Massey products and the Iwasawa theory of fine Selmer groups

Abstract

A central conjecture of Coates and Sujatha predicts that the fine Selmer group of any $p$-adic Galois representation is cotorsion over the relevant Iwasawa algebra with vanishing $\mu$-invariant, generalizing Iwasawa's original conjecture for class groups. In this article, we recast this conjecture in terms of higher Galois cohomological operations called Massey products, stated purely in terms of the residual representation. This characterization implies that if the $\mu$-invariant vanishes for a given $\mathbb{Z}_p$-extension, then it also vanishes for all $\mathbb{Z}_p$-extensions the Greenberg neighbourhood of radius $1/p$. Furthermore, we establish that the unobstructedness of ordinary deformation rings over $\mathbb{Z}_p$-extensions is equivalent to the vanishing of the $\mu$-invariant of the fine Selmer group attached to the adjoint representation. For ordinary deformation rings attached to $2$-dimensional automorphic Galois representations, we establish a close connection between the Noetherian property and the vanishing of the $\mu$-invariant.