809 research outputs found
Information Entropy in Cosmology
The effective evolution of an inhomogeneous cosmological model may be
described in terms of spatially averaged variables. We point out that in this
context, quite naturally, a measure arises which is identical to a fluid model
of the `Kullback-Leibler Relative Information Entropy', expressing the
distinguishability of the local inhomogeneous mass density field from its
spatial average on arbitrary compact domains. We discuss the time-evolution of
`effective information' and explore some implications. We conjecture that the
information content of the Universe -- measured by Relative Information Entropy
of a cosmological model containing dust matter -- is increasing.Comment: LateX, PRLstyle, 4 pages; to appear in PR
Global gravitational instability of FLRW backgrounds - interpreting the dark sectors
The standard model of cosmology is based on homogeneous-isotropic solutions
of Einstein's equations. These solutions are known to be gravitationally
unstable to local inhomogeneous perturbations, commonly described as evolving
on a background given by the same solutions. In this picture, the FLRW
backgrounds are taken to describe the average over inhomogeneous perturbations
for all times. We study in the present article the (in)stability of FLRW dust
backgrounds within a class of averaged inhomogeneous cosmologies. We examine
the phase portraits of the latter, discuss their fixed points and orbital
structure and provide detailed illustrations. We show that FLRW cosmologies are
unstable in some relevant cases: averaged models are driven away from them
through structure formation and accelerated expansion. We find support for the
proposal that the dark components of the FLRW framework may be associated to
these instability sectors. Our conclusion is that FLRW cosmologies have to be
considered critically as for their role to serve as reliable models for the
physical background.Comment: 15 pages, 13 figures, 1 table. Matches published version in CQ
Lagrangian theory of structure formation in relativistic cosmology I: Lagrangian framework and definition of a nonperturbative approximation
In this first paper we present a Lagrangian framework for the description of
structure formation in general relativity, restricting attention to
irrotational dust matter. As an application we present a self-contained
derivation of a general-relativistic analogue of Zel'dovich's approximation for
the description of structure formation in cosmology, and compare it with
previous suggestions in the literature. This approximation is then
investigated: paraphrasing the derivation in the Newtonian framework we provide
general-relativistic analogues of the basic system of equations for a single
dynamical field variable and recall the first-order perturbation solution of
these equations. We then define a general-relativistic analogue of Zel'dovich's
approximation and investigate its implications by functionally evaluating
relevant variables, and we address the singularity problem. We so obtain a
possibly powerful model that, although constructed through extrapolation of a
perturbative solution, can be used to put into practice nonperturbatively, e.g.
problems of structure formation, backreaction problems, nonlinear properties of
gravitational radiation, and light-propagation in realistic inhomogeneous
universe models. With this model we also provide the key-building blocks for
initializing a fully relativistic numerical simulation.Comment: 21 pages, content matches published version in PRD, discussion on
singularities added, some formulas added, some rewritten and some correcte
Relativistic cosmological perturbation scheme on a general background: scalar perturbations for irrotational dust
In standard perturbation approaches and N-body simulations, inhomogeneities
are described to evolve on a predefined background cosmology, commonly taken as
the homogeneous-isotropic solutions of Einstein's field equations
(Friedmann-Lema\^itre-Robertson-Walker (FLRW) cosmologies). In order to make
physical sense, this background cosmology must provide a reasonable description
of the effective, i.e. spatially averaged, evolution of structure
inhomogeneities also in the nonlinear regime. Guided by the insights that (i)
the average over an inhomogeneous distribution of matter and geometry is in
general not given by a homogeneous solution of general relativity, and that
(ii) the class of FLRW cosmologies is not only locally but also globally
gravitationally unstable in relevant cases, we here develop a perturbation
approach that describes the evolution of inhomogeneities on a general
background being defined by the spatially averaged evolution equations. This
physical background interacts with the formation of structures. We derive and
discuss the resulting perturbation scheme for the matter model `irrotational
dust' in the Lagrangian picture, restricting our attention to scalar
perturbations.Comment: 18 pages. Matches published version in CQ
Effective inhomogeneous inflation: curvature inhomogeneities of the Einstein vacuum
We consider spatially averaged inhomogeneous universe models and argue that,
already in the absence of sources, an effective scalar field arises through
foliating and spatially averaging inhomogeneous geometrical curvature
invariants of the Einstein vacuum. This scalar field (the `morphon') acts as an
inflaton, if we prescribe a potential of some generic form. We show that, for
any initially negative average spatial curvature, the morphon is driven through
an inflationary phase and leads - on average - to a spatially flat, homogeneous
and isotropic universe model, providing initial conditions for pre-heating and,
by the same mechanism, a possibly natural self-exit.Comment: 9 pages, 2 figures, to appear in Class. Quant. Grav. as Fast Track
Communicatio
A cosmic equation of state for the inhomogeneous Universe: can a global far-from-equilibrium state explain Dark Energy?
A system of effective Einstein equations for spatially averaged scalar
variables of inhomogeneous cosmological models can be solved by providing a
`cosmic equation of state'. Recent efforts to explain Dark Energy focus on
`backreaction effects' of inhomogeneities on the effective evolution of
cosmological parameters in our Hubble volume, avoiding a cosmological constant
in the equation of state. In this Letter it is argued that, if kinematical
backreaction effects are indeed of the order of the averaged density (or larger
as needed for an accelerating domain of the Universe), then the state of our
regional Hubble volume would have to be in the vicinity of a
far-from-equilibrium state that balances kinematical backreaction and average
density. This property, if interpreted globally, is shared by a stationary
cosmos with effective equation of state . It
is concluded that a confirmed explanation of Dark Energy by kinematical
backreaction may imply a paradigmatic change of cosmology.Comment: 7 pages, matches published version in Class. Quant. Gra
Cosmic Acceleration from Causal Backreaction with Recursive Nonlinearities
We revisit the causal backreaction paradigm, in which the need for Dark
Energy is eliminated via the generation of an apparent cosmic acceleration from
the causal flow of inhomogeneity information coming in towards each observer
from distant structure-forming regions. This second-generation formalism
incorporates "recursive nonlinearities": the process by which
already-established metric perturbations will then act to slow down all future
flows of inhomogeneity information. Here, the long-range effects of causal
backreaction are now damped, weakening its impact for models that were
previously best-fit cosmologies. Nevertheless, we find that causal backreaction
can be recovered as a replacement for Dark Energy via the adoption of larger
values for the dimensionless `strength' of the clustering evolution functions
being modeled -- a change justified by the hierarchical nature of clustering
and virialization in the universe, occurring on multiple cosmic length scales
simultaneously. With this, and with one new model parameter representing the
slowdown of clustering due to astrophysical feedback processes, an alternative
cosmic concordance can once again be achieved for a matter-only universe in
which the apparent acceleration is generated entirely by causal backreaction
effects. One drawback is a new degeneracy which broadens our predicted range
for the observed jerk parameter , thus removing what had
appeared to be a clear signature for distinguishing causal backreaction from
Cosmological Constant CDM. As for the long-term fate of the universe,
incorporating recursive nonlinearities appears to make the possibility of an
`eternal' acceleration due to causal backreaction far less likely; though this
does not take into account gravitational nonlinearities or the large-scale
breakdown of cosmological isotropy, effects not easily modeled within this
formalism.Comment: 53 pages, 7 figures, 3 tables. This paper is an advancement of
previous research on Causal Backreaction; the earlier work is available at
arXiv:1109.4686 and arXiv:1109.515
Performance of the optimized Post-Zel'dovich approximation for CDM models in arbitrary FLRW cosmologies
We investigate the performance of the optimized Post-Zel'dovich approximation
in three cold dark matter cosmologies. We consider two flat models with
(SCDM) and with (CDM) and an open model
with (OCDM). We find that the optimization scheme proposed by
Wei{\ss}, Gottl\"ober & Buchert (1996), in which the performance of the
Lagrangian perturbation theory was optimized only for the Einstein-de Sitter
cosmology, shows the excellent performances not only for SCDM model but also
for both OCDM and CDM models. This universality of the excellent
performance of the optimized Post-Zel'dovich approximation is explained by the
fact that a relation between the Post-Zel'dovich order's growth factor
and Zel'dovich order's one , , is insensitive to the
background cosmologies.Comment: 8 pages, 3 figures, LaTex using aaspp4.sty and epsf.sty, Accepted for
publication in ApJ Letter
Hydrodynamic approach to the evolution of cosmological structures
A hydrodynamic formulation of the evolution of large-scale structure in the
Universe is presented. It relies on the spatially coarse-grained description of
the dynamical evolution of a many-body gravitating system. Because of the
assumed irrelevance of short-range (``collisional'') interactions, the way to
tackle the hydrodynamic equations is essentially different from the usual case.
The main assumption is that the influence of the small scales over the
large-scale evolution is weak: this idea is implemented in the form of a
large-scale expansion for the coarse-grained equations. This expansion builds a
framework in which to derive in a controlled manner the popular ``dust'' model
(as the lowest-order term) and the ``adhesion'' model (as the first-order
correction). It provides a clear physical interpretation of the assumptions
involved in these models and also the possibility to improve over them.Comment: 14 pages, 3 figures. Version to appear in Phys. Rev.
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