1,045 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
Characterization of the glass transition in vitreous silica by temperature scanning small-angle X-ray scattering
The temperature dependence of the x-ray scattering in the region below the
first sharp diffraction peak was measured for silica glasses with low and high
OH content (GE-124 and Corning 7980). Data were obtained upon scanning the
temperature at 10, 40 and 80 K/min between 400 K and 1820 K. The measurements
resolve, for the first time, the hysteresis between heating and cooling through
the glass transition for silica glass, and the data have a better signal to
noise ratio than previous light scattering and differential thermal analysis
data. For the glass with the higher hydroxyl concentration the glass transition
is broader and at a lower temperature. Fits of the data to the
Adam-Gibbs-Fulcher equation provide updated kinetic parameters for this very
strong glass. The temperature derivative of the observed X-ray scattering
matches that of light scattering to within 14%.Comment: EurophysicsLetters, in pres
Ion and polymer dynamics in polymer electrolytes PPO-LiClO4: II. 2H and 7Li NMR stimulated-echo experiment
We use 2H NMR stimulated-echo spectroscopy to measure two-time correlation
functions characterizing the polymer segmental motion in polymer electrolytes
PPO-LiClO4 near the glass transition temperature Tg. To investigate effects of
the salt on the polymer dynamics, we compare results for different ether oxygen
to lithium ratios, namely, 6:1, 15:1, 30:1 and infinity. For all compositions,
we find nonexponential correlation functions, which can be described by a
Kohlrausch function. The mean correlation times show quantitatively that an
increase of the salt concentration results in a strong slowing down of the
segmental motion. Consistently, for the high 6:1 salt concentration, a high
apparent activation energy E_a=4.1eV characterizes the temperature dependence
of the mean correlation times at Tg < T< 1.1T_g, while smaller values E_a=2.5eV
are observed for moderate salt contents. The correlation functions are most
nonexponential for 15:1 PPO-LiClO4, whereas the stretching is reduced for
higher and lower salt concentrations. A similar dependence of the correlation
functions on the evolution time in the presence and in the absence of ions
indicates that addition of salt hardly affects the reorientational mechanism.
For all compositions, mean jump angles of about 15 degree characterize the
segmental reorientation. In addition, comparison of results from 2H and 7Li NMR
stimulated-echo experiments suggests a coupling of ion and polymer dynamics in
15:1 PPO-LiClO4.Comment: 14 pages, 12 figure
Twist glass transition in regioregulated poly(3-alkylthiophenes)s
The molecular structure and dynamics of regioregulated poly(3-butylthiophene)
(P3BT), poly(3-hexylthiophene)(P3HT), and poly(3-dodecylthiophene) (P3DDT) were
investigated using Fourier transform infrared absorption (FTIR), solid state
C nuclear magnetic resonance (NMR), and differential scanning
calorimetry (DSC) measurements. In the DSC measurements, the endothermic peak
was obtained around 340 K in P3BT, and assigned to enthalpy relaxation that
originated from the glass transition of the thiophene ring twist in crystalline
phase from results of FTIR, C cross-polarization and magic-angle
spinning (CPMAS) NMR, C spin-lattice relaxation time measurements, and
centerband-only detection of exchange (CODEX) measurements. We defined this
transition as {\it twist-glass transition}, which is analogous to the plastic
crystal - glassy crystal transition.Comment: 9 pages, 10 figures, 2 tables. Phys.Rev.B, in pres
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
The Einstein static universe with torsion and the sign problem of the cosmological constant
In the field equations of Einstein-Cartan theory with cosmological constant a
static spherically symmetric perfect fluid with spin density satisfying the
Weyssenhoff restriction is considered. This serves as a rough model of space
filled with (fermionic) dark matter. From this the Einstein static universe
with constant torsion is constructed, generalising the Einstein Cosmos to
Einstein-Cartan theory.
The interplay between torsion and the cosmological constant is discussed. A
possible way out of the cosmological constant's sign problem is suggested.Comment: 8 pages, LaTeX; minor layout changes, typos corrected, one new
equation, new reference [5], completed reference [13], two references adde
Hole-burning experiments within solvable glassy models
We reproduce the results of non-resonant spectral hole-burning experiments
with fully-connected (equivalently infinite-dimensional) glassy models that are
generalizations of the mode-coupling approach to nonequilibrium situations. We
show that an ac-field modifies the integrated linear response and the
correlation function in a way that depends on the amplitude and frequency of
the pumping field. We study the effect of the waiting and recovery-times and
the number of oscillations applied. This calculation will help descriminating
which results can and which cannot be attributed to dynamic heterogeneities in
real systems.Comment: 4 pages, 8 figures, RevTe
Inflating wormholes in the braneworld models
The braneworld model, in which our Universe is a three-brane embedded in a
five-dimensional bulk, allows the existence of wormholes, without any violation
of the energy conditions. A fundamental ingredient of traversable wormholes is
the violation of the null energy condition (NEC). However, in the brane world
models, the stress energy tensor confined on the brane, threading the wormhole,
satisfies the NEC. In conventional general relativity, wormholes existing
before inflation can be significantly enlarged by the expanding spacetime. We
investigate the evolution of an inflating wormhole in the brane world scenario,
in which the wormhole is supported by the nonlocal brane world effects. As a
first step in our study we consider the possibility of embedding a
four-dimensional brane world wormhole into a five dimensional bulk. The
conditions for the embedding are obtained by studying the junction conditions
for the wormhole geometry, as well as the full set of the five dimensional bulk
field equations. For the description of the inflation we adopt the chaotic
inflation model. We study the dynamics of the brane world wormholes during the
exponential inflation stage, and in the stage of the oscillating scalar field.
A particular exact solution corresponding to a zero redshift wormhole is also
obtained. The resulting evolution shows that while the physical and geometrical
parameters of a zero redshift wormhole decay naturally, a wormhole satisfying
some very general initial conditions could turn into a black hole, and exist
forever.Comment: 30 pages, no figures, accepted for publication in CQ
Hydrogen-bond equilibria and life times in a supercooled monohydroxy alcohol
Dielectric loss spectra covering 13 decades in frequency were collected for
2-ethyl-1-hexanol, a monohydroxy alcohol that exhibits a prominent Debye-like
relaxation, typical for several classes of hydrogen-bonded liquids. The thermal
variation of the dielectric absorption amplitude agrees well with that of the
hydrogen-bond equilibrium population, experimentally mapped out using near
infrared (NIR) and nuclear magnetic resonance (NMR) measurements. Despite this
agreement, temperature-jump NIR spectroscopy reveals that the hydrogen-bond
switching rate does not define the frequency position of the prominent
absorption peak. This contrasts with widespread notions and models based
thereon, but is consistent with a recent approach.Comment: 4 pages, 4 figure
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