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.
Highlights
Cosmological models allowing for non–trivial inhomogeneities have become a popular tool to analyze cosmological observations without the need of introducing an elusive dark energy source
The preferred configurations are Gpc-scale cosmic void models based on the spherically symmetric Lemaıtre-Tolman (LT) models [2, 3], under the assumption that we live close to a center of a cosmic depression of radius around 1 − 3 Gpc [4, 5, 6]. These configurations are often criticized on the grounds that they violate the Copernican principle, since compliance with the cosmic microwave background (CMB) constraints implies that only one such Gpc structure is allowed and the observer cannot be further away from the origin than ∼ 50 Mpc [7] 1
We show that averaging this inhomogeneous non-spherical configuration leads to a cosmic void that is qualitatively similar to the spherical models discussed by Alexander et al [8]
Summary
Cosmological models allowing for non–trivial inhomogeneities have become a popular tool to analyze cosmological observations without the need of introducing an elusive dark energy source (for a review on a subject and explicit examples the reader is referred to Ref. [1]). The preferred configurations are Gpc-scale cosmic void models based on the spherically symmetric Lemaıtre-Tolman (LT) models [2, 3], under the assumption that we live close to a center of a cosmic depression of radius around 1 − 3 Gpc [4, 5, 6] These configurations are often criticized on the grounds that they violate the Copernican principle, since compliance with the cosmic microwave background (CMB) constraints implies that only one such Gpc structure is allowed and the observer cannot be further away from the origin than ∼ 50 Mpc [7] 1. In this Letter we address this issue by showing that these rather artificial spherical void structures need not exist in its pure form Instead, they approximate configurations that can emerge after coarse-graining and averaging a sufficiently large scale region of a realistic lumpy Universe in which the density distribution is far from spherical.
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