Browsing by Author "Coley, Alan"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Open Access
    Are braneworlds born isotropic?
    (2004) Dunsby, Peter K S; Goheer, Naureen; Bruni, Marco; Coley, Alan
    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 ${cal F}_b$ 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 $tto 0$, showing that the model ${cal F}_b$ 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. Comment: Changed to match published version
  • Thumbnail Image
    Item
    Open Access
    Large-scale perturbations on the brane and the isotropy of the cosmological singularity
    (2004) Goheer, Naureen; Dunsby, Peter K S; Coley, Alan; Bruni, Marco
    We present the complete set of propagation and constraint equations for the kinematic and non-local first order quantities which describe general linear inhomogeneous and anisotropic perturbations of a flat FRW braneworld with vanishing cosmological constant and decompose them in the standard way into their scalar, vector and tensor contributions. A detailed analysis of the perturbation dynamics is performed using dimensionless variables that are specially tailored for the different regimes of interest; namely, the low energy GR regime, the high energy regime and the dark energy regime. Tables are presented for the evolution of all the physical quantities, making it easy to do a detailed comparison of the past asymptotic behaviour of the perturbations of these models. We find results that exactly match those obtained in the analysis of the spatially inhomogeneous $G_{2}$ braneworld cosmologies presented recently; i.e., that isotropization towards the ${cal F}_b$ model occurs for $gamma > 4/3$.