Cosmic anisotropic doomsday in Bianchi type I universes

TL;DR

This paper investigates how anisotropic spacetime in Bianchi type I cosmologies can lead to finite-time future singularities, including a novel 'Vacuum Rip' caused by shear divergence in vacuum models.

Contribution

It identifies and characterizes a new type of finite-time singularity in anisotropic cosmologies, extending understanding of cosmic evolution in Bianchi type I models.

Findings

Vacuum Bianchi I models can rip in finite time due to shear divergence.

Non-vacuum solutions with matter also exhibit anisotropic future singularities.

The 'Vacuum Rip' is driven by shear scalar blow-up in vacuum Kasner cosmologies.

Abstract

In order to investigate if the anisotropy of the spacetime may induce future singularities at a finite value of the cosmic time on the evolution of cosmological models, we study vacuum and non-vacuum Bianchi type I spacetimes exhibiting such future singularities. We show that in the case of Kasner vacuum cosmologies the spacetime may rip itself apart in a finite time, and only in the direction corresponding to the unique scale factor which diverges at this finite value of the cosmic time. The other two directional scale factors and the average scale factor do not diverge and tend to zero at this time, while the directional and average expansion rates also become infinite. Due to the absence of the matter content this anisotropic future singularity is induced by the shear scalar, which also blows up at this time. We call such a singularity "Vacuum Rip". For non-vacuum solutions we discuss fully anisotropic Bianchi type I spacetimes filled with a stiff fluid and ellipsoidal (axisymmetric) cosmological models filled with matter with isotropic and anisotropic barotropic pressure, and characterized by $\sigma/\theta=const$, where $\sigma$ and $\theta$ are the shear scalar and the expansion scalar respectively.