LTB spacetimes in terms of Dirac observables

TL;DR

This paper develops a gauge-invariant Hamiltonian formalism for spherically symmetric spacetimes in General Relativity using Dirac observables and dust matter, connecting it to Lemaitre--Tolman--Bondi metrics for gravitational analysis.

Contribution

It provides a new explicit construction of Dirac observables and a physical Hamiltonian in spherically symmetric GR using dust fields, linking to LTB metrics.

Findings

Solutions correspond to Lemaitre--Tolman--Bondi metrics

The formalism captures all spherically symmetric gravitational experiments

Infinite conserved charges decouple Goldstone bosons

Abstract

The construction of Dirac observables, that is gauge invariant objects, in General Relativity is technically more complicated than in other gauge theories such as the standard model due to its more complicated gauge group which is closely related to the group of spacetime diffeomorphisms. However, the explicit and usually cumbersome expression of Dirac observables in terms of gauge non invariant quantities is irrelevant if their Poisson algebra is sufficiently simple. Precisely that can be achieved by employing the relational formalism and a specific type of matter proposed originally by Brown and Kucha{\v r}, namely pressureless dust fields. Moreover one is able to derive a compact expression for a physical Hamiltonian that drives their physical time evolution. The resulting gauge invariant Hamiltonian system is obtained by Higgs -- ing the dust scalar fields and has an infinite number of conserved charges which force the Goldstone bosons to decouple from the evolution. In previous publications we have shown that explicitly for cosmological perturbations. In this article we analyse the spherically symmetric sector of the theory and it turns out that the solutions are in one--to--one correspondence with the class of Lemaitre--Tolman--Bondi metrics. Therefore the theory is capable of properly describing the whole class of gravitational experiments that rely on the assumption of spherical symmetry.