The Hamiltonian of asymptotically Friedmann-Lemaitre-Robertson-Walker spacetimes

We obtain the correct Hamiltonian which describes the dynamics of classes of asymptotic open Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetimes, which includes Tolman geometries. We calculate the surface term that has to be added to the usual Hamiltonian of General Relativity in order to obtain an improved Hamiltonian with well defined functional derivatives. For asymptotic flat FLRW spaces, this surface term is zero, but for asymptotic negative curvature FLRW spaces it is not null in general. In the particular case of the Tolman geometries, they vanish. The surface term evaluated on a particular solution of Einstein's equations may be viewed as the "energy" of this solution with respect to the FLRW spacetime they approach asymptotically. Our results are obtained for a matter content described by a dust fluid, but they are valid for any perfect fluid, including the cosmological constant.