672 research outputs found
Direct Detection of Galactic Halo Dark Matter
The Milky Way Galaxy contains a large, spherical component which is believed
to harbor a substantial amount of unseen matter. Recent observations indirectly
suggest that as much as half of this ``dark matter'' may be in the form of old,
very cool white dwarfs, the remnants of an ancient population of stars as old
as the Galaxy itself. We conducted a survey to find faint, cool white dwarfs
with large space velocities, indicative of their membership in the Galaxy's
spherical halo component. The survey reveals a substantial, directly observed
population of old white dwarfs, too faint to be seen in previous surveys. This
newly discovered population accounts for at least 2% of the halo dark matter.
It provides a natural explanation for the indirect observations, and represents
a direct detection of Galactic halo dark matter.Comment: 13 pages, 4 figures, 1 table. Note added after Science Express online
publication: This text reflects the correction of a few typographical errors
in the online version of the table. It also includes the new constraint on
the calculation of d_max which accounts for the fact that the survey could
not have detected stars with proper motions below 0.33 arcseconds per year.
Published online at ScienceExpress www.sciencemag.org 22 March 2001;
10.1126/science.1059954; To appear in Science 27 April 200
Granular Packings: Nonlinear elasticity, sound propagation and collective relaxation dynamics
Experiments on isotropic compression of a granular assembly of spheres show
that the shear and bulk moduli vary with the confining pressure faster than the
1/3 power law predicted by Hertz-Mindlin effective medium theories (EMT) of
contact elasticity. Moreover, the ratio between the moduli is found to be
larger than the prediction of the elastic theory by a constant value. The
understanding of these discrepancies has been a longstanding question in the
field of granular matter. Here we perform a test of the applicability of
elasticity theory to granular materials. We perform sound propagation
experiments, numerical simulations and theoretical studies to understand the
elastic response of a deforming granular assembly of soft spheres under
isotropic loading. Our results for the behavior of the elastic moduli of the
system agree very well with experiments. We show that the elasticity partially
describes the experimental and numerical results for a system under
compressional loads. However, it drastically fails for systems under shear
perturbations, particularly for packings without tangential forces and
friction. Our work indicates that a correct treatment should include not only
the purely elastic response but also collective relaxation mechanisms related
to structural disorder and nonaffine motion of grains.Comment: 21 pages, 13 figure
Building Success in Online Educational Programs for Adult Learners
The purpose of this symposium is to explore multiple perspectives on building and maintaining high quality online educational programs in university settings for adult learners
Diagnosis of vertebral fractures in children: is a simplified algorithm-based qualitative technique reliable?
Background Identification of osteoporotic vertebral fractures
allows treatment opportunity reducing future risk. There is no
agreed standardised method for diagnosing paediatric vertebral
fractures.
Objective To evaluate the precision of a modified adult
algorithm-based qualitative (ABQ) technique, applicable to
children with primary or secondary osteoporosis.
Materials and methods Three radiologists independently
assessed lateral spine radiographs of 50 children with
suspected reduction in bone mineral density using a modified
ABQ scoring system and following simplification to include
only clinically relevant parameters, a simplified ABQ score. A
final consensus of all observers using simplified ABQ was
performed as a reference standard for fracture characterisation.
Kappa was calculated for interobserver agreement of the
components of both scoring systems and intraobserver agreement
of simplified ABQ based on a second read of 29 randomly
selected images.
Results Interobserver Kappa for modified ABQ scoring for
fracture detection, severity and shape ranged from 0.34 to
0.49 Kappa for abnormal endplate and position assessment
was 0.27 to 0.38. Inter- and intraobserver Kappa for simplified
ABQ scoring for fracture detection and grade ranged from
0.37 to 0.46 and 0.45 to 0.56, respectively. Inter- and
intraobserver Kappa for affected endplate ranged from 0.31
to 0.41 and 0.45 to 0.51, respectively. Subjectively, observers’
felt simplified ABQ was easier and less time-consuming.
Conclusion Observer reliability of modified and simplified
ABQ was similar, with slight to moderate agreement for fracture
detection and grade/severity. Due to subjective preference
for simplified ABQ, we suggest its use as a semi-objective
measure of diagnosing paediatric vertebral fracture
Diagnostic accuracy of DXA compared to conventional spine radiographs for the detection of vertebral fractures in children
Objectives
In children, radiography is performed to diagnose vertebral fractures and dual energy x-ray absorptiometry (DXA) to
assess bone density. In adults, DXA assesses both. We aimed to establish whether
DXA can replace spine radiographs in assessment of paediatric vertebral fractures.
Methods
Prospectively, lateral spine radiographs and lateral spine DXA of 250 children
performed on the same day were independently scored by three radiologists using
the simplified algorithm based qualitative technique and blinded to results of the
other modality. Consensus radiograph read and second read of 100 random images
were performed. Diagnostic accuracy, inter/intraobserver and intermodality
agreements, patient/carer experience and radiation dose were assessed.
Results
Average sensitivity and specificity (95% confidence interval) in diagnosing one or
more vertebral fractures requiring treatment was 70% (58%-82%) and 97% (94%-
100%) respectively for DXA and 74% (55%-93%) and 96% (95%-98%) for
radiographs. Fleiss’ kappa for interobserver and average kappa for intraobserver
reliability were 0.371 and 0.631 respectively for DXA and 0.418 and 0.621 for
radiographs. Average effective dose was 41.9µSv for DXA and 232.7µSv for
radiographs. Image quality was similar.
Conclusion
Given comparable image quality and non-inferior diagnostic accuracy, lateral spine
DXA should replace conventional radiographs for assessment of vertebral fractures
in children
Why Effective Medium Theory Fails in Granular Materials
Experimentally it is known that the bulk modulus, K, and shear modulus, \mu,
of a granular assembly of elastic spheres increase with pressure, p, faster
than the p^1/3 law predicted by effective medium theory (EMT) based on
Hertz-Mindlin contact forces. To understand the origin of these discrepancies,
we perform numerical simulations of granular aggregates under compression. We
show that EMT can describe the moduli pressure dependence if one includes the
increasing number of grain-grain contacts with p. Most important, the affine
assumption (which underlies EMT), is found to be valid for K(p) but breakdown
seriously for \mu(p). This explains why the experimental and numerical values
of \mu(p) are much smaller than the EMT predictions.Comment: 4 pages, 5 figures, http://polymer.bu.edu/~hmaks
Pressure dependence of the sound velocity in a 2D lattice of Hertz-Mindlin balls: a mean field description
We study the dependence on the external pressure of the velocities
of long wavelength sound waves in a confined 2D h.c.p. lattice of 3D
elastic frictional balls interacting via one-sided Hertz-Mindlin contact
forces, whose diameters exhibit mild dispersion. The presence of an underlying
long range order enables us to build an effective medium description which
incorporates the radial fluctuations of the contact forces acting on a single
site. Due to the non linearity of Hertz elasticity, self-consistency results in
a highly non-linear differential equation for the "equation of state" linking
the effective stiffness of the array with the applied pressure, from which
sound velocities are then obtained. The results are in excellent agreement with
existing experimental results and simulations in the high and intermediate
pressure regimes. It emerges from the analysis that the departure of
from the ideal Hertz behavior must be attributed primarily to the
fluctuations of the stress field, rather than to the pressure dependence of the
number of contacts
How well do Earth system models reproduce the observed aerosol response to rapid emission reductions? A COVID-19 case study
The spring 2020 COVID-19 lockdowns led to a rapid reduction in aerosol and aerosol precursor emissions. These emission reductions provide a unique opportunity for model evaluation and to assess the potential efficacy of future emission control measures. We investigate changes in observed regional aerosol optical depth (AOD) during the COVID-19 lockdowns and use these observed anomalies to evaluate Earth system model simulations forced with COVID-19-like reductions in aerosols and greenhouse gases. Most anthropogenic source regions do not exhibit statistically significant changes in satellite retrievals of total or dust-subtracted AOD, despite the dramatic economic and lifestyle changes associated with the pandemic. Of the regions considered, only India exhibits an AOD anomaly that exceeds internal variability. Earth system models reproduce the observed responses reasonably well over India but initially appear to overestimate the magnitude of response in East China and when averaging over the Northern Hemisphere (0–70∘ N) as a whole. We conduct a series of sensitivity tests to systematically assess the contributions of internal variability, model input uncertainty, and observational sampling to the aerosol signal, and we demonstrate that the discrepancies between observed and simulated AOD can be partially resolved through the use of an updated emission inventory. The discrepancies can also be explained in part by characteristics of the observational datasets. Overall our results suggest that current Earth system models have potential to accurately capture the effects of future emission reductions.</p
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