662 research outputs found
Qualitative Analysis of Causal Cosmological Models
The Einstein's field equations of Friedmann-Robertson-Walker universes filled
with a dissipative fluid described by both the {\em truncated} and {\em
non-truncated} causal transport equations are analyzed using techniques from
dynamical systems theory. The equations of state, as well as the phase space,
are different from those used in the recent literature. In the de Sitter
expansion both the hydrodynamic approximation and the non-thermalizing
condition can be fulfilled simultaneously. For these expansions
turn out to be stable provided a certain parameter of the fluid is lower than
1/2. The more general case is studied in detail as well.Comment: RevTeX file, 23 pages. Accepted for publication in J. Math. Phy
Stability of inflationary solutions driven by a changing dissipative fluid
In this paper the second Lyapunov method is used to study the stability of
the de Sitter phase of cosmic expansion when the source of the gravitational
field is a viscous fluid. Different inflationary scenarios related with
reheating and decay of mini-blackholes into radiation are investigated using an
effective fluid described by time--varying thermodynamical quantities.Comment: 17 pages, LaTeX 2.09, 2 figures. To be published in Classical and
Quantum Gravit
Magnetic relaxation in the Bianchi-I universe
Extended Einstein-Maxwell model and its application to the problem of
evolution of magnetized Bianchi-I Universe are considered. The evolution of
medium magnetization is governed by a relaxation type extended constitutive
equation. The series of exact solutions to the extended master equations is
obtained and discussed. The anisotropic expansion of the Bianchi-I Universe is
shown to become non-monotonic (accelerated/decelerated) in both principal
directions (along the magnetic field and orthogonal to it). A specific type of
expansion, the so-called evolution with hidden magnetic field, is shown to
appear when the magnetization effectively screens the magnetic field and the
latter disappears from the equations for gravitational field.Comment: 32 page
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.
Holographic dark energy in Brans-Dicke theory
In this Letter it is shown that when the holographic dark energy is combined with the Brans-Dicke field equations the transition from decelerated to accelerated expansion of the Universe can be more easily accounted for than when resort to the Einstein field equations is made. Likewise, the coincidence problem of late cosmic acceleration gets more readily softened
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