115 research outputs found
Relativistic dynamics of cylindrical shells of counter-rotating particles
Although infinite cylinders are not astrophysical entities, it is possible to
learn a great deal about the basic qualitative features of generation of
gravitational waves and the behavior of the matter conforming such shells in
the limits of very small radius. We describe the analytical model using kinetic
theory for the matter and the junction conditions through the shell to obtain
its equation of motion. The nature of the static solutions are analyzed, both
for a single shell as well as for two concentric shells. In this second case,
for a time dependent external shell, we integrate numerically the equation of
motion for several values of the constants of the system. Also, a brief
description in terms of the Komar mass is given to account for the
gravitational wave energy emitted by the system.Comment: 19 pages, 8 figure
Gravitational Collapse of Cylindrical Shells Made of Counter-Rotating Dust Particles
The general formulas of a non-rotating dynamic thin shell that connects two
arbitrary cylindrical regions are given using Israel's method. As an
application of them, the dynamics of a thin shell made of counter-rotating dust
particles, which emits both gravitational waves and massless particles when it
is expanding or collapsing, is studied. It is found that when the models
represent a collapsing shell, in some cases the angular momentum of the dust
particles is strong enough to halt the collapse, so that a spacetime
singularity is prevented from forming, while in other cases it is not, and a
line-like spacetime singularity is finally formed on the symmetry axis.Comment: To appear in Phys. Rev.
Comment on "Failure of standard conservation laws at a classical change of signature"
Hellaby & Dray (gr-qc/9404001) have recently claimed that matter conservation
fails under a change of signature, compounding earlier claims that the standard
junction conditions for signature change are unnecessary. In fact, if the field
equations are satisfied, then the junction conditions and the conservation
equations are satisfied. The failure is rather that the authors did not make
sense of the field equations and conservation equations, which are singular at
a change of signature.Comment: 3 pages, Te
Radiation fluid singular hypersurfaces with de Sitter interior as models of charged extended particles in general relativity
In present paper we construct the classical and minisuperspace quantum models
of an extended charged particle. The modelling is based on the radiation fluid
singular hypersurface filled with physical vacuum. We demonstrate that both at
classical and quantum levels such a model can have equilibrium states at the
radius equal to the classical radius of a charged particle. In the cosmological
context the model could be considered also as the primary stationary state,
having the huge internal energy being nonobservable for an external observer,
from which the Universe was born by virtue of the quantum tunnelling.Comment: LaTeX (IOPP style); final versio
Generalized Vaidya Solutions
A large family of solutions, representing, in general, spherically symmetric
Type II fluid, is presented, which includes most of the known solutions to the
Einstein field equations, such as, the monopole-de Sitter-charged Vaidya ones.Comment: Gen. Relativ. Grav. 31 (1), 107-114 (1999
Higher-dimensional Kundt waves and gyratons
We present and analyze exact solutions of the Einstein-Maxwell equations in
higher dimensions which form a large subclass of the Kundt family of
spacetimes. We assume that the cosmological constant may be nonvanishing, and
the matter consists of a background aligned electromagnetic field and an
additional pure radiation (gyratonic) field with a spin. We show that the field
equations reduce to a set of linear equations on the transverse space which can
be solved exactly and expressed in terms of the Green functions. We thus find
explicit exact gyratonic gravitational and electromagnetic fields created by a
radiation beam of null matter with arbitrary profiles of energy density and
angular momenta. In the absence of the gyratonic matter we obtain pure
nonexpanding higher-dimensional gravitational waves.
In particular, we investigate gyratons and waves propagating on backgrounds
which are a direct-product of 2-spaces of constant curvature. Such type D or 0
background spacetimes generalize 4-dimensional Nariai, anti-Nariai and
Plebanski-Hacyan universes, and conformally flat Bertotti-Robinson and
Minkowski spaces. These spacetimes belong to a wider class of spaces which
admit the Kahler structure related to the background magnetic field. The
obtained wave and gyraton solutions are also members of the recently discussed
class of spacetimes with constant scalar invariants (CSI) of the curvature
tensor.Comment: 18 pages, no figure
On Charged Black Holes in Anti-de Sitter Space
We study the region inside the event horizon of charged black holes in five
dimensional asymptotically anti-de Sitter space, using as a probe two-sided
correlators which are dominated by spacelike geodesics penetrating the horizon.
The spacetimes we investigate include the Reissner-Nordstrom black hole and
perturbations thereof. The perturbed spacetimes can be found exactly, enabling
us to perform a local scan of the region between the inner and outer horizons.
Surprisingly, the two-sided correlators we calculate seem to be geometrically
protected from the instability of the inner horizon.Comment: 1+37 pages, 20 ps and eps figures, LaTeX. References added and
changes made to section
Singular shell embedded into a cosmological model
We generalize Israel's formalism to cover singular shells embedded in a
non-vacuum Universe. That is, we deduce the relativistic equation of motion for
a thin shell embedded in a Schwarzschild/Friedmann-Lemaitre-Robertson-Walker
spacetime. Also, we review the embedding of a Schwarzschild mass into a
cosmological model using "curvature" coordinates and give solutions with
(Sch/FLRW) and without the embedded mass (FLRW).Comment: 25 pages, 2 figure
Adsorption and reaction of CO on (Pd–)Al2O3 and (Pd–)ZrO2: vibrational spectroscopy of carbonate formation
γ-Alumina is widely used as an oxide support in catalysis, and palladium nanoparticles supported by alumina represent one of the most frequently used dispersed metals. The surface sites of the catalysts are often probed via FTIR spectroscopy upon CO adsorption, which may result in the formation of surface carbonate species. We have examined this process in detail utilizing FTIR to monitor carbonate formation on γ-alumina and zirconia upon exposure to isotopically labelled and unlabelled CO and CO2. The same was carried out for well-defined Pd nanoparticles supported on Al2O3 or ZrO2. A water gas shift reaction of CO with surface hydroxyls was detected, which requires surface defect sites and adjacent OH groups. Furthermore, we have studied the effect of Cl synthesis residues, leading to strongly reduced carbonate formation and changes in the OH region (isolated OH groups were partly replaced or were even absent). To corroborate this finding, samples were deliberately poisoned with Cl to an extent comparable to that of synthesis residues, as confirmed by Auger electron spectroscopy. For catalysts prepared from Cl-containing precursors a new CO band at 2164 cm−1 was observed in the carbonyl region, which was ascribed to Pd interacting with Cl. Finally, the FTIR measurements were complemented by quantification of the amount of carbonates formed via chemisorption, which provides a tool to determine the concentration of reactive defect sites on the alumina surface
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