Quasi-local energy with respect to a static spacetime
  This article considers the quasi-local energy in reference to a general static spacetime. We follow the approach developed by the authors in [19, 20, 7, 9] and define the quasi-local energy as a difference of surface Hamiltonians, which are derived from the Einstein-Hilbert action. The new quasi-local energy provides an effective gauge independent measurement of how far a spacetime deviates away from the reference static spacetime on a finitely extended region. 
Po-Ning Chen, Mu-Tao Wang, Ye-Kai Wang and Shing-Tung Yau
math.DG gr-qc