The Moduli Stack of Breuil-Kisin Modules with Descent Data for Reductive Groups

Abstract

We introduce and study the moduli stack $\mathcal{Y}$ of Breuil-Kisin modules with $\hat{G}$-structure and descent data, or Breuil-Kisin $(\hat{I}“,\hat{G})$-torsors for short. Specifically, for a dominant cocharacter $\hat{I}¼$, we define the moduli stack ${\mathcal{Y}}^{\le \hat{I}¼}$ of Breuil-Kisin $(\hat{I}“,\hat{G})$-torsors with Hodge-Tate weights bounded by $\hat{I}¼$. We prove that ${\mathcal{Y}}^{\le \hat{I}¼}$ is a $p$-adic formal algebraic stack, and show that it is smoothly equivalent to (the $p$-adic completion of) a twisted Schubert variety ${\mathrm{Gr}}_{\mathcal{G}}^{\le \hat{I}¼}$ in the sense of Pappas-Zhu. This is a reformatted and lightly edited version of the author's PhD thesis, submitted to Northwestern University in August 2024.