arXiv2009
We discuss some geometric problems related to the definitions of quasilocal mass proposed by Brown-York \cite{BYmass1} \cite{BYmass2} and Liu-Yau \cite{LY1} \cite{LY2}. Our discussion consists of three parts. In the first part, we propose a new variational problem on compact manifolds with boundary, which is motivated by the study of Brown-York mass. We prove that critical points of this variation problem are exactly static metrics. In the second part, we derive a derivative formula for the Brown-York mass of a smooth family of closed 2 dimensional surfaces evolving in an ambient three dimensional manifold. As an interesting by-product, we are able to write the ADM mass \cite{ADM61} of an asymptotically flat 3-manifold as the sum of the Brown-York mass of a coordinate sphere $S_r$ and an integral of the scalar curvature plus a geometrically constructed function $Φ(x)$ in the asymptotic region outside $S_r $. In the third part, we prove that for any closed, spacelike, 2-surface $Σ$ in the Minkowski space $\R^{3,1}$ for which the Liu-Yau mass is defined, if $Σ$ bounds a compact spacelike hypersurface in $\R^{3,1}$, then the Liu-Yau mass of $Σ$ is strictly positive unless $Σ$ lies on a hyperplane. We also show that the examples given by Ó Murchadha, Szabados and Tod \cite{MST} are special cases of this result.