Axially symmetric Einstein-Straus models

TL;DR

This paper investigates the possibility of static, axially symmetric regions within a Friedman-Lemaitre cosmology, revealing that such regions are generally spherically symmetric unless specific conditions are met, challenging the robustness of the Einstein-Straus model.

Contribution

It demonstrates that static, axially symmetric regions in cosmology are typically spherically symmetric, showing limitations of the Einstein-Straus model for local and cosmic interactions.

Findings

Most general static region is a moving two-sphere with variable radius.

Imposing common energy-momentum conditions enforces spherical symmetry.

The Einstein-Straus model is an exceptional case, not a general solution.

Abstract

The existence of static and axially symmetric regions in a Friedman-Lemaitre cosmology is investigated under the only assumption that the cosmic time and the static time match properly on the boundary hypersurface. It turns out that the most general form for the static region is a two-sphere with arbitrarily changing radius which moves along the axis of symmetry in a determined way. The geometry of the interior region is completely determined in terms of background objects. When any of the most widely used energy-momentum contents for the interior region is imposed, both the interior geometry and the shape of the static region must become exactly spherically symmetric. This shows that the Einstein-Straus model, which is the generally accepted answer for the null influence of the cosmic expansion on the local physics, is not a robust model and it is rather an exceptional and isolated situation. Hence, its suitability for solving the interplay between cosmic expansion and local physics is doubtful and more adequate models should be investigated.