QUASI-LOCALIZATION OF BONDI-SACHS ENERGY-LOSS

A formula is given for the variation of the Hawking energy along and o ne-parameter family of compact spatial 2-surfaces. A surface for which one null expansion is positive and the other negative has a preferred orientation, with a spatial or null normal direction being called out going or ingoing as the area increases or decreases respectively. A ge ometrically natural way to propagate such a surface through a hypersur face is to choose the foliation such that the null expansions are cons tant over each surface. For such uniformly expanding foliations, the H awking energy is non-decreasing in any outgoing direction, and non-inc reasing in any ingoing direction, assuming the dominant energy conditi on. It follows that the Hawking energy is non-negative if the foliatio n is bounded at the inward end by either a point or a marginal surface , and in the latter case satisfies the Penrose-Gibbons isoperimetric i nequality. The Bondi-Sachs energy may be expressed as a limit of the H awking energy at conformal infinity, and the energy-variation formula reduces at conformal infinity to the Bondi-Sachs energy-loss formula. The relevance to the cosmic censorship hypothesis is discussed.