Non-Spherical Voids: the Best Alternative to Dark Energy?

Abstract

The constraints from current cosmological observations strongly support the ΛCDM model in which late time cosmic dynamics is dominated by a nonzero cosmological constant or by an exotic and elusive source like “dark energy”. However, these constraints can also be met if we assume a non–perturbative treatment of cosmological inhomogeneities and that our location lies within an under–dense or “void” region of at least 300 Mpc characteristic length. Since fitting observational data severely constrains our position to be very near the void center in spherical void models, we propose in this article a toy model of a less idealized non–spherical configuration that may fit this data without the limitations associated with spherical symmetry. In particular, the class of quasi–spherical Szekeres models provides sufficient degrees of freedom to describe the evolution of non–spherical inhomogeneities, including a configuration consisting of several elongated supercluster–like overdense filaments with large underdense regions between them. We summarize a recently published example of such configuration, showing that it yields a reasonable coarse-grained description of realistic observd structures. While the density distribution is not spherically symmetric, its proper volume average yields a spherical density void profile of 250 Mpc that may be further improved to agree with observations. Also, once we consider our location to lie within a non-spherical void, the definition of a “center” location becomes more nuanced, and thus the constraints placed by the fitting of observations on our position with respect to this location become less restrictive.