Bianchi type II models in the presence of perfect fluid and anisotropic dark energy

TL;DR

This paper investigates anisotropic Bianchi type II cosmological models with perfect fluid and anisotropic dark energy, solving Einstein's equations under specific assumptions to explore late-time behavior and anisotropy constraints.

Contribution

It introduces a new exact solution for Bianchi type II models with anisotropic dark energy, highlighting conditions for finite anisotropy and axial symmetry in dark energy.

Findings

Anisotropy converges to finite or zero values in late universe.

Dark energy anisotropy is constrained to be axially symmetric.

Models can produce acceleration with non-divergent anisotropy.

Abstract

Spatially homogeneous but totally anisotropic and non-flat Bianchi type II cosmological model has been studied in general relativity in the presence of two minimally interacting fluids; a perfect fluid as the matter fluid and a hypothetical anisotropic fluid as the dark energy fluid. The Einstein's field equations have been solved by applying two kinematical ans\"{a}tze: we have assumed the variation law for the mean Hubble parameter that yields a constant value of deceleration parameter, and one of the components of the shear tensor has been considered proportional to the mean Hubble parameter. We have particularly dwelled on the accelerating models with non-divergent expansion anisotropy as the Universe evolves. Yielding anisotropic pressure, the fluid we consider in the context of dark energy, can produce results that can be produced in the presence of isotropic fluid in accordance with the \Lambda CDM cosmology. However, the derived model gives additional opportunities by being able to allow kinematics that cannot be produced in the presence of fluids that yield only isotropic pressure. We have obtained well behaving cases where the anisotropy of the expansion and the anisotropy of the fluid converge to finite values (include zero) in the late Universe. We have also showed that although the metric we consider is totally anisotropic, the anisotropy of the dark energy is constrained to be axially symmetric, as long as the overall energy momentum tensor possesses zero shear stress.