Phases of $mathcal{N}=4$ SYM, S-duality and Nilpotent Cones

In this note, I describe the space of vacua $mathcal{V}$ of four dimensional $mathcal{N}=4$ SYM on $mathbb{R}^4$ with gauge group a compact simple Lie Group $G$ as a stratified space. On each stratum, the low energy effective field theory is different. This language allows one to make precise the idea of moving in the space of vacua $mathcal{V}$. A particular subset of the strata of $mathcal{N}=4$ SYM can be efficiently described using the theory of sheets in a Lie algebra. For these strata, I study the conjectural action of S-duality. I also indicate some benefits of using such a language for the study of the available space of vacua $overline{mathcal{V}}$ on the boundary of GL twisted $mathcal{N}=4$ SYM on a half-space $mathbb{R}^3 times mathbb{R}^+$. As an application of boundary symmetry breaking, I indicate how a) the Local Nilpotent Cone arises as part of the available symmetry breaking choices on the boundary of the four dimensional theory and b) the Global Nilpotent Cone arises in the theory reduced down to two dimensions on a Riemann Surface $C$. These geometries play a critical role in the Local and Global Geometric Langlands Program(s).