Are braneworlds born isotropic?

Abstract

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 ${\mathcal{F}}_b$ 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 $\gamma >1$, the dimensionless variables representing generic anisotropic and inhomogeneous perturbations decay as $t\to 0$, showing that the model ${\mathcal{F}}_b$ 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 brane-world paradigm represents a description of the very early universe.

arXiv commentChanged to match published version

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• Dynamics of Brane-World Cosmological Models
• A solution to the anisotropy problem in bouncing cosmologies
• Exact nonlinear inhomogeneities in $\Lambda$CDM cosmology
• Compactifying the state space for alternative theories of gravity
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• Asymptotic analysis of spatially inhomogeneous stiff and ultra-stiff cosmologies
• Brane cosmology, Weyl fluid, and density perturbations
• Cosmic No Hair for Collapsing Universes
• Braneworld Isotropization and Magnetic Fields
• 3+1+1 dimensional covariant gravitational dynamics on an asymmetrically embedded brane
• Cosmic No Hair for Braneworlds with a Bulk Dilaton Field
• The Dynamics of Brane-World Cosmological Models