10,727 research outputs found
Spray-on technique simplifies fabrication of complex thermal insulation blanket
Spray-on process constructs molds used in forming sections of thermal insulation blankets. The process simplifies the fabrication of blankets by eliminating much of the equipment formerly required and decreasing the time involved
Probing the Primordial Power Spectrum with Cluster Number Counts
We investigate how well galaxy cluster number counts can constrain the
primordial power spectrum. Measurements of the primary anisotropies in the
cosmic microwave background (CMB) may be limited, by the presence of
foregrounds from secondary sources, to probing the primordial power spectrum at
wave numbers less than about 0.30 h Mpc^{-1}. We break up the primordial power
spectrum into a number of nodes and interpolate linearly between each node.
This allows us to show that cluster number counts could then extend the
constraints on the form of the primordial power spectrum up to wave numbers of
about 0.45 h Mpc^{-1}. We estimate combinations of constraints from PLANCK and
SPT primary CMB and their respective SZ surveys. We find that their
constraining ability is limited by uncertainties in the mass scaling relations.
We also estimate the constraint from clusters detected from a SNAP like
gravitational lensing survey. As there is an unambiguous and simple
relationship between the filtered shear of the lensing survey and the cluster
mass, it may be possible to obtain much tighter constraints on the primordial
power spectrum in this case.Comment: Clarifications added and a few minor corrections made. Matches
version to appear in PR
Constraints on Dark Energy from Supernovae, Gamma Ray Bursts, Acoustic Oscillations, Nucleosynthesis and Large Scale Structure and the Hubble constant
The luminosity distance vs. redshift law is now measured using supernovae and
gamma ray bursts, and the angular size distance is measured at the surface of
last scattering by the CMB and at z = 0.35 by baryon acoustic oscillations. In
this paper this data is fit to models for the equation of state with w = -1, w
= const, and w(z) = w_0+w_a(1-a). The last model is poorly constrained by the
distance data, leading to unphysical solutions where the dark energy dominates
at early times unless the large scale structure and acoustic scale constraints
are modified to allow for early time dark energy effects. A flat LambdaCDM
model is consistent with all the data.Comment: 19 pages Latex with 8 Postscript figure files. A new reference and
constraint, w vs w' contour plots updated. Version accepted by the the Ap
Testing flatness of the universe with probes of cosmic distances and growth
When using distance measurements to probe spatial curvature, the geometric
degeneracy between curvature and dark energy in the distance-redshift relation
typically requires either making strong assumptions about the dark energy
evolution or sacrificing precision in a more model-independent approach.
Measurements of the redshift evolution of the linear growth of perturbations
can break the geometric degeneracy, providing curvature constraints that are
both precise and model-independent. Future supernova, CMB, and cluster data
have the potential to measure the curvature with an accuracy of
sigma(Omega_K)=0.002, without specifying a particular dark energy
phenomenology. In combination with distance measurements, the evolution of the
growth function at low redshifts provides the strongest curvature constraint if
the high-redshift universe is well approximated as being purely matter
dominated. However, in the presence of early dark energy or massive neutrinos,
the precision in curvature is reduced due to additional degeneracies, and
precise normalization of the growth function relative to recombination is
important for obtaining accurate constraints. Curvature limits from distances
and growth compare favorably to other approaches to curvature estimation
proposed in the literature, providing either greater accuracy or greater
freedom from dark energy modeling assumptions, and are complementary due to the
use of independent data sets. Model-independent estimates of curvature are
critical for both testing inflation and obtaining unbiased constraints on dark
energy parameters.Comment: 23 pages, 11 figures; submitted to Phys. Rev.
Model-Independent Constraints on Dark Energy Density from Flux-averaging Analysis of Type Ia Supernova Data
We reconstruct the dark energy density as a free function from
current type Ia supernova (SN Ia) data (Tonry et al. 2003; Barris et al. 2003;
Knop et al. 2003), together with the Cosmic Microwave Background (CMB) shift
parameter from CMB data (WMAP, CBI, and ACBAR), and the large scale structure
(LSS) growth factor from 2dF galaxy survey data. We parametrize as
a continuous function, given by interpolating its amplitudes at equally spaced
values in the redshift range covered by SN Ia data, and a constant at
larger (where is only weakly constrained by CMB data). We
assume a flat universe, and use the Markov Chain Monte Carlo (MCMC) technique
in our analysis. We find that the dark energy density is constant
for 0 \la z \la 0.5 and increases with redshift for 0.5 \la z \la 1 at
68.3% confidence level, but is consistent with a constant at 95% confidence
level. For comparison, we also give constraints on a constant equation of state
for the dark energy.
Flux-averaging of SN Ia data is required to yield cosmological parameter
constraints that are free of the bias induced by weak gravitational lensing
\citep{Wang00b}. We set up a consistent framework for flux-averaging analysis
of SN Ia data, based on \cite{Wang00b}. We find that flux-averaging of SN Ia
data leads to slightly lower and smaller time-variation in
. This suggests that a significant increase in the number of SNe Ia
from deep SN surveys on a dedicated telescope \citep{Wang00a} is needed to
place a robust constraint on the time-dependence of the dark energy density.Comment: Slightly revised in presentation, ApJ accepted. One color figure
shows rho_X(z) reconstructed from dat
Observation of the dielectric-waveguide mode of light propagation in p-n junctions
Theoretical considerations of the propagation of electromagnetic energy near a p-n junction (1) show that the “sandwich” formed by having a depletion layer bounded by the p and n regions can act as a dielectric waveguide. (1,2
Secondary CMB anisotropies in a universe reionized in patches
In a universe reionized in patches, the Doppler effect from Thomson
scattering off free electrons generates secondary cosmic microwave background
(CMB) anisotropies. For a simple model with small patches and late
reionization, we analytically calculate the anisotropy power spectrum. Patchy
reionization can, in principle, be the main source of anisotropies on arcminute
scales. On larger angular scales, its contribution to the CMB power spectrum is
a small fraction of the primary signal and is only barely detectable in the
power spectrum with even an ideal, i.e. cosmic variance limited, experiment and
an extreme model of reionization. Consequently patchy reionization is unlikely
to affect cosmological parameter estimation from the acoustic peaks in the CMB.
Its detection on small angles would help determine the ionization history of
the universe, in particular the typical size of the ionized region and the
duration of the reionization process.Comment: 7 pages, 2 figures, submitted to Ap
Age problem in holographic dark energy
We study the age problem of the universe with the holographic DE model
introduced in [21], and test the model with some known old high redshift
objects (OHRO). The parameters of the model have been constrained using the
SNIa, CMB and BAO data set. We found that the age of the old quasar APM 08
279+5255 at z = 3.91 can be described by the model.Comment: 13 page
Analysis of recent type Ia supernova data based on evolving dark energy models
We study characters of recent type Ia supernova (SNIa) data using evolving
dark energy models with changing equation of state parameter w. We consider
sudden-jump approximation of w for some chosen redshift spans with double
transitions, and constrain these models based on Markov Chain Monte Carlo
(MCMC) method using the SNIa data (Constitution, Union, Union2) together with
baryon acoustic oscillation A parameter and cosmic microwave background shift
parameter in a flat background. In the double-transition model the Constitution
data shows deviation outside 1 sigma from LCDM model at low (z < 0.2) and
middle (0.2 < z < 0.4) redshift bins whereas no such deviations are noticeable
in the Union and Union2 data. By analyzing the Union members in the
Constitution set, however, we show that the same difference is actually due to
different calibration of the same Union sample in the Constitution set, and is
not due to new data added in the Constitution set. All detected deviations are
within 2 sigma from the LCDM world model. From the LCDM mock data analysis, we
quantify biases in the dark energy equation of state parameters induced by
insufficient data with inhomogeneous distribution of data points in the
redshift space and distance modulus errors. We demonstrate that location of
peak in the distribution of arithmetic means (computed from the MCMC chain for
each mock data) behaves as an unbiased estimator for the average bias, which is
valid even for non-symmetric likelihood distributions.Comment: 12 pages, 6 figures, published in the Phys. Rev.
Power Spectrum Estimators For Large CMB Datasets
Forthcoming high-resolution observations of the Cosmic Microwave Background
(CMB) radiation will generate datasets many orders of magnitude larger than
have been obtained to date. The size and complexity of such datasets presents a
very serious challenge to analysing them with existing or anticipated
computers. Here we present an investigation of the currently favored algorithm
for obtaining the power spectrum from a sky-temperature map --- the quadratic
estimator. We show that, whilst improving on direct evaluation of the
likelihood function, current implementations still inherently scale as the
equivalent of the cube of the number of pixels or worse, and demonstrate the
critical importance of choosing the right implementation for a particular
dataset.Comment: 8 pages LATEX, no figures, corrected misaligned columns in table
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