188 research outputs found
Vente à distance, internet et dynamique des prix.
The share of retail sales made via distance selling has increased steadily, driven by Internet sales. Meanwhile, a large body of research has been devoted to measuring the impact of online shopping on consumer prices. These studies are based primarily on microeconomic data and they reveal contrasting effects due to diverging microeconomic behaviours. This paper aims to use a macro-sector estimation to show how the price-decreasing effects of Internet shopping outweigh the price-increasing effects. In that purpose, we use French price index series and distance selling sales covering about 30 sectors, from 1990 to 2007. We find that downward effects dominate: the recent development of distance selling, due to the development of online selling, results in lower prices.E-Commerce, Price, Competition.
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
Late time solutions for inhomogeneous Lambda-CDM cosmology, their characterization and observation
Assuming homogeneous isotropic Lambda-CDM cosmology allows Lambda, spatial
curvature and dark matter density to be inferred from large scale structure
observations such as supernovae. The purpose of this paper is to extend this to
allow observations to measure or constrain inhomogeneity and anisotropy. We
obtain the general inhomogeneous anisotropic Lambda-CDM solution which is
locally asymptotic to an expanding de Sitter solution as a late time expansion
using Starobinsky's method (analogous to the `holographic renormalization'
technique in AdS/CFT) together with a resummation of the series. The dark
matter is modeled as perfect dust fluid. The terms in the expansion
systematically describe inhomogeneous and anisotropic deformations of an
expanding FLRW solution, and are given as a spatial derivative expansion in
terms of data characterizing the solution - a 3-metric and a perturbation of
that 3-metric. Leading terms describe inhomogeneity and anisotropy on the scale
set by the cosmological constant, approximately the horizon scale today. Higher
terms in the expansion describe shorter scale variations. We compute the
luminosity distance-redshift relation and argue that comparison with current
and future observation would allow a partial reconstruction of the
characterizing data. We also comment on smoothing these solutions noting that
geometric flows (such as Ricci flow) applied to the characterizing data provide
a canonical averaging method.Comment: 15 pages, 2 figures; v2: minor corrections and improvements,
references adde
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
Can the Copernican principle be tested by cosmic neutrino background?
The Copernican principle, stating that we do not occupy any special place in
our universe, is usually taken for granted in modern cosmology. However recent
observational data of supernova indicate that we may live in the under-dense
center of our universe, which makes the Copernican principle challenged. It
thus becomes urgent and important to test the Copernican principle via
cosmological observations. Taking into account that unlike the cosmic photons,
the cosmic neutrinos of different energies come from the different places to us
along the different worldlines, we here propose cosmic neutrino background as a
test of the Copernican principle. It is shown that from the theoretical
perspective cosmic neutrino background can allow one to determine whether the
Copernican principle is valid or not, but to implement such an observation the
larger neutrino detectors are called for.Comment: JHEP style, 10 pages, 4 figures, version to appear in JCA
Role of initial data in spherical collapse
We bring out here the role of initial data in causing the black hole and
naked singularity phases as the final end state of a continual gravitational
collapse. The collapse of a type I general matter field is considered, which
includes most of the known physical forms of matter. It is shown that given the
distribution of the density and pressure profiles at the initial surface from
which the collapse evolves, there is a freedom in choosing rest of the free
functions, such as the velocities of the collapsing shells, so that the end
state could be either a black hole or a naked singularity depending on this
choice. It is thus seen that it is the initial data that determines the end
state of spherical collapse in terms of these outcomes, and we get a good
picture of how these phases come about.Comment: 5 pages, Revtex4, Revised version, To appear in Physical Review
Lemaitre-Tolman-Bondi dust spacetimes: Symmetry properties and some extensions to the dissipative case
We consider extensions of Lemaitre-Tolman-Bondi (LTB) spacetimes to the
dissipative case. For doing that we previously carry out a systematic study on
LTB. This study is based on two different aspects of LTB. On the one hand, a
symmetry property of LTB will be presented. On the other hand, the description
of LTB in terms of some fundamental scalar functions (structure scalars)
appearing in the orthogonal splitting of Riemann tensor will be provided. We
shall consider as "natural" generalizations of LTB (hereafter referred to as
GLTB) either those metrics admitting some similar kind of symmetry as LTB, or
those sharing structure scalars with similar dependence on the metric.Comment: 13 pages RevTex. To appear in Phys. Rev. D. Some references corrected
and update
ZOBOV: a parameter-free void-finding algorithm
ZOBOV (ZOnes Bordering On Voidness) is an algorithm that finds density
depressions in a set of points, without any free parameters, or assumptions
about shape. It uses the Voronoi tessellation to estimate densities, which it
uses to find both voids and subvoids. It also measures probabilities that each
void or subvoid arises from Poisson fluctuations. This paper describes the
ZOBOV algorithm, and the results from its application to the dark-matter
particles in a region of the Millennium Simulation. Additionally, the paper
points out an interesting high-density peak in the probability distribution of
dark-matter particle densities.Comment: 10 pages, 8 figures, MNRAS, accepted. Added explanatory figures, and
better edge-detection methods. ZOBOV code available at
http://www.ifa.hawaii.edu/~neyrinck/vobo
An inhomogeneous alternative to dark energy?
Recently, there have been suggestions that the apparent accelerated expansion
of the universe is not caused by repulsive gravitation due to dark energy, but
is rather a result of inhomogeneities in the distribution of matter. In this
work, we investigate the behaviour of a dust dominated inhomogeneous
Lemaitre-Tolman-Bondi universe model, and confront it with various
astrophysical observations. We find that such a model can easily explain the
observed luminosity distance-redshift relation of supernovae without the need
for dark energy, when the inhomogeneity is in the form of an underdense bubble
centered near the observer. With the additional assumption that the universe
outside the bubble is approximately described by a homogeneous Einstein-de
Sitter model, we find that the position of the first CMB peak can be made to
match the WMAP observations. Whether or not it is possible to reproduce the
entire CMB angular power spectrum in an inhomogeneous model without dark
energy, is still an open question.Comment: 8 pages (REVTeX4), 4 figures. v2: Minor changes to text plus added
some references. Accepted for publication in PR
Dark Energy or Apparent Acceleration Due to a Relativistic Cosmological Model More Complex than FLRW?
We use the Szekeres inhomogeneous relativistic models in order to fit
supernova combined data sets. We show that with a choice of the spatial
curvature function that is guided by current observations, the models fit the
supernova data almost as well as the LCDM model without requiring a dark energy
component. The Szekeres models were originally derived as an exact solution to
Einstein's equations with a general metric that has no symmetries and are
regarded as good candidates to model the true lumpy universe that we observe.
The null geodesics in these models are not radial. The best fit model found is
also consistent with the requirement of spatial flatness at CMB scales. The
first results presented here seem to encourage further investigations of
apparent acceleration using various inhomogeneous models and other constraints
from CMB and large structure need to be explored next.Comment: 6 pages, 1 figure, matches version published in PR
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