1,444 research outputs found
T-spheres as a limit of Lemaitre-Tolman-Bondi solutions
In the Tolman model there exist two quite different branches of solutions -
generic Lemaitre-Tolman-Bondi (LTB) ones and T-spheres as a special case. We
show that, nonetheless, T-spheres can be obtained as a limit of the class of
LTB solutions having no origin and extending to infinity with the areal radius
approaching constant. It is shown that all singularities of T-models are
inherited from those of corresponding LBT solutions. In doing so, the disc type
singularity of a T-sphere is the analog of shell-crossing.Comment: 6 pages. 1 Reference added. To appear in Phys. Rev.
Quintessence and phantom cosmology with non-minimal derivative coupling
We investigate cosmological scenarios with a non-minimal derivative coupling
between the scalar field and the curvature, examining both the quintessence and
the phantom cases in zero and constant potentials. In general, we find that the
universe transits from one de Sitter solution to another, determined by the
coupling parameter. Furthermore, according to the parameter choices and without
the need for matter, we can obtain a Big Bang, an expanding universe with no
beginning, a cosmological turnaround, an eternally contracting universe, a Big
Crunch, a Big Rip avoidance and a cosmological bounce. This variety of
behaviors reveals the capabilities of the present scenario.Comment: 8 pages, 8 figure
Semi-empirical dissipation source functions for ocean waves: Part I, definition, calibration and validation
New parameterizations for the spectra dissipation of wind-generated waves are
proposed. The rates of dissipation have no predetermined spectral shapes and
are functions of the wave spectrum and wind speed and direction, in a way
consistent with observation of wave breaking and swell dissipation properties.
Namely, the swell dissipation is nonlinear and proportional to the swell
steepness, and dissipation due to wave breaking is non-zero only when a
non-dimensional spectrum exceeds the threshold at which waves are observed to
start breaking. An additional source of short wave dissipation due to long wave
breaking is introduced to represent the dissipation of short waves due to
longer breaking waves. Several degrees of freedom are introduced in the wave
breaking and the wind-wave generation term of Janssen (J. Phys. Oceanogr.
1991). These parameterizations are combined and calibrated with the Discrete
Interaction Approximation of Hasselmann et al. (J. Phys. Oceangr. 1985) for the
nonlinear interactions. Parameters are adjusted to reproduce observed shapes of
directional wave spectra, and the variability of spectral moments with wind
speed and wave height. The wave energy balance is verified in a wide range of
conditions and scales, from gentle swells to major hurricanes, from the global
ocean to coastal settings. Wave height, peak and mean periods, and spectral
data are validated using in situ and remote sensing data. Some systematic
defects are still present, but the parameterizations yield the best overall
results to date. Perspectives for further improvement are also given.Comment: revised version for Journal of Physical Oceanograph
A Conformal Mapping and Isothermal Perfect Fluid Model
Instead of conformal to flat spacetime, we take the metric conformal to a
spacetime which can be thought of as ``minimally'' curved in the sense that
free particles experience no gravitational force yet it has non-zero curvature.
The base spacetime can be written in the Kerr-Schild form in spherical polar
coordinates. The conformal metric then admits the unique three parameter family
of perfect fluid solution which is static and inhomogeneous. The density and
pressure fall off in the curvature radial coordinates as for
unbounded cosmological model with a barotropic equation of state. This is the
characteristic of isothermal fluid. We thus have an ansatz for isothermal
perfect fluid model. The solution can also represent bounded fluid spheres.Comment: 10 pages, TeX versio
The Hawking temperature of expanding cosmological black holes
In the context of a debate on the correct expression of the Hawking
temperature of an expanding cosmological black hole, we show that the correct
expression in terms of the Hawking-Hayward quasi-local energy m of the hole is
T=1/(8\pi m(t)). This expression holds for comoving black holes and agrees with
a recent proposal by Saida, Harada, and Maeda.Comment: 5 latex pages, to appear in Phys. Rev. D. Some references adde
Spherically symmetric space-time with the regular de Sitter center
The requirements are formulated which lead to the existence of the class of
globally regular solutions to the minimally coupled GR equations which are
asymptotically de Sitter at the center. The brief review of the resulting
geometry is presented. The source term, invariant under radial boots, is
classified as spherically symmetric vacuum with variable density and pressure,
associated with an r-dependent cosmological term, whose asymptotic in the
origin, dictated by the weak energy condition, is the Einstein cosmological
term. For this class of metrics the ADM mass is related to both de Sitter
vacuum trapped in the origin and to breaking of space-time symmetry. In the
case of the flat asymptotic, space-time symmetry changes smoothly from the de
Sitter group at the center to the Lorentz group at infinity. Dependently on
mass, de Sitter-Schwarzschild geometry describes a vacuum nonsingular black
hole, or G-lump - a vacuum selfgravitating particlelike structure without
horizons. In the case of de Sitter asymptotic at infinity, geometry is
asymptotically de Sitter at both origin and infinity and describes, dependently
on parameters and choice of coordinates, a vacuum nonsingular cosmological
black hole, selfgravitating particlelike structure at the de Sitter background
and regular cosmological models with smoothly evolving vacuum energy density.Comment: Latex, 10 figures, extended version of the plenary talk at V
Friedmann Intern. Conf. on Gravitation and Cosmology, Brazil 2002, to appear
in Int.J.Mod.Phys.
Gravitational field of charged gyratons
We study relativistic gyratons which carry an electric charge. The
Einstein-Maxwell equations in arbitrary dimensions are solved exactly in the
case of a charged gyraton propagating in an asymptotically flat metric.Comment: 11 pages, some new comments and new references added. To appear in
Classical and Quantum Gravit
Charges and fields in a current-carrying wire
Charges and fields in a straight, infinite, cylindrical wire carrying a
steady current are determined in the rest frames of ions and electrons,
starting from the standard assumption that the net charge per unit length is
zero in the lattice frame and taking into account a self-induced pinch effect.
The analysis presented illustrates the mutual consistency of classical
electromagnetism and Special Relativity. Some consequences of the assumption
that the net charge per unit length is zero in the electrons frame are also
briefly discussed
Neutron stars with isovector scalar correlations
Neutron stars with the isovector scalar -field are studied in the
framework of the relativistic mean field () approach in a pure nucleon
plus lepton scheme. The -field leads to a larger repulsion in dense
neutron-rich matter and to a definite splitting of proton and neutron effective
masses. Both features are influencing the stability conditions of the neutron
stars. Two parametrizations for the effective nonlinear Lagrangian density are
used to calculate the nuclear equation of state () and the neutron star
properties, and compared to correlated Dirac-Brueckner results. We conclude
that in order to reproduce reasonable nuclear structure and neutron star
properties within a approach a density dependence of the coupling
constants is required.Comment: 11 pages, 5 figures, revtex4 styl
Impossibility of Unlimited Gravitational Collapse
It is shown that the gravitational field, as a physical field developing in
the Minkowsky space, does not lead to unlimited gravitational collapse of
massive bodies and, hence, excludes a possibility of the formation of the
``black holes''.Comment: 6 page
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