The smooth locus in infinite-level Rapoport-Zink spaces

Abstract

Rapoport-Zink spaces are deformation spaces for $p$-divisible groups with additional structure. At infinite level, they become preperfectoid spaces. Let ${\mathcal{M}}_{\infty }$ be an infinite-level Rapoport-Zink space of EL type, and let ${\mathcal{M}}_{\infty }^{\circ }$ be one geometrically connected component of it. We show that ${\mathcal{M}}_{\infty }^{\circ }$ contains a dense open subset which is cohomologically smooth in the sense of Scholze. This is the locus of $p$-divisible groups which do not have any extra endomorphisms. As a corollary, we find that the cohomologically smooth locus in the infinite-level modular curve $X(p^{\infty }{)}^{\circ }$ is exactly the locus of elliptic curves $E$ with supersingular reduction, such that the formal group of $E$ has no extra endomorphisms.

several minor changes; in particular, some proofs are now explained in more detail; 25 pages