A smooth family of $G_2$-instantons over a generalised Kummer construction

TL;DR

This paper constructs a smooth one-parameter family of $G_2$-instantons on a desingularized $G_2$-orbifold, extending gluing techniques and overcoming obstructions using symmetry, resulting in a novel example of such a family.

Contribution

It extends gluing constructions for $G_2$-instantons to Kummer constructions with non-trivial cokernels, producing the first known smooth 1-parameter family of instantons on a compact $G_2$-manifold.

Findings

Constructed a smooth 1-parameter family of $G_2$-instantons.

Proved the instantons are infinitesimally rigid and non-flat.

Showed the family induces an injective curve in the moduli space.

Abstract

We construct a smooth 1-parameter family of $G_2$-instantons over a generalised Kummer construction desingularising a $G_2$-orbifold discovered by Joyce. For this we extend the gluing construction for $G_2$-instantons developed by Walpuski to Kummer constructions resolving $G_2$-orbifolds whose singular strata are of codimension 6 and to connections (and entire families of connections) whose linearised instanton operator has a non-trivial cokernel. In order to overcome the corresponding obstructions, we utilise a $\mathbb{Z}_2$-action on the ambient manifold. More precisely, we perturb the (family of) pre-glued almost-instantons inside the class of $\mathbb{Z}_2$-invariant connections, which has the advantage that only the $\mathbb{Z}_2$-invariant locus of the cokernel needs to vanish. We then prove that the instantons that we construct over the resolution of the orbifold found by Joyce are all infinitesimally rigid and non-flat. Moreover, we show that the resulting curve into the moduli space of $G_2$-instantons modulo gauge is injective, that is, no two distinct instantons within the family are gauge-equivalent. To the author's knowledge this is the first example of a smooth 1-parameter family of instantons over a compact $G_2$-manifold.