Near horizon limit of the Wang--Yau quasi-local mass

In this article, we compute the limit of the Wang–Yau quasi-local mass on a family of surfaces approaching the apparent horizon (the near horizon limit). Such limit is first considered in [1]. Recently, Pook-Kolb, Zhao, Andersson, Krishnan, and Yau investigated the near horizon limit of the Wang–Yau quasi-local mass in binary black hole mergers in [12] and conjectured that the optimal embeddings approach the isometric embedding of the horizon into ${\mathbb{R}}^3$. Moreover, the quasi-local mass converges to the total mean curvature of the image. The vanishing of the norm of the mean curvature vector implies special properties for the Wang–Yau quasi-local energy and the optimal embedding equation. We utilize these features to prove the existence and uniqueness of the optimal embedding and investigate the minimization of the Wang–Yau quasi-local energy. In particular, we prove the continuity of the quasi-local mass in the near horizon limit.