$G$-torsors on perfectoid spaces

Abstract

For any rigid analytic group variety $G$ over a non-archimedean field $K$ over ${\mathbb{Q}}_p$, we study $G$-torsors on adic spaces over $K$ in the $v$-topology. Our main result is that on perfectoid spaces, $G$-torsors in the étale and $v$-topology are equivalent. This generalises the known cases of $G={\mathbb{G}}_a$ and $G={\mathrm{GL}}_n$ due to Scholze and Kedlaya--Liu. On a general adic space $X$ over $K$, where there can be more $v$-topological $G$-torsors than étale ones, we show that for any open subgroup $U\subseteq G$, any $G$-torsor on $X_v$ admits a reduction of structure group to $U$ étale-locally on $X$. This has applications in the context of the $p$-adic Simpson correspondence: For example, we use it to show that on any adic space, generalised ${\mathbb{Q}}_p$-representations are equivalent to $v$-vector bundles.