Theory of a symmetric tensor field with boundary: Kac-Moody algebras in linearized gravity

In this paper we consider four-dimensional (4D) linearized gravity (LG) with a planar boundary, where the most general boundary conditions are derived following Symanzik’s approach. The boundary breaks diffeomorphism invariance, and this results in a breaking of the corresponding Ward identity. From this, on the boundary we find two conserved currents that form an algebraic structure of the Kac-Moody type, with a central charge proportional to the action “coupling.” Moreover, we identify the boundary degrees of freedom, which are two symmetric rank-2 tensor fields, and derive the symmetry transformations, which are diffeomorphisms. The corresponding most general 3D action is obtained and a contact with the higher dimensional theory is established by requiring that the 3D equations of motion coincide with the 4D boundary conditions. Through this kind of holographic procedure, we find two solutions: LG for a single tensor field and LG for two tensor fields with a mixing term. Curiously, we find that the Symanzik’s 4D boundary term that governs the whole procedure contains a mass term of the Fierz-Pauli type for the bulk graviton.