A parabolic flow for the large volume heterotic $G_2$ system

Abstract

We introduce a geometric flow of conformally coclosed $G_2$-structures, whose fixed points are large volume solutions of the heterotic $G_2$ system, with vanishing scalar torsion class $\tau_0 = 0$. After conformal rescaling, it becomes a flow of coclosed $G_2$-structures, related to Grigorian's modified $G_2$ coflow, which is coupled to a flow for a dilaton function. Our main results establish fundamental short-time existence and Shi-type smoothing properties of this flow, as well as a classification of its fixed points. By a classical rigidity result in the string theory literature, the fixed points on a compact manifold correspond to torsion-free $G_2$-structures, that is, to metrics with holonomy contained in $G_2$. Thus, we establish in the affirmative a folklore question in the special holonomy community, about the existence of a well-posed flow for coclosed $G_2$-structures with fixed points given by torsion-free $G_2$-structures. The flow also satisfies a monotonicity formula for the $G_2$-dilaton functional (volume scale in string theory), which allows us to strengthen the rigidity result with an alternative proof. The monotonicity of the $G_2$-dilaton functional, combined with the Shi-type estimates, leads to a general result on the convergence of nonsingular solutions. A dimension reduction analysis reveals an interesting link with natural flows for $SU(3)$-structures, previously introduced in the literature.