28,221 research outputs found
Non-spherical sources of static gravitational fields: investigating the boundaries of the no-hair theorem
A new, globally regular model describing a static, non spherical gravitating
object in General Relativity is presented. The model is composed by a vacuum
Weyl--Levi-Civita special field - the so called gamma metric - generated by a
regular static distribution of mass-energy. Standard requirements of physical
reasonableness such as, energy, matching and regularity conditions are
satisfied. The model is used as a toy in investigating various issues related
to the directional behavior of naked singularities in static spacetimes and the
blackhole (Schwarschild) limit.Comment: 10 pages, 2 figure
Dissipative fluids out of hydrostatic equilibrium
In the context of the M\"{u}ller-Israel-Stewart second order phenomenological
theory for dissipative fluids, we analyze the effects of thermal conduction and
viscosity in a relativistic fluid, just after its departure from hydrostatic
equilibrium, on a time scale of the order of relaxation times. Stability and
causality conditions are contrasted with conditions for which the ''effective
inertial mass'' vanishes.Comment: 21 pages, 1 postscript figure (LaTex 2.09 and epsfig.sty required)
Submitted to Classical and Quantum Gravit
The use of a conventional wind tunnel as a multigas facility
Hypersonic and supersonic wind tunnels as continuous flow multigas facilitie
Localization of gauge fields in a tachyonic de Sitter thick braneworld
In this work we show that universal gauge vector fields can be localized on
the recently proposed 5D thick tachyonic braneworld which involves a de Sitter
cosmological background induced on the 3-brane. Namely, by performing a
suitable decomposition of the vector field, the resulting 4D effective action
corresponds to a massive gauge field, while the profile along the extra
dimension obeys a Schroedinger-like equation with a Poeschl-Teller potential.
It turns out that the massless zero mode of the gauge field is bound to the
expanding 3-brane and allows us to recover the standard 4D electromagnetic
phenomena of our world. Moreover, this zero mode is separated from the
continuum of Kaluza-Klein (KK) modes by a mass gap determined by the scale of
the expansion parameter. We also were able to analytically solve the
corresponding Schroedinger-like equation for arbitrary mass, showing that KK
massive modes asymptotically behave like plane waves as expected.Comment: 7 pages in latex, no figure
String theory extensions of Einstein-Maxwell fields: the static case
We present a new approach for generation of solutions in the four-dimensional
heterotic string theory with one vector field and in the five-dimensional
bosonic string theory starting from the static Einstein-Maxwell fields. Our
approach allows one to construct the solution classes invariant with respect to
the total subgroup of the three-dimensional charging symmetries of these string
theories. The new generation procedure leads to the extremal
Israel-Wilson-Perjes subclass of string theory solutions in a special case and
provides its natural continuous extension to the realm of non-extremal
solutions. We explicitly calculate all string theory solutions related to
three-dimensional gravity coupled to an effective dilaton field which arises
after an appropriate charging symmetry invariant reduction of the static
Einstein-Maxwell system.Comment: 19 pages in late
On the compressibility equation of state for multicomponent adhesive hard sphere fluids
The compressibility equation of state for a multicomponent fluid of particles
interacting via an infinitely narrow and deep potential, is considered within
the mean spherical approximation (MSA). It is shown that for a class of models
leading to a particular form of the Baxter functions containing
density-independent stickiness coefficient, the compressibility EOS does not
exist, unlike the one-component case. The reason for this is that a direct
integration of the compressibility at fixed composition, cannot be carried out
due to the lack of a reciprocity relation on the second order partial
derivatives of the pressure with respect to two different densities. This is,
in turn, related to the inadequacy of the MSA. A way out to this drawback is
presented in a particular example, leading to a consistent compressibility
pressure, and a possible generalization of this result is discussed.Comment: 13 pages, no figures, accepted for publication Molec. Physics (2002
Charging Interacting Rotating Black Holes in Heterotic String Theory
We present a formulation of the stationary bosonic string sector of the whole
toroidally compactified effective field theory of the heterotic string as a
double Ernst system which, in the framework of General Relativity describes, in
particular, a pair of interacting spinning black holes; however, in the
framework of low--energy string theory the double Ernst system can be
particularly interpreted as the rotating field configuration of two interacting
sources of black hole type coupled to dilaton and Kalb--Ramond fields. We
clarify the rotating character of the --component of the
antisymmetric tensor field of Kalb--Ramond and discuss on its possible torsion
nature. We also recall the fact that the double Ernst system possesses a
discrete symmetry which is used to relate physically different string vacua.
Therefore we apply the normalized Harrison transformation (a charging symmetry
which acts on the target space of the low--energy heterotic string theory
preserving the asymptotics of the transformed fields and endowing them with
multiple electromagnetic charges) on a generic solution of the double Ernst
system and compute the generated field configurations for the 4D effective
field theory of the heterotic string. This transformation generates the
vector field content of the whole low--energy heterotic string
spectrum and gives rise to a pair of interacting rotating black holes endowed
with dilaton, Kalb--Ramond and multiple electromagnetic fields where the charge
vectors are orthogonal to each other.Comment: 15 pages in latex, revised versio
Mixed potentials in radiative stellar collapse
We study the behaviour of a radiating star when the interior expanding,
shearing fluid particles are traveling in geodesic motion. We demonstrate that
it is possible to obtain new classes of exact solutions in terms of elementary
functions without assuming a separable form for the gravitational potentials or
initially fixing the temporal evolution of the model unlike earlier treatments.
A systematic approach enables us to write the junction condition as a Riccati
equation which under particular conditions may be transformed into a separable
equation. New classes of solutions are generated which allow for mixed spatial
and temporal dependence in the metric functions. We regain particular models
found previously from our general classes of solutions.Comment: 10 pages, To appear in J. Math. Phy
Why hyperbolic theories of dissipation cannot be ignored: Comments on a paper by Kostadt and Liu
Contrary to what is asserted in a recent paper by Kostadt and Liu ("Causality
and stability of the relativistic diffusion equation"), experiments can tell
apart (and in fact do) hyperbolic theories from parabolic theories of
dissipation. It is stressed that the existence of a non--negligible relaxation
time does not imply for the system to be out of the hydrodynamic regime.Comment: 8 pages Latex, to appear in Phys.Rev.
On the dual interpretation of zero-curvature Friedmann-Robertson-Walker models
Two possible interpretations of FRW cosmologies (perfect fluid or dissipative
fluid)are considered as consecutive phases of the system. Necessary conditions
are found, for the transition from perfect fluid to dissipative regime to
occur, bringing out the conspicuous role played by a particular state of the
system (the ''critical point '').Comment: 13 pages Latex, to appear in Class.Quantum Gra
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