A new construction of compact torsion-free $G_2$-manifolds by gluing families of Eguchi-Hanson spaces

TL;DR

This paper introduces a novel method for constructing compact torsion-free $G_2$-manifolds by resolving orbifold singularities with Eguchi-Hanson spaces, expanding the toolkit for $G_2$ geometry.

Contribution

It presents a new gluing construction for compact $G_2$-manifolds using Eguchi-Hanson spaces and develops techniques to handle associated elliptic equations.

Findings

Successfully resolves orbifold singularities with Eguchi-Hanson spaces.

Provides methods to produce examples of compact $G_2$-manifolds.

Advances analytic techniques for constructing torsion-free $G_2$-structures.

Abstract

We give a new construction of compact Riemannian 7-manifolds with holonomy $G_2$. Let $M$ be a torsion-free $G_2$-manifold (which can have holonomy a proper subgroup of $G_2$) such that $M$ admits an involution $\iota$ preserving the $G_2$-structure. Then $M/{\langle \iota \rangle}$ is a $G_2$-orbifold, with singular set $L$ an associative submanifold of $M$, where the singularities are locally of the form $\mathbb R^3 \times (\mathbb R^4 / \{\pm 1\})$. We resolve this orbifold by gluing in a family of Eguchi-Hanson spaces, parametrized by a nonvanishing closed and coclosed $1$-form $\lambda$ on $L$. Much of the analytic difficulty lies in constructing appropriate closed $G_2$-structures with sufficiently small torsion to be able to apply the general existence theorem of the first author. In particular, the construction involves solving a family of elliptic equations on the noncompact Eguchi-Hanson space, parametrized by the singular set $L$. We also present two generalizations of the main theorem, and we discuss several methods of producing examples from this construction.