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The field equations and the boundary conditions are inextricably connected and the latter can in no way be considered less important than the formerV. Fock, The theory of space, time and gravitation
[I]t is natural to regulate infinite sized systems by imposing boundary conditions at finite distance, often described as placing the system in a box. This idea has a long history in the gravitational context (see e.g. [15–27]) where it is common to impose a Dirichlet boundary condition, fixing the induced metric at the walls of the box1 .https://arxiv.org/abs/1508.02515