Cohomological nonvanishing for algebraic fundamental groups of ball quotients

Abstract

Suppose $\Gamma < \mathrm{PU}(n,1)$ is a cocompact arithmetic lattice of simplest type with profinite completion $\widehat{\Gamma}$. This paper proves there is an open subgroup $\widehat{\Gamma}_0 \le \widehat{\Gamma}$ such that $H^j(\widehat{\Delta}, \mathbb{F}_p)$ is nontrivial for every open subgroup ${\widehat{\Delta} \le \widehat{\Gamma}_0}$, $j \le 2n$, and sufficiently large prime $p$. If $n \ge 2$, nonvanishing is new for all $j \ge 2$. Consequently, the virtual cohomological dimension of $\widehat{\Gamma}$ is at least $2n$, improving the previous lower bound of $1$. The proof shows there is a profinite fundamental class for the associated ball quotient and that its canonical class is profinite modulo torsion. For congruence $\Gamma$ and $j < \frac{n+1}{2}$, restriction ${H^j(\widehat{\Gamma}, \mathbb{F}_p) \to H^j(\Gamma, \mathbb{F}_p)}$ is shown to be almost surjective in a precise sense; this is related to whether lattices in $\mathrm{PU}(n,1)$ are good in the sense of Serre, which is only known to hold for $n=1$.