On patched completed homology and a conjecture of Venkatesh

Abstract

Let $F$ be a CM field and $\Pi$ a regular algebraic cuspidal cohomological representation of $\mathbf{G}=\operatorname{PGL}_2/F$. A conjecture of Venkatesh describes the structure of the contribution of $\Pi$ to the homology of the locally symmetric spaces associated to $\mathbf{G}$. We investigate this conjecture in the setting of $p$-adic homology with $p$ a totally split prime. Along the way, we elaborate on the relations between Venkatesh's conjecture and completed homology, the Taylor-Wiles method and the $p$-adic local Langlands correspondence. Our main result is a `big $R=T$' theorem in characteristic 0, from which we deduce a variant of the $p$-adic realisation of Venkatesh's conjecture, conditional on various natural conjectures and technical assumptions.