Review on exact and perturbative deformations of the Einstein-Straus model: uniqueness and rigidity results

TL;DR

This review summarizes the uniqueness and rigidity results of the Einstein-Straus model, emphasizing that non-spherical deformations generally require perturbing the background, with a focus on linear perturbations under symmetry assumptions.

Contribution

It compiles and discusses known results on the uniqueness and perturbative stability of the Einstein-Straus model, highlighting the limitations on non-spherical generalizations.

Findings

Non-spherical deformations require perturbing the FLRW background.

Linear perturbations with stationarity and axial symmetry are analyzed.

The model's rigidity limits non-spherical generalizations.

Abstract

The Einstein-Straus model consists of a Schwarzschild spherical vacuole in a Friedman-Lema\^{\i}tre-Robertson-Walker (FLRW) dust spacetime (with or without \Lambda). It constitutes the most widely accepted model to answer the question of the influence of large scale (cosmological) dynamics on local systems. The conclusion drawn by the model is that there is no influence from the cosmic background, since the spherical vacuole is static. Spherical generalizations to other interior matter models are commonly used in the construction of lumpy inhomogeneous cosmological models. On the other hand, the model has proven to be reluctant to admit non-spherical generalizations. In this review, we summarize the known uniqueness results for this model. These seem to indicate that the only reasonable and realistic non-spherical deformations of the Einstein-Straus model require perturbing the FLRW background. We review results about linear perturbations of the Einstein-Straus model, where the perturbations in the vacuole are assumed to be stationary and axially symmetric so as to describe regions (voids in particular) in which the matter has reached an equilibrium regime.