Speaker
Description
Birkhoff's theorem guarantees a unique vacuum sector in classical general relativity. Many effective quantum gravity models lack this property, leading to a fundamental ambiguity in the choice of vacuum solution and complicating phenomenological analyses that may depend on such a choice. This talk discusses a constructive framework that addresses this problem by deriving fully 4D-covariant actions using, on the one hand, the physical properties of the degrees of freedom involved, namely dust and gravity, and, on the other hand, two further guiding principles: spatial diffeomorphism invariance and a geometric guiding principle. In these models, the dynamics decomposes into independent LTB shells. The imposition of spatial diffeormorphism invariance together with a geometric principle, that guarantees a unique static vacuum solution, severely restricts the form of the shell Hamiltonian and the vacuum metric. The class of covariant actions reconstructed in this framework belongs to the class of generalised extended mimetic gravity models. Finally, the extent to which this framework provides a consistent basis for perturbation theory beyond the static sector and offers new insights into the ambiguity of curvature polymerisation in loop quantum cosmology is discussed.
