139 research outputs found
Quintessence Model Building
A short review of some of the aspects of quintessence model building is
presented. We emphasize the role of tracking models and their possible
supersymmetric origin.Comment: 14 pages, to appear in the proceedings of the sixth workshop of the
American University of Pari
Reproducing Cosmic Microwave Background anisotropies with mixed isocurvature perturbations
Recently high quality data of the cosmic microwave background anisotropies
have been published. In this work we study to which extent the cosmological
parameters determined by using this data depend on assumptions about the
initial conditions. We show that for generic initial conditions, not only the
best fit values are very different but, and this is our main result, the
allowed parameter range enlarges dramatically.Comment: 4 pages, 5 figures, submitted to PRL; Major changes following
referees suggestions; the allowed cosmological parameter range enlarges
dramaticall
Bounds on isocurvature perturbations from CMB and LSS data
We obtain very stringent bounds on the possible cold dark matter, baryon and
neutrino isocurvature contributions to the primordial fluctuations in the
Universe, using recent cosmic microwave background and large scale structure
data. In particular, we include the measured temperature and polarization power
spectra from WMAP and ACBAR, as well as the matter power spectrum from the 2dF
galaxy redshift survey. Neglecting the possible effects of spatial curvature,
tensor perturbations and reionization, we perform a Bayesian likelihood
analysis with nine free parameters, and find that the amplitude of the
isocurvature component cannot be larger than about 31% for the cold dark matter
mode, 91% for the baryon mode, 76% for the neutrino density mode, and 60% for
the neutrino velocity mode, at 2-sigma, for uncorrelated models. On the other
hand, for correlated adiabatic and isocurvature components, the fraction could
be slightly larger. However, the cross-correlation coefficient is strongly
constrained, and maximally correlated/anticorrelated models are disfavored.
This puts strong bounds on the curvaton model, independently of the bounds on
non-Gaussianity.Comment: 4 pages, 1 figure, some minor corrections; version accepted in PR
On the detectability of non-trivial topologies
We explore the main physical processes which potentially affect the
topological signal in the Cosmic Microwave Background (CMB) for a range of
toroidal universes. We consider specifically reionisation, the integrated
Sachs-Wolfe (ISW) effect, the size of the causal horizon, topological defects
and primordial gravitational waves. We use three estimators: the information
content, the S/N statistic and the Bayesian evidence. While reionisation has
nearly no effect on the estimators, we show that taking into account the ISW
strongly decreases our ability to detect the topological signal. We also study
the impact of varying the relevant cosmological parameters within the 2 sigma
ranges allowed by present data. We find that only Omega_Lambda, which
influences both ISW and the size of the causal horizon, significantly alters
the detection for all three estimators considered here.Comment: 11 pages, 9 figure
RL-DOVS: Reinforcement Learning for Autonomous Robot Navigation in Dynamic Environments
Autonomous navigation in dynamic environments where people move unpredictably is an essential task for service robots in real-world populated scenarios. Recent works in reinforcement learning (RL) have been applied to autonomous vehicle driving and to navigation around pedestrians. In this paper, we present a novel planner (reinforcement learning dynamic object velocity space, RL-DOVS) based on an RL technique for dynamic environments. The method explicitly considers the robot kinodynamic constraints for selecting the actions in every control period. The main contribution of our work is to use an environment model where the dynamism is represented in the robocentric velocity space as input to the learning system. The use of this dynamic information speeds the training process with respect to other techniques that learn directly either from raw sensors (vision, lidar) or from basic information about obstacle location and kinematics. We propose two approaches using RL and dynamic obstacle velocity (DOVS), RL-DOVS-A, which automatically learns the actions having the maximum utility, and RL-DOVS-D, in which the actions are selected by a human driver. Simulation results and evaluation are presented using different numbers of active agents and static and moving passive agents with random motion directions and velocities in many different scenarios. The performance of the technique is compared with other state-of-the-art techniques for solving navigation problems in environments such as ours
COBE-DMR-Normalized Dark Energy Cosmogony
Likelihood analyses of the COBE-DMR sky maps are used to determine the
normalization of the inverse-power-law-potential scalar field dark energy
model. Predictions of the DMR-normalized model are compared to various
observations to constrain the allowed range of model parameters. Although the
derived constraints are restrictive, evolving dark energy density scalar field
models remain an observationally-viable alternative to the constant
cosmological constant model.Comment: 26 pages, 10 figures, ApJ accepte
Well-proportioned universes suppress CMB quadrupole
A widespread myth asserts that all small universe models suppress the CMB
quadrupole. In actual fact, some models suppress the quadrupole while others
elevate it, according to whether their low-order modes are weak or strong
relative to their high-order modes. Elementary geometrical reasoning shows that
a model's largest dimension determines the rough value ell_min at which the CMB
power spectrum ell(ell + 1)C_ell/(2pi) effectively begins; for cosmologically
relevant models, ell_min < 4. More surprisingly, elementary geometrical
reasoning shows that further reduction of a model's smaller dimensions -- with
its largest dimension held fixed -- serves to elevate modes in the neighborhood
of ell_min relative to the high-ell portion of the spectrum, rather than
suppressing them as one might naively expect. Thus among the models whose
largest dimension is comparable to or less than the horizon diameter, the
low-order C_ell tend to be relatively weak in well-proportioned spaces (spaces
whose dimensions are approximately equal in all directions) but relatively
strong in oddly-proportioned spaces (spaces that are significantly longer in
some directions and shorter in others). We illustrate this principle in detail
for the special cases of rectangular 3-tori and spherical spaces. We conclude
that well-proportioned spaces make the best candidates for a topological
explanation of the low CMB quadrupole observed by COBE and WMAP.Comment: v1: 10 pages, 1 figure. v2: improved exposition of competing
mode-suppression and mode-enhancement effects, coincides with published
version, 12 pages, 1 figur
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
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