Localizing branes with bifurcating bulks

TL;DR

This paper analyzes the evolution of bulk 5-fluid models with embedded branes, revealing bifurcations, stability properties, and conditions for gravity localization, using dynamical systems and phase space analysis.

Contribution

It introduces a new dynamical system formulation for brane-bulk models with fluids, identifies bifurcations and attractors, and demonstrates conditions for gravity localization.

Findings

Linear fluids exhibit a transcritical bifurcation at γ=-1/2.

An attractor exists at λ=1/2 for nonlinear fluids.

Gravity-localizing solutions satisfy all energy conditions.

Abstract

We study the problem of evolution of bulk 5-fluids having an embedded braneworld with a flat, de Sitter, or anti-de Sitter geometry. We introduce new variables to express the Einstein equations as a dynamical system that depends on the equation of state parameter $\gamma$ and exponent $\lambda$. For linear fluids (i.e., $\lambda=1$), our formulation leads to a partial decoupling of the equations and thus to an exact solution. We find that such a fluid develops a transcritical bifurcation around the value $\gamma=-1/2$, and study how this behaviour affects to stability of the solutions. For nonlinear fluids, the situation is more diverse. We find an overall attractor at $\lambda=1/2$ and draw enough phase portraits to exhibit in detail the overall dynamics. We show that the value $\lambda =3/2$ is structurally unstable and typical for other forms of $\lambda$. Consequently, we observe a noticeable dependence of the qualitative behaviour of the solutions on different `polytropic' forms of the fluid bulk. In addition, we prove the existence of a Dulac function for nonlinear fluids, signifying the impossibility of closed orbits in certain subsets of the phase space. We also provide ample numerical evidence of gravity localizing solutions on the brane which satisfy all energy conditions.