18,089 research outputs found
Inflation, Renormalization, and CMB Anisotropies
In single-field, slow-roll inflationary models, scalar and tensorial
(Gaussian) perturbations are both characterized by a zero mean and a non-zero
variance. In position space, the corresponding variance of those fields
diverges in the ultraviolet. The requirement of a finite variance in position
space forces its regularization via quantum field renormalization in an
expanding universe. This has an important impact on the predicted scalar and
tensorial power spectra for wavelengths that today are at observable scales. In
particular, we find a non-trivial change in the consistency condition that
relates the tensor-to-scalar ratio "r" to the spectral indices. For instance,
an exact scale-invariant tensorial power spectrum, n_t=0, is now compatible
with a non-zero ratio r= 0.12 +/- 0.06, which is forbidden by the standard
prediction (r=-8n_t). Forthcoming observations of the influence of relic
gravitational waves on the CMB will offer a non-trivial test of the new
predictions.Comment: 4 pages, jpconf.cls, to appear in the Proceedings of Spanish
Relativity Meeting 2009 (ERE 09), Bilbao (Spain
Raychaudhuri's equation and aspects of relativistic charged collapse
We use the Raychaudhuri equation to probe certain aspects related to the
gravitational collapse of a charged medium. The aim is to identify the stresses
the Maxwell field exerts on the fluid and discuss their potential implications.
Particular attention is given to those stresses that resist contraction. After
looking at the general case, we consider the two opposite limits of poor and
high electrical conductivity. In the former there are electric fields but no
currents, while in the latter the situation is reversed. When the conductivity
is low, we find that the main agents acting against the collapse are the
Coulomb forces triggered by the presence of an excess charge. At the ideal
Magnetohydrodynamic (MHD) limit, on the other hand, the strongest resistance
seems to come from the tension of the magnetic forcelines. In either case, we
discuss whether and how the aforementioned resisting stresses may halt the
contraction and provide a set of conditions making this likely to happen.Comment: Revised version, to appear in PR
Magnetized Tolman-Bondi Collapse
We investigate the gravitational implosion of magnetized matter by studying
the inhomogeneous collapse of a weakly magnetized Tolman-Bondi spacetime. The
role of the field is analyzed by looking at the convergence of neighboring
particle worldlines. In particular, we identify the magnetically related
stresses in the Raychaudhuri equation and use the Tolman-Bondi metric to
evaluate their impact on the collapsing dust. We find that, despite the low
energy level of the field, the Lorentz force dominates the advanced stages of
the collapse, leading to a strongly anisotropic contraction. In addition, of
all the magnetic stresses, those that resist the collapse are found to grow
faster.Comment: 6 pages, RevTex; v2: physical interpretation of the results slightly
changed, references added, version accepted in Phys. Rev. D (2006
Inverse Scattering Transform for the Camassa-Holm equation
An Inverse Scattering Method is developed for the Camassa-Holm equation. As
an illustration of our approach the solutions corresponding to the
reflectionless potentials are explicitly constructed in terms of the scattering
data. The main difference with respect to the standard Inverse Scattering
Transform lies in the fact that we have a weighted spectral problem. We
therefore have to develop different asymptotic expansions.Comment: 17 pages, LaTe
The Casimir Effect for Parallel Plates Revisited
The Casimir effect for a massless scalar field with Dirichlet and periodic
boundary conditions (b.c.) on infinite parallel plates is revisited in the
local quantum field theory (lqft) framework introduced by B.Kay. The model
displays a number of more realistic features than the ones he treated. In
addition to local observables, as the energy density, we propose to consider
intensive variables, such as the energy per unit area , as
fundamental observables. Adopting this view, lqft rejects Dirichlet (the same
result may be proved for Neumann or mixed) b.c., and accepts periodic b.c.: in
the former case diverges, in the latter it is finite, as is shown by
an expression for the local energy density obtained from lqft through the use
of the Poisson summation formula. Another way to see this uses methods from the
Euler summation formula: in the proof of regularization independence of the
energy per unit area, a regularization-dependent surface term arises upon use
of Dirichlet b.c. but not periodic b.c.. For the conformally invariant scalar
quantum field, this surface term is absent, due to the condition of zero trace
of the energy momentum tensor, as remarked by B.De Witt. The latter property
does not hold in tha application to the dark energy problem in Cosmology, in
which we argue that periodic b.c. might play a distinguished role.Comment: 25 pages, no figures, late
Superrevivals in the quantum dynamics of a particle confined in a finite square well potential
We examine the revival features in wave packet dynamics of a particle
confined in a finite square well potential. The possibility of tunneling
modifies the revival pattern as compared to an infinite square well potential.
We study the dependence of the revival times on the depth of the square well
and predict the existence of superrevivals. The nature of these superrevivals
is compared with similar features seen in the dynamics of wavepackets in an
anharmonic oscillator potential.Comment: 8 pages in Latex two-column format with 5 figures (eps). To appear in
Physical Review
Neural Network-Based Equations for Predicting PGA and PGV in Texas, Oklahoma, and Kansas
Parts of Texas, Oklahoma, and Kansas have experienced increased rates of
seismicity in recent years, providing new datasets of earthquake recordings to
develop ground motion prediction models for this particular region of the
Central and Eastern North America (CENA). This paper outlines a framework for
using Artificial Neural Networks (ANNs) to develop attenuation models from the
ground motion recordings in this region. While attenuation models exist for the
CENA, concerns over the increased rate of seismicity in this region necessitate
investigation of ground motions prediction models particular to these states.
To do so, an ANN-based framework is proposed to predict peak ground
acceleration (PGA) and peak ground velocity (PGV) given magnitude, earthquake
source-to-site distance, and shear wave velocity. In this framework,
approximately 4,500 ground motions with magnitude greater than 3.0 recorded in
these three states (Texas, Oklahoma, and Kansas) since 2005 are considered.
Results from this study suggest that existing ground motion prediction models
developed for CENA do not accurately predict the ground motion intensity
measures for earthquakes in this region, especially for those with low
source-to-site distances or on very soft soil conditions. The proposed ANN
models provide much more accurate prediction of the ground motion intensity
measures at all distances and magnitudes. The proposed ANN models are also
converted to relatively simple mathematical equations so that engineers can
easily use them to predict the ground motion intensity measures for future
events. Finally, through a sensitivity analysis, the contributions of the
predictive parameters to the prediction of the considered intensity measures
are investigated.Comment: 5th Geotechnical Earthquake Engineering and Soil Dynamics Conference,
Austin, TX, USA, June 10-13. (2018
The Wisconsin H-Alpha Mapper Northern Sky Survey
The Wisconsin H-Alpha Mapper (WHAM) has surveyed the distribution and
kinematics of ionized gas in the Galaxy above declination -30 degrees. The WHAM
Northern Sky Survey (WHAM-NSS) has an angular resolution of one degree and
provides the first absolutely-calibrated, kinematically-resolved map of the
H-Alpha emission from the Warm Ionized Medium (WIM) within ~ +/-100 km/s of the
Local Standard of Rest. Leveraging WHAM's 12 km/s spectral resolution, we have
modeled and removed atmospheric emission and zodiacal absorption features from
each of the 37,565 spectra. The resulting H-Alpha profiles reveal ionized gas
detected in nearly every direction on the sky with a sensitivity of 0.15 R (3
sigma). Complex distributions of ionized gas are revealed in the nearby spiral
arms up to 1-2 kpc away from the Galactic plane. Toward the inner Galaxy, the
WHAM-NSS provides information about the WIM out to the tangent point down to a
few degrees from the plane. Ionized gas is also detected toward many
intermediate velocity clouds at high latitudes. Several new H II regions are
revealed around early B-stars and evolved stellar cores (sdB/O). This work
presents the details of the instrument, the survey, and the data reduction
techniques. The WHAM-NSS is also presented and analyzed for its gross
properties. Finally, some general conclusions are presented about the nature of
the WIM as revealed by the WHAM-NSS.Comment: 42 pages, 14 figures (Fig 6-9 & 14 are full color); accepted for
publication in 2003, ApJ, 149; Original quality figures (as well as data for
the survey) are available at http://www.astro.wisc.edu/wham
The Dynamics of a Meandering River
We present a statistical model of a meandering river on an alluvial plane
which is motivated by the physical non-linear dynamics of the river channel
migration and by describing heterogeneity of the terrain by noise. We study the
dynamics analytically and numerically. The motion of the river channel is
unstable and we show that by inclusion of the formation of ox-bow lakes, the
system may be stabilised. We then calculate the steady state and show that it
is in agreement with simulations and measurements of field data.Comment: Revtex, 12 pages, 2 postscript figure
Magnetic tension and gravitational collapse
The gravitational collapse of a magnetised medium is investigated by studying
qualitatively the convergence of a timelike family of non-geodesic worldlines
in the presence of a magnetic field. Focusing on the field's tension we
illustrate how the winding of the magnetic forcelines due to the fluid's
rotation assists the collapse, while shear-like distortions in the distribution
of the field's gradients resist contraction. We also show that the relativistic
coupling between magnetism and geometry, together with the tension properties
of the field, lead to a magneto-curvature stress that opposes the collapse.
This tension stress grows stronger with increasing curvature distortion, which
means that it could potentially dominate over the gravitational pull of the
matter. If this happens, a converging family of non-geodesic lines can be
prevented from focusing without violating the standard energy conditions.Comment: Typos corrected. Published versio
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