10,377 research outputs found
Figure of Merit for Dark Energy Constraints from Current Observational Data
Choosing the appropriate figure of merit (FoM) for dark energy (DE)
constraints is key in comparing different DE experiments. Here we show that for
a set of DE parameters {f_i}, it is most intuitive to define FoM =
1/\sqrt{Cov(f1,f2,f3,...)}, where Cov(f1,f2,f3,...) is the covariance matrix of
{f_i}. The {f_i} should be minimally correlated. We demonstrate two useful
choices of {f_i} using 182 SNe Ia (compiled by Riess et al. 2007), [R(z_*),
l_a(z_*), \Omega_b h^2] from the five year Wilkinson Microwave Anisotropy Probe
(WMAP) observations, and SDSS measurement of the baryon acoustic oscillation
(BAO) scale, assuming the HST prior of H_0=72+/-8 km/s Mpc^{-1} and without
assuming spatial flatness. We find that the correlation of (w_0,w_{0.5})
[w_0=w_X(z=0), w_{0.5}=w_X(z=0.5), w_X(a) = 3w_{0.5}-2w_0+3(w_0-w_{0.5})a] is
significantly smaller than that of (w_0,w_a) [w_X(a)=w_0+(1-a)w_a]. In order to
obtain model-independent constraints on DE, we parametrize the DE density
function X(z)=\rho_X(z)/\rho_X(0) as a free function with X_{0.5}, X_{1.0}, and
X_{1.5} [values of X(z) at z=0.5, 1.0, and 1.5] as free parameters estimated
from data. If one assumes a linear DE equation of state, current data are
consistent with a cosmological constant at 68% C.L. If one assumes X(z) to be a
free function parametrized by (X_{0.5}, X_{1.0}, X_{1.5}), current data deviate
from a cosmological constant at z=1 at 68% C.L., but are consistent with a
cosmological constant at 95% C.L.. Future DE experiments will allow us to
dramatically increase the FoM of constraints on (w_0,w_{0.5}) and of (X_{0.5},
X_{1.0}, X_{1.5}). This will significantly shrink the DE parameter space to
enable the discovery of DE evolution, or the conclusive evidence for a
cosmological constant.Comment: 7 pages, 3 color figures. Submitte
Observational Bounds on Modified Gravity Models
Modified gravity provides a possible explanation for the currently observed
cosmic accelaration. In this paper, we study general classes of modified
gravity models. The Einstein-Hilbert action is modified by using general
functions of the Ricci and the Gauss-Bonnet scalars, both in the metric and in
the Palatini formalisms. We do not use an explicit form for the functions, but
a general form with a valid Taylor expansion up to second order about redshift
zero in the Riemann-scalars. The coefficients of this expansion are then
reconstructed via the cosmic expansion history measured using current
cosmological observations. These are the quantities of interest for theoretical
considerations relating to ghosts and instabilities. We find that current data
provide interesting constraints on the coefficients. The next-generation dark
energy surveys should shrink the allowed parameter space for modifed gravity
models quite dramatically.Comment: 23 pages, 5 figures, uses RevTe
A Comparative Study of Dark Energy Constraints from Current Observational Data
We examine how dark energy constraints from current observational data depend
on the analysis methods used: the analysis of Type Ia supernovae (SNe Ia), and
that of galaxy clustering data. We generalize the flux-averaging analysis
method of SNe Ia to allow correlated errors of SNe Ia, in order to reduce the
systematic bias due to weak lensing of SNe Ia. We find that flux-averaging
leads to larger errors on dark energy and cosmological parameters if only SN Ia
data are used. When SN Ia data (the latest compilation by the SNLS team) are
combined with WMAP 7 year results (in terms of our Gaussian fits to the
probability distributions of the CMB shift parameters), the latest Hubble
constant (H_0) measurement using the Hubble Space Telescope (HST), and gamma
ray burst (GRB) data, flux-averaging of SNe Ia increases the concordance with
other data, and leads to significantly tighter constraints on the dark energy
density at z=1, and the cosmic curvature \Omega_k. The galaxy clustering
measurements of H(z=0.35)r_s(z_d) and r_s(z_d)/D_A(z=0.35) (where H(z) is the
Hubble parameter, D_A(z) is the angular diameter distance, and r_s(z_d) is the
sound horizon at the drag epoch) by Chuang & Wang (2011) are consistent with SN
Ia data, given the same pirors (CMB+H_0+GRB), and lead to significantly
improved dark energy constraints when combined. Current data are fully
consistent with a cosmological constant and a flat universe.Comment: 11 pages, 9 figures. Slightly revised version, to appear in PRD.
Supernova flux-averaging code available at
http://www.nhn.ou.edu/~wang/SNcode
Planck priors for dark energy surveys
Although cosmic microwave background (CMB) anisotropy data alone cannot
constrain simultaneously the spatial curvature and the equation of state of
dark energy, CMB data provide a valuable addition to other experimental
results. However computing a full CMB power spectrum with a Boltzmann code is
quite slow; for instance if we want to work with many dark energy and/or
modified gravity models, or would like to optimize experiments where many
different configurations need to be tested, it is possible to adopt a quicker
and more efficient approach.
In this paper we consider the compression of the projected Planck CMB data
into four parameters, R (scaled distance to last scattering surface), l_a
(angular scale of sound horizon at last scattering), Omega_b h^2 (baryon
density fraction) and n_s (powerlaw index of primordial matter power spectrum),
all of which can be computed quickly. We show that, although this compression
loses information compared to the full likelihood, such information loss
becomes negligible when more data is added. We also demonstrate that the method
can be used for scalar field dark energy independently of the parametrisation
of the equation of state, and discuss how this method should be used for other
kinds of dark energy models.Comment: 8 pages, 3 figures, 4 table
Uncorrelated Measurements of the Cosmic Expansion History and Dark Energy from Supernovae
We present a method for measuring the cosmic expansion history H(z) in
uncorrelated redshift bins, and apply it to current and simulated type Ia
supernova data assuming spatial flatness. If the matter density parameter
Omega_m can be accurately measured from other data, then the dark energy
density history X(z)=rho_X(z)/rho_X(0) can trivially be derived from this
expansion history H(z). In contrast to customary ``black box'' parameter
fitting, our method is transparent and easy to interpret: the measurement of
H(z)^{-1} in a redshift bin is simply a linear combination of the measured
comoving distances for supernovae in that bin, making it obvious how systematic
errors propagate from input to output.
We find the Riess et al. (2004) ``gold'' sample to be consistent with the
``vanilla'' concordance model where the dark energy is a cosmological constant.
We compare two mission concepts for the NASA/DOE Joint Dark Energy Mission
(JDEM), the Joint Efficient Dark-energy Investigation (JEDI), and the Supernova
Accelaration Probe (SNAP), using simulated data including the effect of weak
lensing (based on numerical simulations) and a systematic bias from
K-corrections. Estimating H(z) in seven uncorrelated redshift bins, we find
that both provide dramatic improvements over current data: JEDI can measure
H(z) to about 10% accuracy and SNAP to 30-40% accuracy.Comment: 7 pages, 4 color figures. Expanded and revised version; PRD in pres
Object Contour and Edge Detection with RefineContourNet
A ResNet-based multi-path refinement CNN is used for object contour
detection. For this task, we prioritise the effective utilization of the
high-level abstraction capability of a ResNet, which leads to state-of-the-art
results for edge detection. Keeping our focus in mind, we fuse the high, mid
and low-level features in that specific order, which differs from many other
approaches. It uses the tensor with the highest-levelled features as the
starting point to combine it layer-by-layer with features of a lower
abstraction level until it reaches the lowest level. We train this network on a
modified PASCAL VOC 2012 dataset for object contour detection and evaluate on a
refined PASCAL-val dataset reaching an excellent performance and an Optimal
Dataset Scale (ODS) of 0.752. Furthermore, by fine-training on the BSDS500
dataset we reach state-of-the-art results for edge-detection with an ODS of
0.824.Comment: Keywords: Object Contour Detection, Edge Detection, Multi-Path
Refinement CN
Comment on "Single-mode excited entangled coherent states"
In Xu and Kuang (\textit{J. Phys. A: Math. Gen.} 39 (2006) L191), the authors
claim that, for single-mode excited entangled coherent states , \textquotedblleft the photon excitations lead to the
decrease of the concurrence in the strong field regime of and
the concurrence tends to zero when ". This is wrong.Comment: 4 apges, 2 figures, submitted to JPA 15 April 200
A Fair and Secure Cluster Formation Process for Ad Hoc Networks
An efficient approach for organizing large ad hoc networks is to divide the nodes
into multiple clusters and designate, for each cluster, a clusterhead which is responsible for
holding intercluster control information. The role of a clusterhead entails rights and duties.
On the one hand, it has a dominant position in front of the others because it manages the
connectivity and has access to other node¿s sensitive information. But on the other hand, the
clusterhead role also has some associated costs. Hence, in order to prevent malicious nodes
from taking control of the group in a fraudulent way and avoid selfish attacks from suitable
nodes, the clusterhead needs to be elected in a secure way. In this paper we present a novel
solution that guarantees the clusterhead is elected in a cheat-proof manner
Quark mass density- and temperature- dependent model for strange quark matter
It is found that the radius of a stable strangelet decreases as the
temperature increases in a quark mass density-dependent model. To overcome this
difficulty, we extend this model to a quark mass density- and temperature-
dependent model in which the vacuum energy density at zero baryon density limit
B depends on temperature. An ansatz is introduced and the regions for the best
choice of the parameters are studied.Comment: 5 pages, 4 figure
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