Special subvarieties of non-arithmetic ball quotients and Hodge theory

Abstract

Let $\Gamma \subset \mathrm {PU}(1,n)$ be a lattice and $S_\Gamma $ be the associated ball quotient. We prove that, if $S_\Gamma $ contains infinitely many maximal complex totally geodesic subvarieties, then $\Gamma $ is arithmetic. We also prove an Ax–Schanuel Conjecture for $S_\Gamma $, similar to the one recently proven by Mok, Pila and Tsimerman. One of the main ingredients in the proofs is to realise $S_\Gamma $ inside a period domain for polarised integral variations of Hodge structure and interpret totally geodesic subvarieties as unlikely intersections.