Are braneworlds born isotropic?

It has recently been suggested that an isotropic singularity may be a generic feature of brane cosmologies, even in the inhomogeneous case. Using the covariant and gauge-invariant approach we present a detailed analysis of linear perturbations of the isotropic model Fb, which is a past attractor in the phase space of homogeneous Bianchi models on the brane. We find that for matter with an equation of state parameter γ>1, the dimensionless variables representing generic anisotropic and inhomogeneous perturbations decay as t⃗0, showing that the model Fb is asymptotically stable in the past. We conclude that brane universes are born with isotropy naturally built-in, contrary to standard cosmology. The observed large-scale homogeneity and isotropy of the Universe can therefore be explained as a consequence of the initial conditions if the braneworld paradigm represents a description of the very early Universe