Cosmic spherical void via coarse-graining and averaging non-spherical structures

TL;DR

This paper presents a toy model using non-spherical inhomogeneities to simulate cosmic voids, demonstrating that averaging such structures can produce spherical profiles consistent with observations, and relaxes the need for a special observation point.

Contribution

It introduces a non-spherical inhomogeneous model based on the Szekeres solution, showing how coarse-graining yields spherical voids and reduces the significance of our position in the universe.

Findings

A coarse-grained average of non-spherical structures produces a spherical void profile.

Non-spherical models lessen the constraints on our cosmic position.

The model aligns with observed void sizes and profiles.

Abstract

Inhomogeneous cosmological models are able to fit cosmological observations without dark energy under the assumption that we live close to the "center" of a very large-scale under-dense region. Most studies fitting observations by means of inhomogeneities also assume spherical symmetry, and thus being at (or very near) the center may imply being located at a very special and unlikely observation point. We argue that such spherical voids should be treated only as a gross first approximation to configurations that follow from a suitable smoothing out of the non-spherical part of the inhomogeneities on angular scales. In this Letter we present a toy construction that supports the above statement. The construction uses parts of the Szekeres model, which is inhomogeneous and anisotropic thus it also addresses the limitations of spherical inhomogeneities. By using the thin-shell approximation (which means that the Israel-Darmois continuity conditions are not fulfilled between the shells) we construct a model of evolving cosmic structures, containing several elongated supercluster-like structures with underdense regions between them, which altogether provides a reasonable coarse-grained description of cosmic structures. While this configuration is not spherically symmetric, its proper volume average yields a spherical void profile of 250 Mpc that roughly agrees with observations. Also, by considering a non-spherical inhomogeneity, the definition of a "center" location becomes more nuanced, and thus the constraints placed by fitting observations on our position with respect to this location become less restrictive.