14,037 research outputs found
Experimental Tests of General Relativity
Einstein's general theory of relativity is the standard theory of gravity,
especially where the needs of astronomy, astrophysics, cosmology and
fundamental physics are concerned. As such, this theory is used for many
practical purposes involving spacecraft navigation, geodesy, and time transfer.
Here I review the foundations of general relativity, discuss recent progress in
the tests of relativistic gravity in the solar system, and present motivations
for the new generation of high-accuracy gravitational experiments. I discuss
the advances in our understanding of fundamental physics that are anticipated
in the near future and evaluate the discovery potential of the recently
proposed gravitational experiments.Comment: revtex4, 30 pages, 10 figure
Statistical studies of supernova environments
Investigations of the environments of SNe allow statistical constraints to be made on progenitor properties. We review progress that has been made in this field. Pixel statistics using tracers of e.g. star formation within galaxies show differences in the explosion sites of, in particular SNe types II and Ibc (SNe II and SNe Ibc), suggesting differences in population ages. Of particular interest is that SNe Ic are significantly more associated with H-alpha emission than SNe Ib, implying shorter lifetimes for the former. In addition, such studies have shown that the interacting SNe IIn do not explode in regions containing the most massive stars, which suggests that at least a significant fraction of their progenitors arise from the lower end of the core-collapse SN mass range. Host HII region spectroscopy has been obtained for a significant number of core-collapse events, however definitive conclusions have to-date been elusive. Single stellar evolution models predict that the fraction of SNe Ibc to SNe II should increase with increasing metallicity, due to the dependence of mass-loss rates on progenitor metallicity. We present a meta-analysis of host HII region oxygen abundances for CC SNe. It is concluded that the SN II to SN Ibc ratio shows little variation with oxygen abundance, with only a suggestion that the ratio increases in the lowest bin. Radial distributions of different SNe are discussed, where a central excess of SNe Ibc has been observed within disturbed galaxy systems, which is difficult to ascribe to metallicity or selection effects. Environment studies are evolving to enable studies at higher spatial resolutions than previously possible, while in addition the advent of wide-field integral field unit instruments allows galaxy-wide spectral analyses which will provide fruitful results to this field. Some example contemporary results are shown in that direction
VAMPIRE® fundus image analysis algorithms:Validation and diagnostic relevance in hypertensive cats
OBJECTIVES: To validate a retinal imaging software named VAMPIRE® (Vascular Assay and Measurement Platform for Images of the Retina) in feline patients and test the clinical utility in hypertensive cats. ANIMALS STUDIED: One hundred and five healthy cats were enrolled. They represented the normal dataset used in the validation (group 1). Forty-three hypertensive cats with no noticeable retinal abnormalities were enrolled for the clinical validity of the software (group 2). PROCEDURES: Eleven points (4 veins, 4 arteries, and 3 arterial bifurcations) were measured for each digital image. Repeatability and reproducibility of measurements were assessed using two independent operators. Data were statistically analyzed by the Mann-Whiney and Tukey box plot. Significance was considered when P < 0.05. RESULTS: Two hundred and ten retinal images were analyzed for a total of 2310 measurements. Total mean was 9.1 and 6.1 pixels for veins and arteries, respectively. First, second, and third arteriolar bifurcations angles were 73.6°, 76.9°, and 85.4°, respectively. A comparison between groups 1 and 2 showed a statistically significant reduction in arteriolar diameter (mean 3.3 pixels) and branch angle (55°, 47.8° and 59.9°) associated with increasing vein diameter (mean 24.15 pixels). CONCLUSIONS: Current image analysis techniques used in human medicine were investigated in terms of extending their use to veterinary medicine. The VAMPIRE® algorithm proved useful for an objective diagnosis of retinal vasculature changes secondary to systemic hypertension in cats, and could be an additional diagnostic test for feline systemic hypertension
A critical role for the chromatin remodeller CHD7 in anterior mesoderm during cardiovascular development
CHARGE syndrome is caused by spontaneous loss-of-function mutations to the ATP-dependant chromatin remodeller chromodomain-helicase-DNA-binding protein 7 (CHD7). It is characterised by a distinct pattern of congenital anomalies, including cardiovascular malformations. Disruption to the neural crest lineage has previously been emphasised in the aetiology of this developmental disorder. We present evidence for an additional requirement for CHD7 activity in the Mesp1-expressing anterior mesoderm during heart development. Conditional ablation of Chd7 in this lineage results in major structural cardiovascular defects akin to those seen in CHARGE patients, as well as a striking loss of cardiac innervation and embryonic lethality. Genome-wide transcriptional analysis identified aberrant expression of key components of the Class 3 Semaphorin and Slit-Robo signalling pathways in Chd7(fl/fl);Mesp1-Cre mutant hearts. CHD7 localises at the Sema3c promoter in vivo, with alteration of the local chromatin structure seen following Chd7 ablation, suggestive of direct transcriptional regulation. Furthermore, we uncover a novel role for CHD7 activity upstream of critical calcium handling genes, and demonstrate an associated functional defect in the ability of cardiomyocytes to undergo excitation-contraction coupling. This work therefore reveals the importance of CHD7 in the cardiogenic mesoderm for multiple processes during cardiovascular development
A test of general relativity from the three-dimensional orbital geometry of a binary pulsar
Binary pulsars provide an excellent system for testing general relativity
because of their intrinsic rotational stability and the precision with which
radio observations can be used to determine their orbital dynamics.
Measurements of the rate of orbital decay of two pulsars have been shown to be
consistent with the emission of gravitational waves as predicted by general
relativity, providing the most convincing evidence for the self-consistency of
the theory to date. However, independent verification of the orbital geometry
in these systems was not possible. Such verification may be obtained by
determining the orientation of a binary pulsar system using only classical
geometric constraints, permitting an independent prediction of general
relativistic effects. Here we report high-precision timing of the nearby binary
millisecond pulsar PSR J0437-4715, which establish the three-dimensional
structure of its orbit. We see the expected retardation of the pulse signal
arising from the curvature of space-time in the vicinity of the companion
object (the `Shapiro delay'), and we determine the mass of the pulsar and its
white dwarf companion. Such mass determinations contribute to our understanding
of the origin and evolution of neutron stars.Comment: 5 pages, 2 figure
Lagrangian Reachabililty
We introduce LRT, a new Lagrangian-based ReachTube computation algorithm that
conservatively approximates the set of reachable states of a nonlinear
dynamical system. LRT makes use of the Cauchy-Green stretching factor (SF),
which is derived from an over-approximation of the gradient of the solution
flows. The SF measures the discrepancy between two states propagated by the
system solution from two initial states lying in a well-defined region, thereby
allowing LRT to compute a reachtube with a ball-overestimate in a metric where
the computed enclosure is as tight as possible. To evaluate its performance, we
implemented a prototype of LRT in C++/Matlab, and ran it on a set of
well-established benchmarks. Our results show that LRT compares very favorably
with respect to the CAPD and Flow* tools.Comment: Accepted to CAV 201
On the environments of Type Ia supernovae within host galaxies
We present constraints on supernovae type Ia (SNe Ia) progenitors through an analysis of the environments found at the explosion sites of 102 events within star-forming host galaxies. Hα and GALEX near-UV images are used to trace on-going and recent star formation (SF), while broad band B,R, J,K imaging is also analysed. Using pixel statistics we find that SNe Ia show the lowest degree of association with Hα emission of all supernova types.
It is also found that they do not trace near-UV emission. As the latter traces SF on timescales less than 100Myr, this rules out any extreme ‘prompt’ delay-times as the dominant progenitor channel of SNe Ia. SNe Ia best trace the B-band light distribution of their host galaxies. This
implies that the population within star-forming galaxies is dominated by relatively young progenitors. Splitting SNe by their (B-V) colours at maximumlight, ‘redder’ events show a higher degree of association to H II regions and are found more centrally within hosts. We discuss possible explanations of this result in terms of line of sight extinction and progenitor effects. No evidence for correlations between SN stretch and environment properties is observed.
Key words: supernovae: general, galaxies: statistic
An evaluation of strategies used by the Landscapes and Policy Hub to achieve interdisciplinary and transdisciplinary research
The report presents an evaluation of the Landscapes and Policy Hub's approach to interdisciplinary and transdisciplinary research. The hub was a national, four year, $15 million collaborative research program. The focus of the evaluation was for researchers to reflect on the effectiveness of strategies used by the hub to facilitate interdisciplinarity (where researchers from different disciplines work together to solve problems) and transdisciplinarity (where researchers from different disciplines work in partnership with research users to solve problems).
The evaluation was commissioned in the final phase of the hub’s life in the interests of improving performance of future interdisciplinary and transdisciplinary research. It was based on a number of strategies that had been implemented by the hub to encourage and facilitate interdisciplinary research occurring in partnership with research users. The aim of the evaluation was to improve performance of future interdisciplinary and transdisciplinary research. Six recommendations are presented
Pathwise Sensitivity Analysis in Transient Regimes
The instantaneous relative entropy (IRE) and the corresponding instanta-
neous Fisher information matrix (IFIM) for transient stochastic processes are
pre- sented in this paper. These novel tools for sensitivity analysis of
stochastic models serve as an extension of the well known relative entropy rate
(RER) and the corre- sponding Fisher information matrix (FIM) that apply to
stationary processes. Three cases are studied here, discrete-time Markov
chains, continuous-time Markov chains and stochastic differential equations. A
biological reaction network is presented as a demonstration numerical example
The connection between superconducting phase correlations and spin excitations in YBaCuO: A magnetic field study
One of the most striking universal properties of the
high-transition-temperature (high-) superconductors is that they are all
derived from the hole-doping of their insulating antiferromagnetic (AF) parent
compounds. From the outset, the intimate relationship between magnetism and
superconductivity in these copper-oxides has intrigued researchers. Evidence
for this link comes from neutron scattering experiments that show the
unambiguous presence of short-range AF correlations (excitations) in cuprate
superconductors. Even so, the role of such excitations in the pairing mechanism
and superconductivity is still a subject of controversy. For
YBaCuO, where controls the hole-doping level, the most
prominent feature in the magnetic excitations spectra is the ``resonance''.
Here we show that for underdoped YBaCuO, where and
are below the optimal values, modest magnetic fields suppress the resonance
significantly, much more so for fields approximately perpendicular rather than
parallel to the CuO planes. Our results indicate that the resonance
measures pairing and phase coherence, suggesting that magnetism plays an
important role in the superconductivity of cuprates. The persistence of a field
effect above favors mechanisms with preformed pairs in the normal state
of underdoped cuprates.Comment: 12 pages, 4 figures, Nature (in press
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