Topology of asymptotically conical Calabi--Yau and G2 manifolds and desingularization of nearly Kähler and nearly G2 conifolds

Abstract: A natural approach to the construction of nearly G2 manifolds lies in resolving nearly G2 spaces with isolated conical singularities by gluing in asymptotically conical G2 manifolds modelled on the same cone. If such a resolution exits, one expects there to be a family of nearly G2 manifolds, whose endpoint is the original nearly G2 conifold and whose parameter is the scale of the glued in asymptotically conical G2 manifold. We show that in many cases such a curve does not exist. The non-existence result is based on a topological result for asymptotically conical G2 manifolds: if the rate of the metric is below -7/2, then the G2 4-form is exact if and only if the manifold is isometric to the 7-dimensional Euclidean space. A similar construction is possible in the nearly K\"ahler case, which we investigate in the same manner with similar results. In this case, the non-existence results is based on a topological result for asymptotically conical Calabi--Yau 6-manifolds: if the rate of the metric is below -3, then the square of the K\"ahler form and the complex volume form can only be simultaneously exact, if the manifold is isometric to the 6-dimensional Euclidean space.