- 09/14/2018: Infinitely many new families of complete cohomogeneity one G
_{2}-manifolds - 04/11/2018: Examples of collapsing 4-dimensional hyperkähler metrics
- 09/11/2017: ALC manifolds with special holonomy
- 01/11/2017: Non-compact G
_{2}manifolds collapsing to asymptotically conical Calabi-Yau 3-folds - 09/08/2016: ALF spaces and collapsing Ricci-flat metrics on the K3 surface

### September 14, 2018

TITLE: Infinitely many new families of complete cohomogeneity one G_{2}-manifolds

ABSTRACT: I will present joint work with Mark Haskins and Johannes Nordström on cohomogeneity one metrics, that is, holonomy metrics acted upon by a group of isometries with generic orbits of codimension one. We construct infinitely many new one-parameter families of simply connected complete noncompact -manifolds with controlled geometry at infinity. The generic member of each family has so-called asymptotically locally conical (ALC) geometry. At a special parameter value, the nature of the asymptotic geometry changes, and we obtain a unique member of each family with asymptotically conical (AC) geometry. On approach to a second special parameter value, the family of metrics collapses to an AC Calabi-Yau 3-fold. We also construct a closely related singular holonomy space with an isolated conical singularity in its interior and ALC geometry at infinity. Our infinitely many new simply connected AC manifolds are particularly noteworthy: only the three classic examples constructed by Bryant and Salamon in 1989 were previously known.

### April 11, 2018

TITLE: Examples of collapsing 4-dimensional hyperkähler metrics

ABSTRACT: This talk is complementary to Ruobing Zhang’s lectures and Jeff Viaclovsky’s talk. I will survey older constructions of collapsing sequences of hyperkähler metrics on K3 surfaces, focusing in particular on Gross-Wilson’s study of Kähler Ricci-flat metrics on elliptic K3 surfaces and on my work on collapse to a 3-dimensional limit.

### September 11, 2017

TITLE: ALC manifolds with special holonomy

ABSTRACT: At the meeting in January, I described a new analytic construction, obtained in joint work with Mark Haskins and Johannes Nordström, of complete non-compact -manifolds. The construction starts with an asymptotically conical (AC) Calabi-Yau 3-fold B and produces a 1-parameter family of complete -metrics on a suitable circle bundle over B. These -metrics have asymptotically locally conical (ALC) geometry, the natural generalisation of the asymptotic geometry of ALF hyperkähler 4-manifolds to higher dimensions. In this talk I will describe some initial steps in two different directions related to that result. Firstly, I will show how in our construction of ALC -manifolds rigid compact holomorphic curves in the AC Calabi-Yau 3-fold lift to rigid associative submanifolds in the ALC -manifold. Secondly, I will describe the analogous construction of ALC Spin(7)-holonomy metrics on suitable circle bundles over AC -manifolds.

### January 10, 2017 (jointly with Mark Haskins)

TITLE: Non-compact G_{2} manifolds collapsing to asymptotically conical Calabi-Yau 3-folds

ABSTRACT:We will present a new analytic construction of complete non-compact holonomy metrics, that yields infinitely many families of examples. The underlying 7-manifolds are all circle bundles over asymptotically conical (AC) Calabi-Yau 3-folds endowed with circle-invariant metrics. Their geometry at infinity is that of a circle bundle over a Calabi-Yau cone with circle fibres of fixed finite length. The manifolds we construct are therefore 7-dimensional analogues of 4-dimensional ALF hyperkähler metrics. Physicists have termed metrics with such asymptotics ALC (asymptotically locally conical). Within the Melrose school, ALC metrics form a special subclass of so-called fibred boundary metrics.

The dimensional reduction of the equations for holonomy in the presence of a Killing field was considered by Apostolov-Salamon and by several groups of physicists. We reinterpret the dimensionally-reduced equations in terms of a pair consisting of an SU(3) structure (with tightly constrained torsion) on the 6-dimensional orbit space coupled to an abelian Calabi-Yau monopole on this 6-manifold. We solve this nonlinear coupled system of PDEs by considering the geometric limit in which the circle fibres of the associated circle-invariant holonomy metrics collapse. In this collapsed limit, given the asymptotically conical Calabi-Yau 3-fold to which the 7-dimensional metrics collapse, the problem comes close to linearising and the crux of the matter is therefore to develop a sufficiently good understanding of solutions to these linearised equations in suitable spaces of functions. For this we need the well-known Fredholm theory for elliptic operators acting on weighted Sobolev spaces on asymptotically conical Calabi-Yau 3-folds and an extension of this theory to the setting of weighted Sobolev spaces on ALC manifolds.

The holonomy metrics we construct should be thought of as arising from (a suitable perturbation of) abelian Hermitian-Yang-Mills connections on AC Calabi-Yau 3-folds, especially those that arise as crepant resolutions of Calabi-Yau cones. From the physics viewpoint our solutions correspond to M theory uplifts of type IIA solutions in the absence of D6-branes. The existence of such a rich spectrum of “no brane” solutions does not seem to have been anticipated on the physics side, and is a new feature of holonomy metrics compared to the 4-dimensional hyperkähler setting. In particular all our examples provide instances of families of holonomy metrics that collapse with bounded curvature to Calabi-Yau 3-folds.

Time permitting we will outline extensions of the construction described here and how it fits into a broader programme.

The talk is closely related to the talks by Carron, Collins and Acharya and is based on joint work with Mark Haskins and Johannes Nordström.

### September 8, 2016

TITLE: ALF spaces and collapsing Ricci-flat metrics on the K3 surface

ABSTRACT: The Kummer construction of Kähler Ricci-flat metrics on the (smooth 4-manifold underlying a complex) surface provides the prototypical example of the formation of orbifold singularities in non-collapsing sequences of Einstein 4-manifolds. Much less is known about the structure of the singularities forming along sequences of collapsing Einstein metrics.

I will describe the construction of large families of Ricci-flat metrics on the surface collapsing to the quotient of a flat 3-torus by an involution. The collapse occurs with bounded curvature away from finitely many points. The geometry around these points is modelled by ALF gravitational instantons (complete non-compact hyperkähler 4-manifolds with decaying curvature and cubic volume growth).

This 4-dimensional construction serves as a good warm-up for the much more involved case of collapsing families of holonomy metrics described in the talk of Mark Haskins.