571 research outputs found
Gravitational induced particle production through a nonminimal curvature-matter coupling
We consider the possibility of a gravitationally induced particle production
through the mechanism of a nonminimal curvature-matter coupling. An interesting
feature of this gravitational theory is that the divergence of the
energy-momentum tensor is nonzero. As a first step in our study we reformulate
the model in terms of an equivalent scalar-tensor theory, with two arbitrary
potentials. By using the formalism of open thermodynamic systems, we interpret
the energy balance equations in this gravitational theory from a thermodynamic
point of view, as describing irreversible matter creation processes. The
particle number creation rates, the creation pressure, and the entropy
production rates are explicitly obtained as functions of the scalar field and
its potentials, as well as of the matter Lagrangian. The temperature evolution
laws of the newly created particles are also obtained. The cosmological
implications of the model are briefly investigated, and it is shown that the
late-time cosmic acceleration may be due to particle creation processes.
Furthermore, it is also shown that due to the curvature--matter coupling,
during the cosmological evolution a large amount of comoving entropy is also
produced.Comment: 15 pages; accepted for publication in the European Physical Journal
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
Dual interacting cosmologies and late accelerated expansion
In this paper we show that by considering a universe dominated by two
interacting components a superaccelerated expansion can be obtained from a
decelerated one by applying a dual transformation that leaves the Einstein's
field equations invariant.Comment: 13 pages, 1 figura, version to match published articl
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.
Cosmological perturbations in warm inflationary models with viscous pressure
Scalar and tensorial cosmological perturbations generated in warm
inflationary scenarios whose matter-radiation fluid is endowed with a viscous
pressure are considered. Recent observational data from the WMAP experiment are
employed to restrict the parameters of the model. Although the effect of this
pressure on the matter power spectrum is of the order of a few percent, it may
be detected in future experiments.Comment: 20 pages, to be published in the Physcal Review
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