1,166 research outputs found
Bounds on the basic physical parameters for anisotropic compact general relativistic objects
We derive upper and lower limits for the basic physical parameters
(mass-radius ratio, anisotropy, redshift and total energy) for arbitrary
anisotropic general relativistic matter distributions in the presence of a
cosmological constant. The values of these quantities are strongly dependent on
the value of the anisotropy parameter (the difference between the tangential
and radial pressure) at the surface of the star. In the presence of the
cosmological constant, a minimum mass configuration with given anisotropy does
exist. Anisotropic compact stellar type objects can be much more compact than
the isotropic ones, and their radii may be close to their corresponding
Schwarzschild radii. Upper bounds for the anisotropy parameter are also
obtained from the analysis of the curvature invariants. General restrictions
for the redshift and the total energy (including the gravitational
contribution) for anisotropic stars are obtained in terms of the anisotropy
parameter. Values of the surface redshift parameter greater than two could be
the main observational signature for anisotropic stellar type objects.Comment: 18 pages, no figures, accepted for publication in CQ
Minimum mass-radius ratio for charged gravitational objects
We rigorously prove that for compact charged general relativistic objects
there is a lower bound for the mass-radius ratio. This result follows from the
same Buchdahl type inequality for charged objects, which has been extensively
used for the proof of the existence of an upper bound for the mass-radius
ratio. The effect of the vacuum energy (a cosmological constant) on the minimum
mass is also taken into account. Several bounds on the total charge, mass and
the vacuum energy for compact charged objects are obtained from the study of
the Ricci scalar invariants. The total energy (including the gravitational one)
and the stability of the objects with minimum mass-radius ratio is also
considered, leading to a representation of the mass and radius of the charged
objects with minimum mass-radius ratio in terms of the charge and vacuum energy
only.Comment: 19 pages, accepted by GRG, references corrected and adde
Can dark matter be a Bose-Einstein condensate?
We consider the possibility that the dark matter, which is required to
explain the dynamics of the neutral hydrogen clouds at large distances from the
galactic center, could be in the form of a Bose-Einstein condensate. To study
the condensate we use the non-relativistic Gross-Pitaevskii equation. By
introducing the Madelung representation of the wave function, we formulate the
dynamics of the system in terms of the continuity equation and of the
hydrodynamic Euler equations. Hence dark matter can be described as a
non-relativistic, Newtonian Bose-Einstein gravitational condensate gas, whose
density and pressure are related by a barotropic equation of state. In the case
of a condensate with quartic non-linearity, the equation of state is polytropic
with index . To test the validity of the model we fit the Newtonian
tangential velocity equation of the model with a sample of rotation curves of
low surface brightness and dwarf galaxies, respectively. We find a very good
agreement between the theoretical rotation curves and the observational data
for the low surface brightness galaxies. The deflection of photons passing
through the dark matter halos is also analyzed, and the bending angle of light
is computed. The bending angle obtained for the Bose-Einstein condensate is
larger than that predicted by standard general relativistic and dark matter
models. Therefore the study of the light deflection by galaxies and the
gravitational lensing could discriminate between the Bose-Einstein condensate
dark matter model and other dark matter models.Comment: 20 pages, 7 figures, accepted for publication in JCAP, references
adde
Solar system tests of brane world models
The classical tests of general relativity (perihelion precession, deflection
of light, and the radar echo delay) are considered for the Dadhich, Maartens,
Papadopoulos and Rezania (DMPR) solution of the spherically symmetric static
vacuum field equations in brane world models. For this solution the metric in
the vacuum exterior to a brane world star is similar to the Reissner-Nordstrom
form of classical general relativity, with the role of the charge played by the
tidal effects arising from projections of the fifth dimension. The existing
observational solar system data on the perihelion shift of Mercury, on the
light bending around the Sun (obtained using long-baseline radio
interferometry), and ranging to Mars using the Viking lander, constrain the
numerical values of the bulk tidal parameter and of the brane tension.Comment: 14 pages, to appear in Classical and Quantum Gravity. V2, minor
corrections and references adde
Multi-Atom Quasiparticle Scattering Interference for Superconductor Energy-Gap Symmetry Determination
Complete theoretical understanding of the most complex superconductors
requires a detailed knowledge of the symmetry of the superconducting energy-gap
, for all momenta on the Fermi surface
of every band . While there are a variety of techniques for determining
, no general method existed to measure the signed
values of . Recently, however, a new technique based
on phase-resolved visualization of superconducting quasiparticle interference
(QPI) patterns centered on a single non-magnetic impurity atom, was introduced.
In principle, energy-resolved and phase-resolved Fourier analysis of these
images identifies wavevectors connecting all k-space regions where
has the same or opposite sign. But use of a single
isolated impurity atom, from whose precise location the spatial phase of the
scattering interference pattern must be measured is technically difficult. Here
we introduce a generalization of this approach for use with multiple impurity
atoms, and demonstrate its validity by comparing the
it generates to the determined from single-atom
scattering in FeSe where energy-gap symmetry is established. Finally,
to exemplify utility, we use the multi-atom technique on LiFeAs and find
scattering interference between the hole-like and electron-like pockets as
predicted for of opposite sign
Solidity of Viscous Liquids
Recent NMR experiments on supercooled toluene and glycerol by Hinze and
Bohmer show that small rotation angles dominate with only few large molecular
rotations. These results are here interpreted by assuming that viscous liquids
are solid-like on short length scales. A characteristic length, the "solidity
length", separates solid-like behavior from liquid-like behavior.Comment: Plain RevTex file, no figure
Symptoms of gastroesophageal reflux disease in severely mentally retarded people: a systematic review
<p>Abstract</p> <p>Background</p> <p>Gastroesophageal reflux disease (GERD) occurs when stomach acid frequently backs up (or refluxes) into the gullet (or esophagus), and it has serious consequences for the quality of life. Usually this is felt as heartburn. Because severely mentally retarded people usually do not utter complaints of heartburn, it requires a high index of suspicion to discover possible GERD. Therefore it is relevant for care professionals such as nurses to have knowledge of those with a higher risk of GERD and of the possible manifestations of GERD.</p> <p>Methods</p> <p>Using a predefined search method, electronic databases were searched for studies relating the presence of symptoms to the presence of GERD. Relevant data were extracted and the methodological quality of the studies assessed. The results of the included studies were synthesized and conclusions about the level of evidence were drawn.</p> <p>Results</p> <p>Nineteen studies were found relating symptoms to the presence of GERD. Only four were of good methodological quality. The studies were very diverse concerning the studied population, the study method, and the kind of symptoms examined. This makes it difficult to synthesize the results of the studies. There is evidence that patients with cerebral palsy, patients using anticonvulsive drugs, and those with an IQ lower than 35 more frequently have GERD. There is also evidence that vomiting, rumination and hematemesis are associated with a higher risk of the presence of GERD, whereas there is no clear scientific evidence that particular behavior symptoms are indicative for GERD.</p> <p>Conclusion</p> <p>The possible manifestations of GERD are many and varied. A guideline will be made for care professionals to aid systematic observation of possible manifestations of GERD.</p
Sharp bounds on the critical stability radius for relativistic charged spheres
In a recent paper by Giuliani and Rothman \cite{GR}, the problem of finding a
lower bound on the radius of a charged sphere with mass M and charge Q<M is
addressed. Such a bound is referred to as the critical stability radius.
Equivalently, it can be formulated as the problem of finding an upper bound on
M for given radius and charge. This problem has resulted in a number of papers
in recent years but neither a transparent nor a general inequality similar to
the case without charge, i.e., M\leq 4R/9, has been found. In this paper we
derive the surprisingly transparent inequality
The
inequality is shown to hold for any solution which satisfies
where and are the radial- and tangential pressures respectively
and is the energy density. In addition we show that the inequality
is sharp, in particular we show that sharpness is attained by infinitely thin
shell solutions.Comment: 20 pages, 1 figur
Electronic and optical properties of LiBC
LiBC, a semiconducting ternary borocarbide constituted of the lightest
elements only, has been synthesized and characterized by x-ray powder
diffraction, dielectric spectroscopy, and conductivity measurements. Utilizing
an infrared microscope the phonon spectrum has been investigated in single
crystals. The in-plane B-C stretching mode has been detected at 150 meV,
noticeably higher than in AlB2, a non-superconducting isostructural analog of
MgB2. It is this stretching mode, which reveals a strong electron-phonon
coupling in MgB2, driving it into a superconducting state below 40 K, and is
believed to mediate predicted high-temperature superconductivity in hole-doped
LiBC [H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88,
127001 (2002)].Comment: 4 pages, 4 figure
Nonequilibrium Magnetization Dynamics of Nickel
Ultrafast magnetization dynamics of nickel has been studied for different
degrees of electronic excitation, using pump-probe second-harmonic generation
with 150 fs/800 nm laser pulses of various fluences. Information about the
electronic and magnetic response to laser irradiation is obtained from sums and
differences of the SHG intensity for opposite magnetization directions. The
classical M(T)-curve can be reproduced for delay times larger than the electron
thermalization time of about 280 fs, even when electrons and lattice have not
reached thermal equilibrium. Further we show that the transient magnetization
reaches its minimum approx. 50 fs before electron thermalization is completed.Comment: 8 pages, 5 figures, revte
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