Non-stationary Energy of Perfect Fluid Sources in General Relativity

Abstract: The ADM energy for asymptotically flat spacetimes or its generalizations to asymptotically non-flat spacetimes measure the energy content of a stationary spacetime, such as a single black hole. Such a stationary energy is given as a geometric invariant of the spatial hypersurface of the spacetime and is expressed as an integral on the boundary of the hypersurface. For non-stationary spacetimes, there is a refinement of the ADM energy, the so-called Dain's invariant that measures the non-stationary part, the gravitational radiation component, of the total energy. Dain's invariant uses the metric and the extrinsic curvature of the spatial hypersurface together with the so-called approximate Killing initial data and vanishes for stationary spacetimes. In our earlier work [Phys.Rev.D 101 (2020)2, 024035], we gave a reformulation of the non-stationary energy for vacuum spacetimes in the Hamiltonian form of General Relativity written succinctly in the Fischer-Marsden form. That formulation is relevant for merging black holes or other compact sources. Here we extend this formulation to non-vacuum spacetimes with a perfect fluid source. This is expected to be relevant for spacetimes that have a compact star, say a neutron star colliding with a black hole or another non-vacuum object.