Article

# A gradient expansion for cosmological backreaction

Journal of Cosmology and Astroparticle Physics (Impact Factor: 6.04). 12/2011; 2012(03). DOI: 10.1088/1475-7516/2012/03/026

Source: arXiv

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**ABSTRACT:**The Affleck–Dine mechanism is an attractive scenario for generating the observed baryon asymmetry of the universe utilizing flat directions in the scalar potential of supersymmetric theories. In this mini review, we describe this mechanism in its original version, its explicit realization within the minimal supersymmetric standard model and its variants. We discuss the formation of a condensate along the flat directions in the inflationary era, its post-inflationary evolution leading to baryogenesis and its fate. In some cases the condensate may fragment into non-topological solitons, known as Q-balls, during its evolution. In models of gravity-mediated supersymmetry breaking, the Q-balls can be long-lived, in which case their decay will be the source of all baryons and dark matter in the form of the lightest supersymmetric particle. In models of gauge-mediated supersymmetry breaking, the Q-balls can be absolutely stable and form dark matter that can be searched for directly.New Journal of Physics 12/2012; 14(12):125013. · 4.06 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Kinematical and dynamical properties of a generic inhomogeneous cosmological model, spatially averaged with respect to free-falling (generalized fundamental) observers, are investigated for the matter model irrotational dust. Paraphrasing a previous Newtonian investigation, we present a relativistic generalization of a backreaction model based on volume-averaging the Relativistic Zeldovich Approximation. In this model we investigate the effect of kinematical backreaction on the evolution of cosmological parameters as they are defined in an averaged inhomogeneous cosmology, and we show that the backreaction model interpolates between orthogonal symmetry properties by covering subcases of the plane-symmetric solution, the Lemaitre-Tolman-Bondi solution and the Szekeres solution. We so obtain a powerful model that lays the foundations for quantitatively addressing curvature inhomogeneities as they would be interpreted as Dark Energy or Dark Matter in a quasi-Newtonian cosmology. The present model, having a limited architecture due to an assumed FLRW background, is nevertheless capable of replacing 1/4 of the needed amount for Dark Energy on domains of 200 Mpc in diameter for typical (one-sigma) fluctuations in a CDM initial power spectrum. However, the model is far from explaining Dark Energy on larger scales (spatially), where a 6% effect on 400 Mpc domains is identified that can be traced back to an on average negative intrinsic curvature today. One drawback of the quantitative results presented is the fact that the epoch when backreaction is effective on large scales and leads to volume acceleration lies in the future. We discuss this issue in relation to the initial spectrum, the Dark Matter problem, the coincidence problem, and the fact that large-scale Dark Energy is an effect on the past light cone (not spatial), and we pinpoint key elements of future research.Physical review D: Particles and fields 03/2013; 87(12). - [Show abstract] [Hide abstract]

**ABSTRACT:**We describe the irrotational dust component of the universe in terms of a relativistic gradient expansion and transform the resulting synchronous metric to a Newtonian coordinate system. The two metrics are connected via a space-like displacement field and a time-like perturbation, providing a relativistic generalization of the transformation from Lagrangian to Eulerian coordinates. The relativistic part of the displacement field generates already at initial time a non-local density perturbation at second order. This is a purely relativistic effect since it originates from space-time mixing. We give two options, the passive and the active approach, on how to include the relativistic corrections for example in N-body simulations. In the passive approach we treat the corrections as a non-Gaussian modification of the initial Gaussian field (primordial non-Gaussianity could be incorporated as well). The induced non-Gaussianity depends on scale and the redshift at which initial conditions are set, with f_NL ~ few for small enough scales and redshifts. In the active approach we show how to use the relativistic trajectory to obtain the initial displacement and velocity of particles for N-body simulations without modifying the initial Gaussian field.Physical review D: Particles and fields 04/2013; 87(12).

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