178 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.
Can we avoid dark energy?
The idea that we live near the centre of a large, nonlinear void has
attracted attention recently as an alternative to dark energy or modified
gravity. We show that an appropriate void profile can fit both the latest
cosmic microwave background and supernova data. However, this requires either a
fine-tuned primordial spectrum or a Hubble rate so low as to rule these models
out. We also show that measurements of the radial baryon acoustic scale can
provide very strong constraints. Our results present a serious challenge to
void models of acceleration.Comment: 5 pages, 4 figures; minor changes; version published in Phys. Rev.
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Precision cosmology defeats void models for acceleration
The suggestion that we occupy a privileged position near the centre of a
large, nonlinear, and nearly spherical void has recently attracted much
attention as an alternative to dark energy. Putting aside the philosophical
problems with this scenario, we perform the most complete and up-to-date
comparison with cosmological data. We use supernovae and the full cosmic
microwave background spectrum as the basis of our analysis. We also include
constraints from radial baryonic acoustic oscillations, the local Hubble rate,
age, big bang nucleosynthesis, the Compton y-distortion, and for the first time
include the local amplitude of matter fluctuations, \sigma_8. These all paint a
consistent picture in which voids are in severe tension with the data. In
particular, void models predict a very low local Hubble rate, suffer from an
"old age problem", and predict much less local structure than is observed.Comment: 22 pages, 12 figures; v2 adds models in closed backgrounds;
conclusions strengthened; version accepted to Phys. Rev.
Can decaying modes save void models for acceleration?
The unexpected dimness of Type Ia supernovae (SNe), apparently due to
accelerated expansion driven by some form of dark energy or modified gravity,
has led to attempts to explain the observations using only general relativity
with baryonic and cold dark matter, but by dropping the standard assumption of
homogeneity on Hubble scales. In particular, the SN data can be explained if we
live near the centre of a Hubble-scale void. However, such void models have
been shown to be inconsistent with various observations, assuming the void
consists of a pure growing mode. Here it is shown that models with significant
decaying mode contribution today can be ruled out on the basis of the expected
cosmic microwave background spectral distortion. This essentially closes one of
the very few remaining loopholes in attempts to rule out void models, and
strengthens the evidence for Hubble-scale homogeneity.Comment: 11 pages, 3 figures; discussion expanded, appendix added; version
accepted to Phys. Rev.
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
The growth of structure in the Szekeres inhomogeneous cosmological models and the matter-dominated era
This study belongs to a series devoted to using Szekeres inhomogeneous models
to develop a theoretical framework where observations can be investigated with
a wider range of possible interpretations. We look here into the growth of
large-scale structure in the models. The Szekeres models are exact solutions to
Einstein's equations that were originally derived with no symmetries. We use a
formulation of the models that is due to Goode and Wainwright, who considered
the models as exact perturbations of an FLRW background. Using the Raychaudhuri
equation, we write for the two classes of the models, exact growth equations in
terms of the under/overdensity and measurable cosmological parameters. The new
equations in the overdensity split into two informative parts. The first part,
while exact, is identical to the growth equation in the usual linearly
perturbed FLRW models, while the second part constitutes exact non-linear
perturbations. We integrate numerically the full exact growth rate equations
for the flat and curved cases. We find that for the matter-dominated era, the
Szekeres growth rate is up to a factor of three to five stronger than the usual
linearly perturbed FLRW cases, reflecting the effect of exact Szekeres
non-linear perturbations. The growth is also stronger than that of the
non-linear spherical collapse model, and the difference between the two
increases with time. This highlights the distinction when we use general
inhomogeneous models where shear and a tidal gravitational field are present
and contribute to the gravitational clustering. Additionally, it is worth
observing that the enhancement of the growth found in the Szekeres models
during the matter-dominated era could suggest a substitute to the argument that
dark matter is needed when using FLRW models to explain the enhanced growth and
resulting large-scale structures that we observe today (abridged)Comment: 18 pages, 4 figures, matches PRD accepted versio
Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae
It is now a known fact that if we happen to be living in the middle of a
large underdense region, then we will observe an "apparent acceleration", even
when any form of dark energy is absent. In this paper, we present a "Minimal
Void" scenario, i.e. a "void" with minimal underdensity contrast (of about
-0.4) and radius (~ 200-250 Mpc/h) that can, not only explain the supernovae
data, but also be consistent with the 3-yr WMAP data. We also discuss
consistency of our model with various other measurements such as Big Bang
Nucleosynthesis, Baryon Acoustic Oscillations and local measurements of the
Hubble parameter, and also point out possible observable signatures.Comment: Minor numerical errors and typos corrected, references adde
Dynamics of a lattice Universe
We find a solution to Einstein field equations for a regular toroidal lattice
of size L with equal masses M at the centre of each cell; this solution is
exact at order M/L. Such a solution is convenient to study the dynamics of an
assembly of galaxy-like objects. We find that the solution is expanding (or
contracting) in exactly the same way as the solution of a
Friedman-Lema\^itre-Robertson-Walker Universe with dust having the same average
density as our model. This points towards the absence of backreaction in a
Universe filled with an infinite number of objects, and this validates the
fluid approximation, as far as dynamics is concerned, and at the level of
approximation considered in this work.Comment: 14 pages. No figure. Accepted version for Classical and Quantum
Gravit
Gauging the cosmic microwave background
We provide a new derivation of the anisotropies of the cosmic microwave
background (CMB), and find an exact expression that can be readily expanded
perturbatively. Close attention is paid to gauge issues, with the motivation to
examine the effect of super-Hubble modes on the CMB. We calculate a transfer
function that encodes the behaviour of the dipole, and examine its
long-wavelength behaviour. We show that contributions to the dipole from
adiabatic super-Hubble modes are strongly suppressed, even in the presence of a
cosmological constant, contrary to claims in the literature. We also introduce
a naturally defined CMB monopole, which exhibits closely analogous
long-wavelength behaviour. We discuss the geometrical origin of this
super-Hubble suppression, pointing out that it is a simple reflection of
adiabaticity, and hence argue that it will occur regardless of the matter
content.Comment: 17 pages, 5 figures; minor changes; version accepted to Phys. Rev.
Synthesis by sol-gel route of oxyapatite powders for dense ceramics. Applications as electrolytes for solid oxide fuel cells
Abstract Solid oxide fuel cells have considerable interest in recent years, because of their high efficiency and environmentally friendly nature. Such systems required oxygen-conducting electrolytes and now the most common electrolyte is yttria stabilized zirconia (YSZ). This compound exhibits high oxide ion conductivity at elevated temperatures (850-1000°C). However, this high working temperature causes problems in terms of materials selection and lifetime. One solution is to develop new oxide ions conductors exhibiting high oxide ion conductivity at intermediary temperatures (700-800°C). Recent work has identified Ln 10-x Si 6 O 26±z (Ln = rare earths) as a good fast oxide ion conductor. Undoped and doped Ln 10-x B 6 O 26±z (B = Si or Ge) oxides are currently prepared by solid-state methods. In that work, we propose a sol-gel process to synthesize powders of La 9.33 Si 6 O 26 type-silicated apatites. The main advantage is to decrease the crystallization temperature in comparison to the conventional methods, allowing the synthesis of reactive powders with nanometric particles size. These oxides are synthesized using silicon alkoxide and lanthanum nitride as precursors. In the litterature, no study refers to the synthesis of mixed oxides with silicon alcoxides. However, there are several studies on sol-gel synthesis of glasses with this precursor. In this study, several processing parameters have been investigated (the hydrolysis ratio, the concentration of metallic precursors in the sol and the role of organic compounds) in order to synthesize pure phases after the decomposition of the sols. Pure powders of La 9.33 Si 6 O 26 type-silicated apatites are obtained at 800°C. These powders were used to prepare ceramics. Several processing parameters as morphology of powders (agglomeration, particle sizes) and, heating profiles have been studied on the densification. Dense ceramics (90-95%) have been prepared at temperatures around 1400°C. The used of sol-gel powders allow the decrease of the sintering temperature of about 200°C
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