Deformations and desingularizations of conically singular associative submanifolds

Abstract: The proposals of Joyce [Joy18], and Doan and Walpuski [DW19] on counting closed associative submanifolds of $G_2$-manifolds depend on various conjectural transitions. This article contributes to the transitions arising from the degenerations of associative submanifolds into conically singular (CS) associative submanifolds. First, we study the moduli space of CS associative submanifolds with singularities at a finite number of points locally modeled on associative cones in $\mathbf R^7$ and their transversality in a co-closed $G_2$-manifold as well as in a path of co-closed $G_2$-structures. We prove that for a generic co-closed $G_2$-structure and for a generic path of co-closed $G_2$-structures, there are no CS associative submanifolds having singularities modeled on cones with stability-index greater than $0$ and $1$, respectively. We establish that associative cones with links null-torsion holomorphic curves in $S^6$ have stability-index greater than $4$, and all special Lagrangian cones in $\mathbf C^3$ have stability-index greater than equal to $1$ with equality only holding for the Harvey-Lawson $T^2$-cone and a transverse pair of planes. Second, we study the desingularizations of CS associative submanifolds in a one parameter family of co-closed $G_2$-structures. Consequently, we derive desingularization results relating the above transitions for CS associative submanifolds with Harvey-Lawson $T^2$-cone singularity and for associative submanifolds with transverse self-intersection.