Late-time behaviour of the tilted Bianchi type VIh models

TL;DR

This paper investigates the late-time evolution of tilted Bianchi type VI_h cosmological models, identifying equilibrium points, stability, and novel attractors like the Mussel attractor through analytical and numerical methods.

Contribution

It provides a comprehensive analysis of the asymptotic behavior of Bianchi type VI_h models, including the discovery of new equilibrium solutions and the characterization of complex attractors.

Findings

Existence of vacuum plane-wave attractors.

Identification of a parameter 'loophole' with no stable equilibrium.

Discovery of the Mussel attractor as a stable closed orbit.

Abstract

We study tilted perfect fluid cosmological models with a constant equation of state parameter in spatially homogeneous models of Bianchi type VI_h using dynamical systems methods and numerical experimentation, with an emphasis on their future asymptotic evolution. We determine all of the equilibrium points of the type VI_h state space (which correspond to exact self-similar solutions of the Einstein equations, some of which are new), and their stability is investigated. We find that there are vacuum plane-wave solutions that act as future attractors. In the parameter space, a `loophole' is shown to exist in which there are no stable equilibrium points. We then show that a Hopf-bifurcation can occur resulting in a stable closed orbit (which we refer to as the Mussel attractor) corresponding to points both inside the loophole and points just outside the loophole; in the former case the closed curves act as late-time attractors while in the latter case these attracting curves will co-exist with attracting equilibrium points. In the special Bianchi type III case, centre manifold theory is required to determine the future attractors. Comprehensive numerical experiments are carried out to complement and confirm the analytical results presented. We note that the Bianchi type VI_h case is of particular interest in that it contains many different subcases which exhibit many of the different possible future asymptotic behaviours of Bianchi cosmological models.