7 research outputs found
Correspondence between kinematical backreaction and scalar field cosmologies - the `morphon field'
Spatially averaged inhomogeneous cosmologies in classical general relativity
can be written in the form of effective Friedmann equations with sources that
include backreaction terms. In this paper we propose to describe these
backreaction terms with the help of a homogeneous scalar field evolving in a
potential; we call it the `morphon field'. This new field links classical
inhomogeneous cosmologies to scalar field cosmologies, allowing to reinterpret,
e.g., quintessence scenarios by routing the physical origin of the scalar field
source to inhomogeneities in the Universe. We investigate a one-parameter
family of scaling solutions to the backreaction problem. Subcases of these
solutions (all without an assumed cosmological constant) include
scale-dependent models with Friedmannian kinematics that can mimic the presence
of a cosmological constant or a time-dependent cosmological term. We explicitly
reconstruct the scalar field potential for the scaling solutions, and discuss
those cases that provide a solution to the Dark Energy and coincidence
problems. In this approach, Dark Energy emerges from morphon fields, a
mechanism that can be understood through the proposed correspondence: the
averaged cosmology is characterized by a weak decay (quintessence) or growth
(phantom quintessence) of kinematical fluctuations, fed by `curvature energy'
that is stored in the averaged 3-Ricci curvature. We find that the late-time
trajectories of those models approach attractors that lie in the future of a
state that is predicted by observational constraints.Comment: 36 pages and 6 Figures, matches published version in Class.Quant.Gra
Cosmological Backreaction from Perturbations
We reformulate the averaged Einstein equations in a form suitable for use
with Newtonian gauge linear perturbation theory and track the size of the
modifications to standard Robertson-Walker evolution on the largest scales as a
function of redshift for both Einstein de-Sitter and Lambda CDM cosmologies. In
both cases the effective energy density arising from linear perturbations is of
the order of 10^-5 the matter density, as would be expected, with an effective
equation of state w ~ -1/19. Employing a modified Halofit code to extend our
results to quasilinear scales, we find that, while larger, the deviations from
Robertson-Walker behaviour remain of the order of 10^-5.Comment: 15 pages, 8 figures; replaced by version accepted by JCA
LTB solutions in Newtonian gauge: from strong to weak fields
Lemaitre-Tolman-Bondi (LTB) solutions are used frequently to describe the
collapse or expansion of spherically symmetric inhomogeneous mass distributions
in the Universe. These exact solutions are obtained in the synchronous gauge
where nonlinear dynamics (with respect to the FLRW background) induce large
deviations from the FLRW metric. In this paper we show explicitly that this is
a gauge artefact (for realistic sub-horizon inhomogeneities). We write down the
nonlinear gauge transformation from synchronous to Newtonian gauge for a
general LTB solution using the fact that the peculiar velocities are small. In
the latter gauge we recover the solution in the form of a weakly perturbed FLRW
metric that is assumed in standard cosmology. Furthermore we show how to obtain
the LTB solutions directly in Newtonian gauge and illustrate how the Newtonian
approximation remains valid in the nonlinear regime where cosmological
perturbation theory breaks down. Finally we discuss the implications of our
results for the backreaction scenario.Comment: 17 page
Flat Universe with Hyperbolic Voids
The properties of geodesics flow are studied in a Friedmann-Robertson-Walker
metric perturbed due to the inhomogeneities of matter. The basic, averaged
Jacobi equation is derived, which reveals that the low density regions (voids)
are able to induce hyperbolicity, even if the global curvature of the Universe
is zero or slightly positive. It is shown that the energy independence is a
characteristic property of these geometric effects. The importance of these
conclusions is determined by the temperature independent ellipticity of
excursion sets and regions of different randomness found in Kolmogorov CMB
maps.Comment: Europhys.Lett., in pres
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc
Apparent and average acceleration of the Universe
In this paper we consider the relation between the volume deceleration
parameter obtained within the Buchert averaging scheme and the deceleration
parameter derived from the supernova observation. This work was motivated by
recent findings that showed that there are models which despite
have volume deceleration parameter . This opens the possibility
that backreaction and averaging effects may be used as an interesting
alternative explanation to the dark energy phenomenon.
We have calculated in some Lema\^itre--Tolman models. For those
models which are chosen to be realistic and which fit the supernova data, we
find that , while those models which we have been able to find
which exhibit turn out to be unrealistic. This indicates that
care must be exercised in relating the deceleration parameter to observations.Comment: 15 pages, 5 figures; matches published versio