12,044 research outputs found
The Holography Hypothesis in Pre-Big-Bang Cosmology with String Sources
The holographic ratio in Pre-big bang string cosmology is obtained in the
presence string sources. An iterative procedure is adopted to solve the
equations of motion and derive the ratio in four dimensional world. First the
zeroth order ratio is computed in the remote past, i.e. at , then
the holographic ratio is obtained taking into account the evolution of the
backgrounds following the iterative procedure. The corrections to the zeroth
order value of the ratio depends on the form of the initial number distribution
of the strings chosen. Moreover, we estimate the holographic ratio in the
recent past (i.e. when ) and in the remote past (i.e. when
), , in different dimensions in the
Einstein frame and in the string frame. We find that in the first case it has
similar time dependences in both the frames, especially in four dimensions the
ratio is explicitly computed to be the same in the two cases, whereas for
case the time dependence is different.Comment: 33 pages, latex, five figure
Low-energy quenching of positronium by helium
Very low-energy scattering of orthopositronium by helium has been
investigated for simultaneous study of elastic cross section and pick-off
quenching rate using a model exchange potential. The present calculational
scheme, while agrees with the measured cross section of Skalsey et al,
reproduces successfully the parameter ^ 1Z_{\makebox{eff}}, the effective
number of electrons per atom in a singlet state relative to the positron.
Together with the fact that this model potential also leads to an agreement
with measured medium energy cross sections of this system, this study seems to
resolve the long-standing discrepancy at low energies among different
theoretical calculations and experimental measurements.Comment: 4 latex pages, 3 postscript figure
Black soliton in a quasi-one-dimensional trapped fermion-fermion mixture
Employing a time-dependent mean-field-hydrodynamic model we study the
generation of black solitons in a degenerate fermion-fermion mixture in a
cigar-shaped geometry using variational and numerical solutions. The black
soliton is found to be the first stationary vibrational excitation of the
system and is considered to be a nonlinear continuation of the vibrational
excitation of the harmonic oscillator state. We illustrate the stationary
nature of the black soliton, by studying different perturbations on it after
its formation.Comment: 7 pages, 10 figure
Quantum Gravity Equation In Schroedinger Form In Minisuperspace Description
We start from classical Hamiltonian constraint of general relativity to
obtain the Einstein-Hamiltonian-Jacobi equation. We obtain a time parameter
prescription demanding that geometry itself determines the time, not the matter
field, such that the time so defined being equivalent to the time that enters
into the Schroedinger equation. Without any reference to the Wheeler-DeWitt
equation and without invoking the expansion of exponent in WKB wavefunction in
powers of Planck mass, we obtain an equation for quantum gravity in
Schroedinger form containing time. We restrict ourselves to a minisuperspace
description. Unlike matter field equation our equation is equivalent to the
Wheeler-DeWitt equation in the sense that our solutions reproduce also the
wavefunction of the Wheeler-DeWitt equation provided one evaluates the
normalization constant according to the wormhole dominance proposal recently
proposed by us.Comment: 11 Pages, ReVTeX, no figur
Time in Quantum Gravity
The Wheeler-DeWitt equation in quantum gravity is timeless in character. In
order to discuss quantum to classical transition of the universe, one uses a
time prescription in quantum gravity to obtain a time contained description
starting from Wheeler-DeWitt equation and WKB ansatz for the WD wavefunction.
The approach has some drawbacks. In this work, we obtain the time-contained
Schroedinger-Wheeler-DeWitt equation without using the WD equation and the WKB
ansatz for the wavefunction. We further show that a Gaussian ansatz for SWD
wavefunction is consistent with the Hartle-Hawking or wormhole dominance
proposal boundary condition. We thus find an answer to the small scale boundary
conditions.Comment: 12 Pages, LaTeX, no figur
Initial results of finger imaging using Photoacoustic Computed Tomography
We present a photoacoustic computed tomography investigation on a healthy
human finger, to image blood vessels with a focus on vascularity across the
interphalangeal joints. The cross-sectional images were acquired using an
imager specifically developed for this purpose. The images show rich detail of
the digital blood vessels with diameters between 100 m and 1.5 mm in
various orientations and at various depths. Different vascular layers in the
skin including the subpapillary plexus could also be visualized. Acoustic
reflections on the finger bone of photoacoustic signals from skin were visible
in sequential slice images along the finger except at the location of the joint
gaps. Not unexpectedly, the healthy synovial membrane at the joint gaps was not
detected due to its small size and normal vascularization. Future research will
concentrate on studying digits afflicted with rheumatoid arthritis to detect
the inflamed synovium with its heightened vascularization, whose
characteristics are potential markers for disease activity.Comment: 2 figure
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