625 research outputs found
Dark Energy Evolution and the Curvature of the Universe from Recent Observations
We discuss the constraints on the time-varying equation of state for dark
energy and the curvature of the universe using observations of type Ia
supernovae from Riess et al. and the most recent Supernova Legacy Survey
(SNLS), the baryon acoustic oscillation peak detected in the SDSS luminous red
galaxy survey and cosmic microwave background. Due to the degeneracy among the
parameters which describe the time dependence of the equation of state and the
curvature of the universe, the constraints on them can be weakened when we try
to constrain them simultaneously, in particular when we use a single
observational data. However, we show that we can obtain relatively severe
constraints when we use all data sets from observations above even if we
consider the time-varying equation of state and do not assume a flat universe.
We also found that the combined data set favors a flat universe even if we
consider the time variation of dark energy equation of state.Comment: 17 pages, 12 figures, references adde
Duality and cosmological compactification of superstrings with unbroken supersymmetry
The cosmological compactification of D=10, N=1 supergravity-super-Yang-Mills
theory obtained from superstring theory is studied. The constraint of unbroken
N=1 supersymmetry is imposed. A duality transformation is performed on the
resulting consistency conditions. The original equations as well as the
transformed equations are solved numerically to obtain new configurations with
a nontrivial scale factor and a dynamical dilaton. It is shown that various
classes of solutions are possible, which include cosmological solutions with no
initial singularity.Comment: Latex2e file, 24 pages including 10 figures as tex file
The Holographic dark energy reexamined
We have reexamined the holographic dark energy model by considering the
spatial curvature. We have refined the model parameter and observed that the
holographic dark energy model does not behave as phantom model. Comparing the
holographic dark energy model to the supernova observation alone, we found that
the closed universe is favored. Combining with the Wilkinson Microwave
Anisotropy Probe (WMAP) data, we obtained the reasonable value of the spatial
curvature of our universe.Comment: divided into sections, add one figure, some typos corrected,
references added, Accepted for publication in PRD; v3: some typos corrected,
title change
Late acceleration and crossing in induced gravity
We study the cosmological evolution on a brane with induced gravity within a
bulk with arbitrary matter content. We consider a Friedmann-Robertson-Walker
brane, invariantly characterized by a six-dimensional group of isometries. We
derive the effective Friedmann and Raychaudhuri equations. We show that the
Hubble expansion rate on the brane depends on the covariantly defined
integrated mass in the bulk, which determines the energy density of the
generalized dark radiation. The Friedmann equation has two branches,
distinguished by the two possible values of the parameter \ex=\pm 1. The
branch with \ex=1 is characterized by an effective cosmological constant and
accelerated expansion for low energy densities. Another remarkable feature is
that the contribution from the generalized dark radiation appears with a
negative sign. As a result, the presence of the bulk corresponds to an
effective negative energy density on the brane, without violation of the weak
energy condition. The transition from a period of domination of the matter
energy density by non-relativistic brane matter to domination by the
generalized dark radiation corresponds to a crossing of the phantom divide
.Comment: 7 pages, no figures, RevTex 4.0; (v2) new references are added, minor
corrections and expanded discussion; (v3) additional comments at the end of
section III, minor corrections and several new references are added, to match
published version in Phys. Rev.
Scalar Field Dark Energy Perturbations and their Scale Dependence
We estimate the amplitude of perturbation in dark energy at different length
scales for a quintessence model with an exponential potential. It is shown that
on length scales much smaller than hubble radius, perturbation in dark energy
is negligible in comparison to that in in dark matter. However, on scales
comparable to the hubble radius () the
perturbation in dark energy in general cannot be neglected. As compared to the
CDM model, large scale matter power spectrum is suppressed in a
generic quintessence dark energy model. We show that on scales , this suppression is primarily due to different background
evolution compared to CDM model. However, on much larger scales
perturbation in dark energy can effect matter power spectrum significantly.
Hence this analysis can act as a discriminator between CDM model and
other generic dark energy models with .Comment: 12 pages, 13 figures, added new section, accepted for publication in
Phys. Rev.
Student Observations of Postsecondary Classroom Instruction: Accessibility Challenges and Collaborative Feedback.
For deaf students, accessible classroom design is often provided through external services such as interpreters or speech-to-text providers. An approach based in Universal Design for Learning (UDL), though, seeks to engage students in learning by creating classroom spaces with accessibility integrated into the pedagogical approach. Realizing these goals requires participation from one of the most valuable, yet underused, resources: the students themselves. To that end, this paper examines a student-faculty collaborative approach to increasing accessibility for deaf students in postsecondary classrooms. Results of this study suggest that student observers are able to provide concrete and constructive feedback on strategies to increase classroom accessibility
Equation of state description of the dark energy transition between quintessence and phantom regimes
The dark energy crossing of the cosmological constant boundary (the
transition between the quintessence and phantom regimes) is described in terms
of the implicitly defined dark energy equation of state. The generalizations of
the models explicitly constructed to exhibit the crossing provide the insight
into the cancellation mechanism which makes the transition possible.Comment: 3 pages, talk given at TAUP200
String dynamics near a Kaluza-Klein black hole
The dynamics of a string near a Kaluza-Klein black hole are studied.
Solutions to the classical string equations of motion are obtained using the
world sheet velocity of light as an expansion parameter. The electrically and
magnetically charged cases are considered separately. Solutions for string
coordinates are obtained in terms of the world-sheet coordinate . It is
shown that the Kaluza-Klein radius increases/decreases with for
electrically/magnetically charged black hole.Comment: Latex2e file with six postscript figures. Minor changes, more
accurate numerical results and updated reference
A modified scoring system to describe gross pathology in the rabbit model of tuberculosis
<p>Abstract</p> <p>Background</p> <p>The rabbit model is an ideal means to study the pathogenesis of tuberculosis due to its semblance to the disease in humans. We have previously described the results using a bronchoscopic route of infection with live bacilli as a reliable means of generating lung cavities in sensitized rabbits. The role of sensitization in the development of disease outcomes has been well established in several animal models. We have described here the varying gross pathology that result from lack of sensitization with heat-killed <it>M. bovis </it>prior to high-dose bronchoscopic infection with live bacilli.</p> <p>Results</p> <p>Rabbits lacking sensitization did not generate lung cavities, but instead formed solely a tuberculoid pneumonia that replaced the normal lung parenchyma in the area of infection. Extrapulmonary dissemination was seen in approximately equal frequency and distribution in both rabbit populations. Notable differences include the lack of intestinal lesions in non-sensitized rabbits likely due to the lack of ingestion of expectorated bacilli from cavitary lesions. The experiment also employed a modified scoring system developed initially in the primate model of tuberculosis to allow for the quantification of findings observed at necropsy.</p> <p>Conclusions</p> <p>To date, no such scoring system has been employed in the rabbit model to describe gross pathology. The quantitative methodology would allow for rapid comparative analyses and standardization of thoracic and extrapulmonary pathology that could be evaluated for statistical significance. The aim is to use such a scoring system as the foundation for all future rabbit studies describing gross pathology at all stages in TB pathogenesis.</p
Direct reconstruction of the quintessence potential
We describe an algorithm which directly determines the quintessence potential
from observational data, without using an equation of state parametrisation.
The strategy is to numerically determine observational quantities as a function
of the expansion coefficients of the quintessence potential, which are then
constrained using a likelihood approach. We further impose a model selection
criterion, the Bayesian Information Criterion, to determine the appropriate
level of the potential expansion. In addition to the potential parameters, the
present-day quintessence field velocity is kept as a free parameter. Our
investigation contains unusual model types, including a scalar field moving on
a flat potential, or in an uphill direction, and is general enough to permit
oscillating quintessence field models. We apply our method to the `gold' Type
Ia supernovae sample of Riess et al. (2004), confirming the pure cosmological
constant model as the best description of current supernovae
luminosity-redshift data. Our method is optimal for extracting quintessence
parameters from future data.Comment: 9 pages RevTeX4 with lots of incorporated figure
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