Measured Asymptotic Expanders and Rigidity for Roe Algebras

Abstract

In this paper, we give a new geometric condition in terms of measured asymptotic expanders to ensure rigidity of Roe algebras. Consequently, we obtain the rigidity for all bounded geometry spaces that coarsely embed into some |$L^p$|-space for |$p\in [1,\infty )$|⁠. Moreover, we also verify rigidity for the box spaces constructed by Arzhantseva–Tessera and Delabie–Khukhro even though they do not coarsely embed into any |$L^p$|-space. The key step in our proof of rigidity is showing that a block-rank-one (ghost) projection on a sparse space |$X$| belongs to the Roe algebra |$C^{\ast }(X)$| if and only if |$X$| consists of (ghostly) measured asymptotic expanders. As a by-product, we also deduce that ghostly measured asymptotic expanders are new sources of counterexamples to the coarse Baum–Connes conjecture.