On density of modular points in pseudo-deformation rings

Abstract

Given a continuous, odd, reducible and semi-simple $2$-dimensional representation ${\bar{\rho }}_0$ of $G_{\mathbb{Q},Np}$ over a finite field of odd characteristic $p$, we study the relation between the universal deformation ring of the pseudo-representation corresponding to ${\bar{\rho }}_0$ (pseudo-deformation ring) and the big $p$-adic Hecke algebra to prove that the maximal reduced quotient of the pseudo-deformation ring is isomorphic to the local component of the big $p$-adic Hecke algebra corresponding to ${\bar{\rho }}_0$ if a certain global Galois cohomology group has dimension $1$. This partially extends the results of Böckle to the case of residually reducible representations. We give an application of our main theorem to the structure of Hecke algebras modulo $p$. As another application of our methods and results, we prove a result about non-optimal levels of newforms lifting ${\bar{\rho }}_0$ in the spirit of Diamond-Taylor. This also gives a partial answer to a conjecture of Billerey-Menares.

arXiv commentv3, 50 pages, made changes and corrections following comments given by referees, added a subsection in the introduction about the hypotheses of the main theorem, added more details to some proofs for better exposition and added a subsection in Section 2 listing various background results from the literature that are used in the article. Comments are welcome