Hyperkähler manifolds are one of the simplest examples of Einstein manifolds. They are Ricci-flat Riemannian manifolds with special holonomy. In dimension 4, hyperkähler 4-manifolds can be purely described by a triple of symplectic 2-forms that satisfy the pointwise orthonormal condition with respect to the wedge product.
In this dissertation, we proved the compactness of a set of hyperkähler 4-manifolds with boundary under Cheeger-Gromov topology, where we assume only geometric control on the boundary and topological conditions. We showed that our proof can be extended to Einstein 4-manifolds with boundary by assuming only additional topological conditions.
Furthermore, we discuss the period map for K3 surfaces in a differential geometric setting. We gave a simple proof for the surjectivity of the period map, without invoking Yau's theorem on the Calabi conjecture or any algebraic geometry. The key is to show that when a sequence of hyperkähler metrics has bounded period in some sense, then the sequence has a convergent subsequence under Cheeger-Gromov topology.
This thesis studies a special class of Calabi-Yau metrics and singular K\"ahler-Einstein metrics that exhibit cone structures. Building on Donaldson--Sun's 2-step degeneration theory, we make progress on the precise asymptotic behavior of these metrics. The following results are proved, revealing interesting distinctions between the global and local settings.In the global setting, every complete Calabi-Yau metric with Euclidean volume growth and quadratic curvature decay, is polynomially asymptotic to its unique tangent cone at infinity. In the local setting, let $(X,p,\omega)$ be a singular K\"ahler-Einstein metric with an isolated singularity. An algebraic criterion for polynomial convergence to tangent cones is established under certain assumptions. Additionally, examples are provided where the polynomial convergence rate does not hold.
In this thesis, we also study a special class of Hermitian-Yang-Mills connections over complete non-compact K\"ahler manifolds. We introduce the notion of stability for a pair of classes that generalizes the standard slope stability. Under the assumption that both the holomorphic vector bundle and the ambient manifold can be compactified, and the K\"ahler metric satisfies certain asymptotic behavior, we prove that this new stability condition is both sufficient and necessary for the existence of projectively Hermitian-Yang-Mills metrics.
Aubin and Yau proved that every compact Kahler manifold with negative first Chern class admits a unique metric g such that Ric(g) =-g. Understanding how families of these metrics degenerate gives insight into their geometry and is important for understanding the compactification of the moduli space of negative Kahler-Einstein metrics. I study a special class of such families in complex dimension two. Following the work of Sun and Zhang in the Calabi-Yau case, I construct a Kahler-Einstein neck region interpolating between canonical metrics on components of the central fiber. This provides a model for the limiting geometry of metrics in the family.