Approximation properties for noncommutative $L^p$-spaces of high rank lattices and nonembeddability of expanders

Abstract

This article contains two rigidity type results for $\mathrm{SL}(n,\mathbb{Z})$ for large $n$ that share the same proof. Firstly, we prove that for every $p \in [1,\infty]$ different from $2$, the noncommutative $L^p$-space associated with $\mathrm{SL}(n,\mathbb{Z})$ does not have the completely bounded approximation property for sufficiently large $n$ depending on $p$. The second result concerns the coarse embeddability of expander families constructed from $\mathrm{SL}(n,\mathbb{Z})$. Let $X$ be a Banach space and suppose that there exist $\beta < \frac{1}{2}$ and $C>0$ such that the Banach-Mazur distance to a Hilbert space of all $k$-dimensional subspaces of $X$ is bounded above by $C k^\beta$. Then the expander family constructed from $\mathrm{SL}(n,\mathbb{Z})$ does not coarsely embed into $X$ for sufficiently large $n$ depending on $X$. More generally, we prove that both results hold for lattices in connected simple real Lie groups with sufficiently high real rank.