94 research outputs found
Kerr Geodesics, the Penrose Process and Jet Collimation by a Black Hole
We re-examine the possibility that astrophysical jet collimation may arise
from the geometry of rotating black holes and the presence of high-energy
particles resulting from a Penrose process, without the help of magnetic
fields. Our analysis uses the Weyl coordinates, which are revealed better
adapted to the desired shape of the jets. We numerically integrate the
2D-geodesics equations. We give a detailed study of these geodesics and give
several numerical examples. Among them are a set of perfectly collimated
geodesics with asymptotes parallel to the axis, with
only depending on the ratios and
, where and are the parameters of the Kerr black hole,
the particle energy and the Carter's constant.Comment: Accepted by Astronomy and Astrophysics. AA style with 3 EPS figures.
Content amended after AA's refereeing. Discussion of geodesics also corrected
and expanded earlier. Conclusions amended accordingl
Generalised equilibrium of cosmological fluids in second-order thermodynamics
Combining the second-order entropy flow vector of the causal Israel-Stewart
theory with the conformal Killing-vector property of , where
is the four-velocity of the medium and T its equilibrium temperature, we
investigate generalized equilibrium states for cosmological fluids with
nonconserved particle number. We calculate the corresponding equilibrium
particle production rate and show that this quantity is reduced compared with
the results of the previously studied first-order theory. Generalized
equilibrium for massive particles turns out to be compatible with a dependence
of the fluid energy density on the scale factor a
of the Robertson-Walker metric and may be regarded as a realization of
so-called K-matter.Comment: 17 pages, iopfts.tex file, submitted to Class. Quantum Gra
A study of the angular size-redshift relation for models in which decays as the energy density
By modifying the Chen and Wu ansatz, we have investigated some Friedmann
models in which varies as . In order to test the consistency of
the models with observations, we study the angular size - redshift relation for
256 ultracompact radio sources selected by Jackson and Dodgson. The angular
sizes of these sources were determined by using very long-baseline
interferometry in order to avoid any evolutionary effects. The models fit the
data very well and require an accelerating universe with a positive
cosmological constant. Open, flat and closed models are almost equally
probable, though the open model provides a comparatively better fit to the
data. The models are found to have intermediate density and imply the existence
of dark matter, though not as much as in the canonical Einstein-de Sitter
model.Comment: LaTex, 15 pages including 2 figures (Revised version appeared in CQG
Scaling dependence on the fluid viscosity ratio in the selective withdrawal transition
In the selective withdrawal experiment fluid is withdrawn through a tube with
its tip suspended a distance S above a two-fluid interface. At sufficiently low
withdrawal rates, Q, the interface forms a steady state hump and only the upper
fluid is withdrawn. When Q is increased (or S decreased), the interface
undergoes a transition so that the lower fluid is entrained with the upper one,
forming a thin steady-state spout. Near this transition the hump curvature
becomes very large and displays power-law scaling behavior. This scaling allows
for steady-state hump profiles at different flow rates and tube heights to be
scaled onto a single similarity profile. I show that the scaling behavior is
independent of the viscosity ratio.Comment: 33 Pages, 61 figures, 1 tabl
Qualitative Viscous Cosmology
The Full (non--truncated) Israel--Stewart theory of bulk viscosity is applied
to dissipative FRW spacetimes. Dimensionless variables and dimensionless
equations of state are used to write the Einstein--thermodynamic equations as a
plane autonomous system and the qualitative behaviour of this system is
determined. Entropy production in these models is also discussed.Comment: 13 pages, REVTeX, accepted for publication in Physical Review
Dissipative cosmological solutions
The exact general solution to the Einstein equations in a homogeneous
Universe with a full causal viscous fluid source for the bulk viscosity index
is found. We have investigated the asymptotic stability of Friedmann
and de Sitter solutions, the former is stable for and the latter for
. The comparison with results of the truncated theory is made. For
, it was found that families of solutions with extrema no longer remain
in the full case, and they are replaced by asymptotically Minkowski evolutions.
These solutions are monotonic.Comment: 17 pages, LaTeX 2.09, 1 figure. To be published in Classical and
Quantum Gravit
Temperature Evolution Law of Imperfect Relativistic Fluids
The first-order general relativistic theory of a generic dissipative
(heat-conducting, viscous, particle-creating) fluid is rediscussed from a
unified covariant frame-independent point of view. By generalizing some
previous works in the literature, we derive a formula for the temperature
variation rate, which is valid both in Eckart's (particle) and in the
Landau-Lifshitz (energy) frames. Particular attention is paid to the case of
gravitational particle creation and its possible cross-effect with the bulk
viscosity mechanism.Comment: 14 pages, no figure, revte
Cosmological particle production, causal thermodynamics, and inflationary expansion
Combining the equivalence between cosmological particle creation and an
effective viscous fluid pressure with the fact that the latter represents a
dynamical degree of freedom within the second-order Israel-Stewart theory for
imperfect fluids, we reconsider the possibility of accelerated expansion in
fluid cosmology. We find an inherent self-limitation for the magnitude of an
effective bulk pressure which is due to adiabatic (isentropic) particle
production. For a production rate which depends quadratically on the Hubble
rate we confirm the existence of solutions which describe a smooth transition
from inflationary to noninflationary behavior and discuss their interpretation
within the model of a decaying vacuum energy density. An alternative
formulation of the effective imperfect fluid dynamics in terms of a minimally
coupled scalar field is given. The corresponding potential is discussed and an
entropy equivalent for the scalar field is found.Comment: 16 pages, revtex file, submitted to Phys. Rev.
Entropy-Corrected Holographic Dark Energy
The holographic dark energy (HDE) is now an interesting candidate of dark
energy, which has been studied extensively in the literature. In the derivation
of HDE, the black hole entropy plays an important role. In fact, the
entropy-area relation can be modified due to loop quantum gravity or other
reasons. With the modified entropy-area relation, we propose the so-called
``entropy-corrected holographic dark energy'' (ECHDE) in the present work. We
consider many aspects of ECHDE and find some interesting results. In addition,
we briefly consider the so-called ``entropy-corrected agegraphic dark energy''
(ECADE).Comment: 11 pages, 2 tables, revtex4; v2: references adde
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