Boundary effects in General Relativity with tetrad variables
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Boundary effects in General Relativity with tetrad variables Roberto Oliveri1
· Simone Speziale2
Received: 14 May 2020 / Accepted: 26 August 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract 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. The additional gauge symmetry of internal Lorentz transformations leaves however an imprint: the boundary variation differs from the metric one by an exact 3-form. On the one hand, this difference helps in the variational principle: gluing hypersurfaces to determine the action boundary terms for given boundary conditions is simpler, including the most general case of non-orthogonal corners. On the other hand, it affects the construction of Hamiltonian surface charges with covariant phase space methods, which end up being generically different from the metric ones, in both first and second-order formalisms. This situation is treated in the literature gauge-fixing the tetrad to be adapted to the hypersurface or introducing a fine-tuned internal Lorentz transformation depending non-linearly on the fields. We point out and explore the alternative approach of dressing the bare symplectic potential to recover the value of all metric charges, and not just for isometries. Surface charges can also be constructed using a cohomological prescription: in this case we find that the exact 3-form mismatch plays no role, and tetrad and metric charges are equal. This prescription leads however to different charges whether one uses a first-order or second-order Lagrangian, and only for isometries one recovers the same charges.
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Simone Speziale [email protected] Roberto Oliveri [email protected]
1
CEICO, Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Praha 8, Czech Republic
2
CNRS, CPT, UMR 7332, Aix Marseille Univ., Univ. de Toulon, 13288 Marseille, France 0123456789().: V,-vol
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83
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R. Oliveri, S. Speziale
Keywords Tetrad general relativity · Variational principle · Surface charges
Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Variation of the gravitational action with boundaries . . . . . . . . . . . . . . . 2.1 Matching the symplectic potentials: the dressing 2-form . . . . . . . . . . . 3 Geometric decomposition of the boundary variation . . . . . . . . . . . . . . . 3.1 Non-null hypersurfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Preliminaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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