Abstract
A class of boundary conditions for canonical general relativity is proposed and studied at the quasi-local level. It is shown that for untrapped or marginal surfaces, fixing the area element on the 2-surface (rather than the induced 2-metric) and the angular momentum surface density is enough to have a functionally differentiable Hamiltonian, thus providing definition of conserved quantities for the quasi-local regions. If on the boundary the evolution vector normal to the 2-surface is chosen to be proportional to the dual expansion vector, we obtain a generalization of the Hawking energy associated with a generalized Kodama vector. This vector plays the same role for the stationary untrapped boundary conditions that the stationary Killing vector plays for stationary black holes. When the dual expansion vector is null, the boundary conditions reduce to those given by the non-expanding horizons and null trapping horizons.
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