The Big Bang could be anisotropic. The case of Bianchi I model

TL;DR

This paper explores anisotropic Bianchi I cosmological models with matter and dark energy, showing that such models generally evolve from an anisotropic Big Bang to a nearly isotropic universe, consistent with observations.

Contribution

It provides a general solution for anisotropic Bianchi I models with arbitrary EoS and analyzes their evolution, including singularities and isotropization processes.

Findings

Universe begins with an anisotropic Big Bang

Models tend to become more isotropic over time

Inflation enhances isotropization, making models observationally indistinguishable from isotropic ones

Abstract

We consider an evolution of anisotropic cosmological model on the example of the Bianchi type I homogeneous universe. It is filled by the mixture of matter and dark energy with an arbitrary barotropic equation of state (EoS). The general solution for this case is found and analyzed. A complete list of possible future singularities for this model is given. Some new solution were obtained for a particular EoSs, e.g. for the Bianchi type I {\Lambda}CDM homogeneous model. It is shown that all special cases corresponding to different EoSs have common properties, provided that now or at another moment of time the Universe is expanding, and the density of the mixture is positive. Then the evolution always begins with an anisotropic "Big Bang" which happened a finite time ago. After that the universe is constantly expanding and, in all cases, with rare exceptions, becomes more isotropic. A particularly strong isotropization is associated with the epoch of inflation. After its completion, the expansion of the universe becomes almost isotropic, and this case cannot be distinguished from isotropic by astronomical observations. This fact allows us to consider an anisotropic cosmological model as a possible candidate for the description of the observed Universe despite the isotropic pattern of expansion.