Stabilization of Large Scale Structure by Adhesive Gravitational Clustering

The interplay between gravitational and dispersive forces in a multi-streamed medium leads to an effect which is exposed in the present note as the genuine driving force of stabilization of large-scale structure. The conception of `adhesive gravitational clustering' is advanced to interlock the fairly well-understood epoch of formation of large-scale structure and the onset of virialization into objects that are dynamically in equilibrium with their large-scale structure environment. The classical `adhesion model' is opposed to a class of more general models traced from the physical origin of adhesion in kinetic theory.Comment: LaTeX 8 pages, incl. 2 figures; uses paspconf.sty, epsf.sty, matches published version, `From Stars to the Universe', Workshop on Cosmology, Shanghai 199

Backreaction Issues in Relativistic Cosmology and the Dark Energy Debate

The effective evolution of an inhomogeneous universe model in Einstein's theory of gravitation may be described in terms of spatially averaged scalar variables. This evolution can be modeled by solutions of a set of Friedmann equations for an effective scale factor, with matter and backreaction source terms, where the latter can be represented by a minimally coupled scalar field (`morphon field'). We review the basic steps of a description of backreaction effects in relativistic cosmology that lead to refurnishing the standard cosmological equations, but also lay down a number of unresolved issues in connection with their interpretation within observational cosmology.Comment: 17 pages; Lecture provided at the XII. Brazilian School of Cosmology and Gravitation, Mangaratiba, Rio de Janeiro, Brazil, September 2006; matches version to be published by AI

Towards physical cosmology: geometrical interpretation of Dark Energy, Dark Matter and Inflation without fundamental sources

We outline the key-steps towards the construction of a physical, fully relativistic cosmology, in which we aim to trace Dark Energy and Dark Matter back to physical properties of space. The influence of inhomogeneities on the effective evolution history of the Universe is encoded in backreaction terms and expressed through spatially averaged geometrical invariants. These are absent and interpreted as missing dark fundamental sources in the standard model. In the inhomogeneous case they can be interpreted as energies of an emerging scalar field (the morphon). These averaged invariants vanish for a homogeneous geometry, where the morphon is in an unstable equilibrium state. If this state is perturbed, the morphon can act as a classical inflaton in the Early Universe and its de-balanced energies can mimic the dark sources in the Late Universe, depending on spatial scale as Dark Energy or as Dark Matter, respectively. We lay down a line of arguments that is qualitatively conclusive, and we outline open problems of quantitative nature, related to the interpretation of observations.Comment: 14 pages, 6 figures; presented at the International Conference on Two Cosmological Models, Universidad Iberoamericana Ciudad de M\'exico - Department of Physics and Mathematics, November 19 (2010

Multiscale approach to inhomogeneous cosmologies

The backreaction of inhomogeneities on the global expansion history of the Universe suggests a possible link of the formation of structures to the recent accelerated expansion. In this paper, the origin of this conjecture is illustrated and a model without Dark Energy that allows for a more explicit investigation of this link is discussed. Additionally to this conceptually interesting feature, the model leads to a LCDM-like distance-redshift relation that is consistent with SN data.Comment: 5 pages, 4 figures, contributed talk at the Workshop: New Directions in Modern Cosmology, Leiden, The Netherlands, 27.9.-1.10. (2010

Lagrangian perturbations and the matter bispectrum I: fourth-order model for non-linear clustering

We investigate the Lagrangian perturbation theory of a homogeneous and isotropic universe in the non-relativistic limit, and derive the solutions up to the fourth order. These solutions are needed for example for the next-to-leading order correction of the (resummed) Lagrangian matter bispectrum, which we study in an accompanying paper. We focus on flat cosmologies with a vanishing cosmological constant, and provide an in-depth description of two complementary approaches used in the current literature. Both approaches are solved with two different sets of initial conditions---both appropriate for modelling the large-scale structure. Afterwards we consider only the fastest growing mode solution, which is not affected by either of these choices of initial conditions. Under the reasonable approximation that the linear density contrast is evaluated at the initial Lagrangian position of the fluid particle, we obtain the nth-order displacement field in the so-called initial position limit: the nth order displacement field consists of 3(n-1) integrals over n linear density contrasts, and obeys self-similarity. Then, we find exact relations between the series in Lagrangian and Eulerian perturbation theory, leading to identical predictions for the density contrast and the peculiar-velocity divergence up to the fourth order.Comment: 31 pages, matches published version in JCAP, added an extra section which discusses and motivates the choice of initial conditions, extended the title for the sake of precisio

Cosmological parameters are dressed

In the context of the averaging problem in relativistic cosmology, we provide a key to the interpretation of cosmological parameters by taking into account the actual inhomogeneous geometry of the Universe. We discuss the relation between `bare' cosmological parameters determining the cosmological model, and the parameters interpreted by observers with a ``Friedmannian bias'', which are `dressed' by the smoothed-out geometrical inhomogeneities of the surveyed spatial region.Comment: LateX, PRLstyle, 4 pages; submitted to PR

Multiscale cosmology and structure-emerging Dark Energy: A plausibility analysis

Cosmological backreaction suggests a link between structure formation and the expansion history of the Universe. In order to quantitatively examine this connection, we dynamically investigate a volume partition of the Universe into over-- and underdense regions. This allows us to trace structure formation using the volume fraction of the overdense regions \lambda_{\CM} as its characterizing parameter. Employing results from cosmological perturbation theory and extrapolating the leading mode into the nonlinear regime, we construct a three--parameter model for the effective cosmic expansion history, involving \lambda_{\CM_{0}}, the matter density \Omega_{m}^{\CD_{0}}, and the Hubble rate H_{\CD_{0}} of today's Universe. Taking standard values for \Omega_{m}^{\CD_{0}} and H_{\CD_{0}} as well as a reasonable value for \lambda_{\CM_{0}}, that we derive from $N$--body simulations, we determine the corresponding amounts of backreaction and spatial curvature. We find that the obtained values that are sufficient to generate today's structure also lead to a $\Lambda$CDM--like behavior of the scale factor, parametrized by the same parameters \Omega_{m}^{\CD_{0}} and H_{\CD_{0}}, but without a cosmological constant. However, the temporal behavior of \lambda_{\CM} does not faithfully reproduce the structure formation history. Surprisingly, however, the model matches with structure formation with the assumption of a low matter content, \Omega_{m}^{\CD_{0}}\approx3\%, a result that hints to a different interpretation of part of the backreaction effect as kinematical Dark Matter. (truncated)Comment: 25 pages, 10 figures, includes calculation of luminosity distances, matches published version in Phys. Rev.

Regional averaging and scaling in relativistic cosmology

Averaged inhomogeneous cosmologies lie at the forefront of interest, since cosmological parameters like the rate of expansion or the mass density are to be considered as volume-averaged quantities and only these can be compared with observations. For this reason the relevant parameters are intrinsically scale-dependent and one wishes to control this dependence without restricting the cosmological model by unphysical assumptions. In the latter respect we contrast our way to approach the averaging problem in relativistic cosmology with shortcomings of averaged Newtonian models. Explicitly, we investigate the scale-dependence of Eulerian volume averages of scalar functions on Riemannian three-manifolds. We propose a complementary view of a Lagrangian smoothing of (tensorial) variables as opposed to their Eulerian averaging on spatial domains. This program is realized with the help of a global Ricci deformation flow for the metric. We explain rigorously the origin of the Ricci flow which, on heuristic grounds, has already been suggested as a possible candidate for smoothing the initial data set for cosmological spacetimes. The smoothing of geometry implies a renormalization of averaged spatial variables. We discuss the results in terms of effective cosmological parameters that would be assigned to the smoothed cosmological spacetime.Comment: LateX, IOPstyle, 48 pages, 11 figures; matches published version in C.Q.