84 research outputs found
Coherent forward stimulated Brillouin scattering of a spatially incoherent laser beam in a plasma and its effect on beam spray
A statistical model for forward stimulated Brillouin scattering (FSBS) is
developed for a spatially incoherent, monochromatic, laser beam propagating in
a plasma. A threshold for the average power in a speckle is found, well below
the self-focusing one, above which the laser beam spatial incoherence can not
prevent the coherent growth of FSBS. Three-dimensional simulations confirm its
existence and reveal the onset of beam spray above it. From these results, we
propose a new figure of merit for the control of the propagation through a
plasma of a spatially incoherent laser beam.Comment: submitted to PR
Cosmic microwave background multipole alignments in slab topologies
Several analyses of the microwave sky maps from the Wilkinson Microwave
Anisotropy Probe (WMAP) have drawn attention to alignments amongst the
low-order multipoles. Amongst the various possible explanations, an effect of
cosmic topology has been invoked by several authors. We focus on an alignment
of the first four multipoles (\ell = 2 to 5) found by Land and Magueijo (2005),
and investigate the distribution of their alignment statistic for a set of
simulated cosmic microwave background maps for cosmologies with slab-like
topology. We find that this topology does offer a modest increase in the
probability of the observed value, but that even for the smallest topology
considered the probability of the observed value remains below one percent.Comment: 6 pages RevTex with 6 figures included. Minor changes to match
version accepted as Physical Review D Rapid Communicatio
Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background
Cosmology's standard model posits an infinite flat universe forever expanding
under the pressure of dark energy. First-year data from the Wilkinson Microwave
Anisotropy Probe (WMAP) confirm this model to spectacular precision on all but
the largest scales (Bennett {\it et al.}, 2003 ; Spergel {\it et al.}, 2003).
Temperature correlations across the microwave sky match expectations on scales
narrower than , yet vanish on scales wider than .
Researchers are now seeking an explanation of the missing wide-angle
correlations (Contaldi {\it et al.}, 2003 ; Cline {\it et al.}, 2003). One
natural approach questions the underlying geometry of space, namely its
curvature (Efstathiou, 2003) and its topology (Tegmark {\it et al.}, 2003). In
an infinite flat space, waves from the big bang would fill the universe on all
length scales. The observed lack of temperature correlations on scales beyond
means the broadest waves are missing, perhaps because space itself
is not big enough to support them.
Here we present a simple geometrical model of a finite, positively curved
space -- the Poincar\'e dodecahedral space -- which accounts for WMAP's
observations with no fine-tuning required. Circle searching (Cornish, Spergel
and Starkman, 1998) may confirm the model's topological predictions, while
upcoming Planck Surveyor data may confirm its predicted density of . If confirmed, the model will answer the ancient question of
whether space is finite or infinite, while retaining the standard
Friedmann-Lema\^\i{}tre foundation for local physics.Comment: 10 pages, 4 figures. This is a slightly longer version of the paper
published in Nature 425, p. 593, 200
Dark Energy, Induced Gravity and Broken Scale Invariance
We study the cosmological evolution of an induced gravity model with a
self-interacting scalar field and in the presence of matter and
radiation. Such model leads to Einstein Gravity plus a cosmological constant as
a stable attractor among homogeneous cosmologies and is therefore a viable
dark-energy (DE) model for a wide range of scalar field initial conditions and
values for its positive coupling to the Ricci curvature .Comment: 6 pages, 5 figures, 1 table: final version accepted for publication
in PL
A Dynamical Study of the Friedmann Equations
Cosmology is an attracting subject for students but usually difficult to deal
with if general relativity is not known. In this article, we first recall the
Newtonian derivation of the Friedmann equations which govern the dynamics of
our universe and discuss the validity of such a derivation. We then study the
equations of evolution of the universe in terms of a dynamical system. This
sums up the different behaviors of our universe and enables to address some
cosmological problems.Comment: Needs IOP LaTeX class; 17 pages, 9 figure
Cosmological observations in scalar-tensor quintessence
The framework for considering the astronomical and cosmological observations
in the context of scalar-tensor quintessence in which the quintessence field
also accounts for a time dependence of the gravitational constant is developed.
The constraints arising from nucleosynthesis, the variation of the constant,
and the post-Newtonian measurements are taken into account. A simple model of
supernovae is presented in order to extract the dependence of their light
curves with the gravitational constant; this implies a correction when fitting
the luminosity distance. The properties of perturbations as well as CMB
anisotropies are also investigated.Comment: 26 pages, 22 figures, to appear in PR
Quintessence with two energy scales
We study quintessence models using low energy supergravity inspired from
string theory. We consider effective supergravity with two scales m_S, the
string scale, and m_PL, the Planck scale and show that quintessence naturally
arises from a supersymmetry breaking hidden sector. As long as supersymmetry is
broken by the -term of a Polonyi-like field coupled to the quintessence
field in the K\"ahler potential we find that the Ratra-Peebles potential and
its supergravity version are generic predictions. This requires that the string
scale decouples from the Planck scale, m_S << m_PL. In the context of
supergravity, the potential possesses a minimum induced by the supergravity
corrections to the Ratra-Peebles potential at low redshifts. We study the
physical consequences of the presence of this minimum.Comment: 16 pages, 9 figures, minor changes matching published version.
Accepted for publication in PR
Reheating After Quintessential Inflation and Gravitational Waves
We investigate the dependence of the gravitational wave spectrum from
quintessential inflation on the reheating process. We consider two extreme
reheating processes. One is the gravitational reheating by particle creation in
the expanding universe in which the beginning of the radiation dominated epoch
is delayed due to the presence of the epoch of domination of the kinetic energy
of the inflaton (kination). The other is the instant preheating considered by
Felder et al. in which the Universe becomes radiation dominated soon after the
end of inflation. We find that the spectrum of the gravitational waves at MHz is quite sensitive to the reheating process. This result is not
limited to quintessential inflation but applicable to various inflation models.
Conversely, the detection or non-detection of primordial gravitational waves at
100 MHz would provide useful information regarding the reheating process
in inflation.Comment: 15 pages, 5 figures, to be published in CQ
Radiative Corrections to the Inflaton Potential as an Explanation of Suppressed Large Scale Power in Density Perturbations and the Cosmic Microwave Background
The Wilkinson Microwave Anisotropy Probe microwave background data suggest
that the primordial spectrum of scalar curvature fluctuations is suppressed at
small wavenumbers. We propose a UV/IR mixing effect in small-field inflationary
models that can explain the observable deviation in WMAP data from the
concordance model. Specifically, in inflationary models where the inflaton
couples to an asymptotically free gauge theory, the radiative corrections to
the effective inflaton potential can be anomalously large. This occurs for
small values of the inflaton field which are of the order of the gauge theory
strong coupling scale. Radiative corrections cause the inflaton potential to
blow up at small values of the inflaton field. As a result, these corrections
can violate the slow-roll condition at the initial stage of the inflation and
suppress the production of scalar density perturbations.Comment: 20 pages, 2 figures, v2: refs added, v3: JCAP versio
Constraints on the Detectability of Cosmic Topology from Observational Uncertainties
Recent observational results suggest that our universe is nearly flat and
well modelled within a CDM framework. The observed values of
and inevitably involve uncertainties. Motivated
by this, we make a systematic study of the necessary and sufficient conditions
for undetectability as well as detectability (in principle) of cosmic topology
(using pattern repetition) in presence of such uncertainties. We do this by
developing two complementary methods to determine detectability for nearly flat
universes. Using the first method we derive analytical conditions for
undetectability for infinite redshift, the accuracy of which is then confirmed
by the second method. Estimates based on WMAP data together with other
measurements of the density parameters are used to illustrate both methods,
which are shown to provide very similar results for high redshifts.Comment: 16 pages, 1 figure, LaTeX2
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