General Bianchi IX dynamics in bouncing braneworld cosmology: homoclinic chaos and the BKL conjecture

Abstract

We examine the dynamics of a Bianchi IX model on a 4-dim brane embedded in a 5-dim conformally flat empty bulk with a timelike extra dimension. Einstein's equations on the brane reduces to a 6-dim Hamiltonian dynamical system with additional terms that implement nonsingular bounces in the model. The phase space of the model has two critical points (a saddle-center-center and a center-center-center) in a finite region of phase space, and two asymptotic de Sitter critical points, one acting as an attractor to late-time dynamics. The saddle-center-center engenders in the phase space the topology of stable and unstable 4-dim cylinders $R \times S^3$, where $R$ is a saddle direction and $S^3$ is the center manifold of unstable periodic orbits (the nonlinear extension of the center-center sector). By a proper canonical transformation we separate the degrees of freedom of the dynamics into one degree connected with the expansion/contraction of the scales of the model, and two rotational degrees of freedom connected to the $S^3$. The typical dynamical flow is then an oscillatory mode about a neighborhood of the cylinders. For the stable and unstable cylinders the oscillatory motion about the separatrix towards the bounce leads to the homoclinic transversal intersection of the cylinders, as shown numerically in two distinct experiments. We show that the homoclinic intersection manifold has the topology of $R \times S^2$ consisting of homoclinic orbits biasymptotic to the center manifold $S^3$. This behavior defines a {\it chaotic saddle} associated with $S^3$, indicating that the intersection points of the cylinders have the nature of a Cantor set with a compact support $S^2$, characterizing chaos in the model. We discuss the oscillatory approach to the bounce together with its chaotic behavior, and analogous features present in the BKL conjecture in general relativity.