38 research outputs found
Radio-optical properties of extragalactic populations in the VIPERS Survey
The radio and optical characteristics of faint radio sources are investigated thanks to the VIPERS spectroscopic and photometric data, and radio data from the VLA FIRST survey at 1.4 GHz
Testing homogeneity with galaxy number counts : light-cone metric and general low-redshift expansion for a central observer in a matter dominated isotropic universe without cosmological constant
As an alternative to dark energy it has been suggested that we may be at the
center of an inhomogeneous isotropic universe described by a
Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. In order to
test this hypothesis we calculate the general analytical formula to fifth order
for the redshift spherical shell mass. Using the same analytical method we
write the metric in the light-cone by introducing a gauge invariant quantity
which together with the luminosity distance completely
determine the light-cone geometry of a LTB model.Comment: 13 page
Corrections to the apparent value of the cosmological constant due to local inhomogeneities
Supernovae observations strongly support the presence of a cosmological
constant, but its value, which we will call apparent, is normally determined
assuming that the Universe can be accurately described by a homogeneous model.
Even in the presence of a cosmological constant we cannot exclude nevertheless
the presence of a small local inhomogeneity which could affect the apparent
value of the cosmological constant. Neglecting the presence of the
inhomogeneity can in fact introduce a systematic misinterpretation of
cosmological data, leading to the distinction between an apparent and true
value of the cosmological constant. We establish the theoretical framework to
calculate the corrections to the apparent value of the cosmological constant by
modeling the local inhomogeneity with a solution. Our assumption
to be at the center of a spherically symmetric inhomogeneous matter
distribution correspond to effectively calculate the monopole contribution of
the large scale inhomogeneities surrounding us, which we expect to be the
dominant one, because of other observations supporting a high level of isotropy
of the Universe around us.
By performing a local Taylor expansion we analyze the number of independent
degrees of freedom which determine the local shape of the inhomogeneity, and
consider the issue of central smoothness, showing how the same correction can
correspond to different inhomogeneity profiles. Contrary to previous attempts
to fit data using large void models our approach is quite general. The
correction to the apparent value of the cosmological constant is in fact
present for local inhomogeneities of any size, and should always be taken
appropriately into account both theoretically and observationally.Comment: 16 pages,new sections added analyzing central smoothness and accuracy
of the Taylor expansion approach, Accepted for publication by JCAP. An essay
based on this paper received honorable mention in the 2011 Essay Context of
the Gravity Research Foundatio
Testing the Copernican and Cosmological Principles in the local universe with galaxy surveys
Cosmological density fields are assumed to be translational and rotational
invariant, avoiding any special point or direction, thus satisfying the
Copernican Principle. A spatially inhomogeneous matter distribution can be
compatible with the Copernican Principle but not with the stronger version of
it, the Cosmological Principle which requires the additional hypothesis of
spatial homogeneity. We establish criteria for testing that a given density
field, in a finite sample at low redshifts, is statistically and/or spatially
homogeneous. The basic question to be considered is whether a distribution is,
at different spatial scales, self-averaging. This can be achieved by studying
the probability density function of conditional fluctuations. We find that
galaxy structures in the SDSS samples, the largest currently available, are
spatially inhomogeneous but statistically homogeneous and isotropic up to ~ 100
Mpc/h. Evidences for the breaking of self-averaging are found up to the largest
scales probed by the SDSS data. The comparison between the results obtained in
volumes of different size allows us to unambiguously conclude that the lack of
elf-averaging is induced by finite-size effects due to long-range correlated
fluctuations. We finally discuss the relevance of these results from the point
of view of cosmological modeling.Comment: 12 pages, 3 figures, accepted for publication in JCA
Can the cosmological constant be mimicked by smooth large-scale inhomogeneities for more than one observable?
As an alternative to dark energy it has been suggested that we may be at the
center of an inhomogeneous isotropic universe described by a
Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. In order to
test such an hypothesis we calculate the low redshift expansion of the
luminosity distance and the redshift spherical shell mass density
for a central observer in a LTB space without cosmological constant and
show how they cannot fit the observations implied by a model if
the conditions to avoid a weak central singularity are imposed, i.e. if the
matter distribution is smooth everywhere. Our conclusions are valid for any
value of the cosmological constant, not only for as
implied by previous proofs that has to be positive in a smooth LTB
space, based on considering only the luminosity distance.
The observational signatures of smooth LTB matter dominated models are
fundamentally different from the ones of models not only because
it is not possible to reproduce a negative apparent central deceleration
, but because of deeper differences in their space-time geometry
which make impossible the inversion problem when more than one observable is
considered, and emerge at any redshift, not only for .Comment: 18 pages, corrected a typo in the definition of the energy density
which doesn't change the conclusion, references adde
Supernovae data and perturbative deviation from homogeneity
We show that a spherically symmetric perturbation of a dust dominated
FRW universe in the Newtonian gauge can lead to an apparent
acceleration of standard candles and provide a fit to the magnitude-redshift
relation inferred from the supernovae data, while the perturbation in the
gravitational potential remains small at all scales. We also demonstrate that
the supernovae data does not necessarily imply the presence of some additional
non-perturbative contribution by showing that any Lemaitre-Tolman-Bondi model
fitting the supernovae data (with appropriate initial conditions) will be
equivalent to a perturbed FRW spacetime along the past light cone.Comment: 8 pages, 3 figures; v2: 1 figure added, references added/updated,
minor modifications and clarifications, matches published versio
Supernovae as seen by off-center observers in a local void
Inhomogeneous universe models have been proposed as an alternative
explanation for the apparent acceleration of the cosmic expansion that does not
require dark energy. In the simplest class of inhomogeneous models, we live
within a large, spherically symmetric void. Several studies have shown that
such a model can be made consistent with many observations, in particular the
redshift--luminosity distance relation for type Ia supernovae, provided that
the void is of Gpc size and that we live close to the center. Such a scenario
challenges the Copernican principle that we do not occupy a special place in
the universe. We use the first-year Sloan Digital Sky Survey-II supernova
search data set as well as the Constitution supernova data set to put
constraints on the observer position in void models, using the fact that
off-center observers will observe an anisotropic universe. We first show that a
spherically symmetric void can give good fits to the supernova data for an
on-center observer, but that the two data sets prefer very different voids. We
then continue to show that the observer can be displaced at least fifteen
percent of the void scale radius from the center and still give an acceptable
fit to the supernova data. When combined with the observed dipole anisotropy of
the cosmic microwave background however, we find that the data compells the
observer to be located within about one percent of the void scale radius. Based
on these results, we conclude that considerable fine-tuning of our position
within the void is needed to fit the supernova data, strongly disfavouring the
model from a Copernican principle point of view.Comment: 20 pages, 6 figures, matches the published versio
Stationary Configurations Imply Shift Symmetry: No Bondi Accretion for Quintessence / k-Essence
In this paper we show that, for general scalar fields, stationary
configurations are possible for shift symmetric theories only. This symmetry
with respect to constant translations in field space should either be manifest
in the original field variables or reveal itself after an appropriate field
redefinition. In particular this result implies that neither k-Essence nor
Quintessence can have exact steady state / Bondi accretion onto Black Holes. We
also discuss the role of field redefinitions in k-Essence theories. Here we
study the transformation properties of observables and other variables in
k-Essence and emphasize which of them are covariant under field redefinitions.
Finally we find that stationary field configurations are necessarily linear in
Killing time, provided that shift symmetry is realized in terms of these field
variables.Comment: 8 page
How close can an Inhomogeneous Universe mimic the Concordance Model?
Recently, spatially inhomogeneous cosmological models have been proposed as
an alternative to the LCDM model, with the aim of reproducing the late time
dynamics of the Universe without introducing a cosmological constant or dark
energy. This paper investigates the possibility of distinguishing such models
from the standard LCDM using background or large scale structure data. It also
illustrates and emphasizes the necessity of testing the Copernican principle in
order to confront the tests of general relativity with the large scale
structure.Comment: 15 pages, 7 figure
The Crossing Statistic: Dealing with Unknown Errors in the Dispersion of Type Ia Supernovae
We propose a new statistic that has been designed to be used in situations
where the intrinsic dispersion of a data set is not well known: The Crossing
Statistic. This statistic is in general less sensitive than `chi^2' to the
intrinsic dispersion of the data, and hence allows us to make progress in
distinguishing between different models using goodness of fit to the data even
when the errors involved are poorly understood. The proposed statistic makes
use of the shape and trends of a model's predictions in a quantifiable manner.
It is applicable to a variety of circumstances, although we consider it to be
especially well suited to the task of distinguishing between different
cosmological models using type Ia supernovae. We show that this statistic can
easily distinguish between different models in cases where the `chi^2'
statistic fails. We also show that the last mode of the Crossing Statistic is
identical to `chi^2', so that it can be considered as a generalization of
`chi^2'.Comment: 14 pages, 5 figures. Paper restructured and extended and new
interpretation of the method presented. New results concerning model
selection. Treatment and error-analysis made fully model independent.
References added. Accepted for publication in JCA