The big projective module as a nearby cycles sheaf

Abstract

We give a new geometric construction of the big projective module in the principal block of the BGG category $\mathcal{O}$, or rather the corresponding $\mathcal{D}$-module on the flag variety. Namely, given a one-parameter family of nondegenerate additive characters of the unipotent radical of a Borel subgroup which degenerate to the trivial character, there is a corresponding one-parameter family of Whittaker sheaves. We show that the unipotent nearby cycles functor applied to this family yields the big projective $\mathcal{D}$-module.