The gravitational Hamiltonian, first order action, Poincar\'e charges and surface terms

TL;DR

This paper develops a consistent Hamiltonian formulation for asymptotically flat gravity using the Holst action, deriving ADM energy-momentum and Poincaré charges within connection variables, ensuring well-defined variational principles.

Contribution

It provides the first complete Hamiltonian formulation for connection-tetrad gravity with asymptotically flat boundary conditions starting from a well-posed action.

Findings

Derivation of the gravitational Hamiltonian from the Holst action with surface terms.

Recovery of ADM energy-momentum from covariant surface terms.

Expression of Poincaré generators in Ashtekar-Barbero variables.

Abstract

We consider the issue of attaining a consistent Hamiltonian formulation, after a 3+1 splitting, of a well defined action principle for asymptotically flat gravity. More precisely, our starting point is the gravitational first order Holst action with surface terms and fall-off conditions that make the variational principle and the covariant phase space formulation well defined for asymptotically flat spacetimes. Keeping all surface terms and paying due attention to subtleties that arise from the different cut-offs at infinity, we give a derivation of the gravitational Hamiltonian starting from this action. The 3+1 decomposition and time gauge fixing results in a well defined Hamiltonian action and a well defined Hamiltonian formulation for the standard -and more general- asymptotic ADM conditions. Unlike the case of the Einstein-Hilbert action with Gibbons-Hawking-York or Hawking-Horowitz terms, here we {\it {do}} recover the ADM energy-momentum from the covariant surface term also when more general variations respecting asymptotic flatness are allowed. Additionally, our strategy yields a derivation of the parity conditions for connection variables independent of the conditions given by Regge and Teitelboim for ADM variables. Finally, we exhibit the other Poincar\'e generators in terms of real Ashtekar-Barbero variables. We complement previous constructions in self-dual variables by pointing out several subtleties and refining the argument showing that -on shell- they coincide with the ADM charges. Our results represent the first consistent treatment of the Hamiltonian formulation for the connection-tetrad gravitational degrees of freedom, starting from a well posed action, in the case of asymptotically flat boundary conditions.