Covariant phase space with null boundaries

Kai Shi,Xuan Wang,Yihong Xiu,Hongbao Zhang

Figure 2. The null hypersurface ${{ \mathcal N }}_{1}$ is the inner boundary of the theory such as the black hole event horizon, and ${{ \mathcal N }}_{2}$ is the outer boundary, whose limit can be thought of as the null infinity in an asymptotically flat spacetime.