions $Γ\curvearrowright B$ 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 $N' \cap M^ω$ 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 $Γ\curvearrowright (X, μ)$ of any bi-exact countable discrete group on a standard probability space, the corresponding group measure space factor ${\rm L}^\infty(X) \rtimes Γ$ has no nontrivial central sequence. Using recent results of Boutonnet-Ioana-Salehi Golsefidy [BISG15], we construct, for every $0 < λ\leq 1$, a type III$_λ$ strongly ergodic essentially free nonsingular action $\mathbf F_\infty \curvearrowright (X_λ, μ_λ)$ of the free group $\mathbf F_\infty$ on a standard probability space so that the corresponding group measure space type III$_λ$ factor ${\rm L}^\infty(X_λ, μ_λ) \rtimes \mathbf F_\infty$ 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.

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