93 research outputs found
Energy and Momentum densities of cosmological models, with equation of state , in general relativity and teleparallel gravity
We calculated the energy and momentum densities of stiff fluid solutions,
using Einstein, Bergmann-Thomson and Landau-Lifshitz energy-momentum complexes,
in both general relativity and teleparallel gravity. In our analysis we get
different results comparing the aforementioned complexes with each other when
calculated in the same gravitational theory, either this is in general
relativity and teleparallel gravity. However, interestingly enough, each
complex's value is the same either in general relativity or teleparallel
gravity. Our results sustain that (i) general relativity or teleparallel
gravity are equivalent theories (ii) different energy-momentum complexes do not
provide the same energy and momentum densities neither in general relativity
nor in teleparallel gravity. In the context of the theory of teleparallel
gravity, the vector and axial-vector parts of the torsion are obtained. We show
that the axial-vector torsion vanishes for the space-time under study.Comment: 15 pages, no figures, Minor typos corrected; version to appear in
International Journal of Theoretical Physic
Energy and Momentum Distributions of Kantowski and Sachs Space-time
We use the Einstein, Bergmann-Thomson, Landau-Lifshitz and Papapetrou
energy-momentum complexes to calculate the energy and momentum distributions of
Kantowski and Sachs space-time. We show that the Einstein and Bergmann-Thomson
definitions furnish a consistent result for the energy distribution, but the
definition of Landau-Lifshitz do not agree with them. We show that a signature
switch should affect about everything including energy distribution in the case
of Einstein and Papapetrou prescriptions but not in Bergmann-Thomson and
Landau-Lifshitz prescriptions.Comment: 12 page
On the energy of charged black holes in generalized dilaton-axion gravity
In this paper we calculate the energy distribution of some charged black
holes in generalized dilaton-axion gravity. The solutions correspond to charged
black holes arising in a Kalb-Ramond-dilaton background and some existing
non-rotating black hole solutions are recovered in special cases. We focus our
study to asymptotically flat and asymptotically non-flat types of solutions and
resort for this purpose to the M{\o}ller prescription. Various aspects of
energy are also analyzed.Comment: LaTe
Analysis of a three-component model phase diagram by Catastrophe Theory: Potentials with two Order Parameters
In this work we classify the singularities obtained from the Gibbs potential
of a lattice gas model with three components, two order parameters and five
control parameters applying the general theorems provided by Catastrophe
Theory. In particular, we clearly establish the existence of Landau potentials
in two variables or, in other words, corank 2 canonical forms that are
associated to the hyperbolic umbilic, D_{+4}, its dual the elliptic umbilic,
D_{-4}, and the parabolic umbilic, D_5, catastrophes. The transversality of the
potential with two order parameters is explicitely shown for each case. Thus we
complete the Catastrophe Theory analysis of the three-component lattice model,
initiated in a previous paper.Comment: 17 pages, 3 EPS figures, Latex file, continuation of Phys. Rev. B57,
13527 (1998) (cond-mat/9707015), submitted to Phys. Rev.
Hawking Radiation as Tunneling for Extremal and Rotating Black Holes
The issue concerning semi-classical methods recently developed in deriving
the conditions for Hawking radiation as tunneling, is revisited and applied
also to rotating black hole solutions as well as to the extremal cases. It is
noticed how the tunneling method fixes the temperature of extremal black hole
to be zero, unlike the Euclidean regularity method that allows an arbitrary
compactification period. A comparison with other approaches is presented.Comment: 17 pages, Latex document, typos corrected, four more references,
improved discussion in section
Macrospin approximation and quantum effects in models for magnetization reversal
The thermal activation of magnetization reversal in magnetic nanoparticles is
controlled by the anisotropy-energy barrier. Using perturbation theory, exact
diagonalization and stability analysis of the ferromagnetic spin-s Heisenberg
model with coupling or single-site anisotropy, we study the effects of quantum
fluctuations on the height of the energy barrier. Opposed to the classical
case, there is no critical anisotropy strength discriminating between reversal
via coherent rotation and via nucleation/domain-wall propagation. Quantum
fluctuations are seen to lower the barrier depending on the anisotropy
strength, dimensionality and system size and shape. In the weak-anisotropy
limit, a macrospin model is shown to emerge as the effective low-energy theory
where the microscopic spins are tightly aligned due to the ferromagnetic
exchange. The calculation provides explicit expressions for the anisotropy
parameter of the effective macrospin. We find a reduction of the
anisotropy-energy barrier as compared to the classical high spin-s limit.Comment: 10 pages, 11 figure
On parton distributions in a photon gas
In some cases it may be useful to know parton distributions in a photon gas.
This may be relevant, e.g., for the analysis of interactions of high energy
cosmic ray particles with the cosmic microwave background radiation. The latter
can be considered as a gas of photons with an almost perfect blackbody
spectrum. An approach to finding such parton distributions is described. The
survival probability of ultra-high energy neutrinos traveling through this
radiation is calculated.Comment: 5 pages, 4 figures, EPJ style files. Some changes in the text. Two
new sections discussing ultra-high energy neutrino damping in the cosmic
microwave background radiation are include
The Energy of Regular Black Hole in General Relativity Coupled to Nonlinear Electrodynamics
According to the Einstein, Weinberg, and M{\o}ller energy-momentum complexes,
we evaluate the energy distribution of the singularity-free solution of the
Einstein field equations coupled to a suitable nonlinear electrodynamics
suggested by Ay\'{o}n-Beato and Garc\'{i}a. The results show that the energy
associated with the definitions of Einstein and Weinberg are the same, but
M{\o}ller not. Using the power series expansion, we find out that the first two
terms in the expression are the same as the energy distributions of the
Reissner-Nordstr\"{o}m solution, and the third term could be used to survey the
factualness between numerous solutions of the Einstein field eqautions coupled
to a nonlinear electrodynamics.Comment: 11 page
Energy Contents of Some Well-Known Solutions in Teleparallel Gravity
In the context of teleparallel equivalent to General Relativity, we study
energy and its relevant quantities for some well-known black hole solutions.
For this purpose, we use the Hamiltonian approach which gives reasonable and
interesting results. We find that our results of energy exactly coincide with
several prescriptions in General Relativity. This supports the claim that
different energy-momentum prescriptions can give identical results for a given
spacetime. We also evaluate energy-momentum flux of these solutions.Comment: 16 pages, accepted for publication in Astrophys. Space Sc
Adiabatic following criterion, estimation of the nonadiabatic excitation fraction and quantum jumps
An accurate theory describing adiabatic following of the dark, nonabsorbing
state in the three-level system is developed. An analytical solution for the
wave function of the particle experiencing Raman excitation is found as an
expansion in terms of the time varying nonadiabatic perturbation parameter. The
solution can be presented as a sum of adiabatic and nonadiabatic parts. Both
are estimated quantitatively. It is shown that the limiting value to which the
amplitude of the nonadiabatic part tends is equal to the Fourier component of
the nonadiabatic perturbation parameter taken at the Rabi frequency of the
Raman excitation. The time scale of the variation of both parts is found. While
the adiabatic part of the solution varies slowly and follows the change of the
nonadiabatic perturbation parameter, the nonadiabatic part appears almost
instantly, revealing a jumpwise transition between the dark and bright states.
This jump happens when the nonadiabatic perturbation parameter takes its
maximum value.Comment: 33 pages, 8 figures, submitted to PRA on 28 Oct. 200
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