Ultrapowers of spectral subspaces

Abstract

We prove, for any W$^*$-probability space $(M,φ)$ where $M$ is a type $\mathrm{III}_1$ factor, any nontrivial, proper closed $F\subseteq \mathbb{R}$, and any nonprincipal ultrafilter $\mathcal{U}$ on $\mathbb{N}$, that the ultrapower $M(σ^φ,F)^{\mathcal{U}}$ of the spectral subspace $M(σ^φ,F)$ is a proper subset of the spectral subspace $M^{\mathcal{U}}(σ^{φ^{\mathcal{U}}},F)$. We discuss the model-theoretic implications of this result.