5 research outputs found
Averaging inhomogeneities in scalar-tensor cosmology
The backreaction of inhomogeneities on the cosmic dynamics is studied in the
context of scalar-tensor gravity. Due to terms of indefinite sign in the
non-canonical effective energy tensor of the Brans-Dicke-like scalar field,
extra contributions to the cosmic acceleration can arise. Brans-Dicke and
metric f(R) gravity are presented as specific examples. Certain representation
problems of the formalism peculiar to these theories are pointed out.Comment: Comments and references added. 14 page
Chaplygin gas and effective description of inhomogeneous universe models in general relativity
In the framework of spatially averaged inhomogeneous cosmologies in classical
general relativity, effective Einstein equations govern the dynamics of
averaged scalar variables in a scale--dependent way. A particular cosmology may
be characterized by a cosmic equation of state, closing the hierarchy of
effective equations. In this context a natural candidate is provided by the
Chaplygin gas, standing for a unified description of dark energy and dark
matter. In this paper, we suppose that the inhomogeneous properties of matter
and geometry obey the Chaplygin equation of state. The most extreme
interpretation assumes that both dark energy and dark matter are not included
as additional sources, but are both manifestations of spatial geometrical
properties. This feature is an important conceptual difference in comparison
with the standard approach of a Friedmann-Lema\^itre-Robertson-Walker universe
filled with dust and another fundamental field characterized by the Chaplygin
equation of state. We finally discuss the consequences of the resulting
scenario for effective cosmological parameters in order to establish the
framework of a future confrontation with observations, and we note that the
standard Chaplygin gas may not be ruled out by them.Comment: 21 pages, 5 figures, matches published version in CQ
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
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
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