Asymptotic freeness in tracial ultraproducts

TL;DR

This paper establishes new asymptotic freeness results in tracial ultraproduct von Neumann algebras, with applications to maximal amenability, Gamma absorption, and constructing a ${ m II_1}$ factor lacking property Gamma.

Contribution

It introduces novel asymptotic freeness results in tracial ultraproducts and applies them to derive absorption theorems and construct a unique ${ m II_1}$ factor.

Findings

Proves asymptotic freeness of relative commutants in ultraproducts of free product von Neumann algebras.

Derives a general absorption result in tracial amalgamated free products.

Constructs a ${ m II_1}$ factor without property Gamma not elementary equivalent to free products.

Abstract

We prove novel asymptotic freeness results in tracial ultraproduct von Neumann algebras. In particular, we show that whenever $M = M_1 \ast M_2$ is a tracial free product von Neumann algebra and $u_1 \in \mathscr U(M_1)$, $u_2 \in \mathscr U(M_2)$ are Haar unitaries, the relative commutants $\{u_1\}' \cap M^{\mathcal U}$ and $\{u_2\}' \cap M^{\mathcal U}$ are freely independent in the ultraproduct $M^{\mathcal U}$. Our proof relies on Mei-Ricard's results [MR16] regarding $\operatorname{L}^p$-boundedness (for all $1 < p < +\infty$) of certain Fourier multipliers in tracial amalgamated free products von Neumann algebras. We derive two applications. Firstly, we obtain a general absorption result in tracial amalgamated free products that recovers several previous maximal amenability/Gamma absorption results. Secondly, we prove a new lifting theorem which we combine with our asymptotic freeness results and Chifan-Ioana-Kunnawalkam Elayavalli's recent construction [CIKE22] to provide the first example of a ${\rm II_1}$ factor that does not have property Gamma and is not elementary equivalent to any free product of diffuse tracial von Neumann algebras.