Smooth moduli spaces of associative submanifolds

Abstract

Let $M^7$ be a smooth manifold equipped with a $G_2$-structure $ϕ$, and $Y^3$ be an closed compact $ϕ$-associative submanifold. In \cite{McL}, R. McLean proved that the moduli space $\bm_{Y,ϕ}$ of the $ϕ$-associative deformations of $Y$ has vanishing virtual dimension. In this paper, we perturb $ϕ$ into a $G_2$-structure $ψ$ in order to ensure the smoothness of $\bm_{Y,ψ}$ near $Y$. If $Y$ is allowed to have a boundary moving in a fixed coassociative submanifold $X$, it was proved in \cite{GaWi} that the moduli space $\bm_{Y,X}$ of the associative deformations of $Y$ with boundary in $X$ has finite virtual dimension. We show here that a generic perturbation of the boundary condition $X$ into $X'$ gives the smoothness of $\bm_{Y,X'}$. In another direction, we use the Bochner technique to prove a vanishing theorem that forces $\bm_Y$ or $\bm_{Y,X}$ to be smooth near $Y$. For every case, some explicit families of examples will be given.