Boundary effects in General Relativity with tetrad variables

Understand

Varying the gravitational Lagrangian produces a boundary contribution that has various physical applications.

• It determines the right boundary terms to be added to the action once boundary conditions are specified, and defines the symplectic structure of covariant phase space methods.
• We study general boundary variations using tetrads instead of the metric.
• This choice streamlines many calculations, especially in the case of null hypersurfaces with arbitrary coordinates, where we show that the spin-1 momentum coincides with the rotational 1-form of isolated horizons.