Hypertrace and entropy gap characterizations of property (T) for $\mathrm{II}_1$ factors

Abstract

We establish a hypertrace characterization of property (T) for $\mathrm{II}_1$ factors: Given a $\mathrm{II}_1$ factors $M$, $M$ does not have property (T) if and only if there exists a von Neumann algebra $\mathcal{A}$ with $M\subset \mathcal{A}$ such that $\mathcal{A}$ admits a $M$-hypertrace but no normal hypertrace. For $M$ without property (T), such an inclusion $M\subset \mathcal{A}$ also admits almost vanishing Furstenberg entropy. With the same construction of $M\subset \mathcal{A}$, we also establish similar characterizations of Haagerup property for $\mathrm{II}_1$ factors.