8,750 research outputs found
Brane cosmology with a van der Waals equation of state
The evolution of a Universe confined onto a 3-brane embedded in a
five-dimensional space-time is investigated where the cosmological fluid on the
brane is modeled by the van der Waals equation of state. It is shown that the
Universe on the brane evolves in such a manner that three distinct periods
concerning its acceleration field are attained: (a) an initial accelerated
epoch where the van der Waals fluid behaves like a scalar field with a negative
pressure; (b) a past decelerated period which has two contributions, one of
them is related to the van der Waals fluid which behaves like a matter field
with a positive pressure, whereas the other contribution comes from a term of
the Friedmann equation on the brane which is inversely proportional to the
scale factor to the fourth power and can be interpreted as a radiation field,
and (c) a present accelerated phase due to a cosmological constant on the
brane.Comment: 9 pages, 2 figures, to be published in General Relativity and
Gravitatio
A model for a non-minimally coupled scalar field interacting with dark matter
In this work we investigate the evolution of a Universe consisted of a scalar
field, a dark matter field and non-interacting baryonic matter and radiation.
The scalar field, which plays the role of dark energy, is non-minimally coupled
to space-time curvature, and drives the Universe to a present accelerated
expansion. The non-relativistic dark matter field interacts directly with the
dark energy and has a pressure which follows from a thermodynamic theory. We
show that this model can reproduce the expected behavior of the density
parameters, deceleration parameter and luminosity distance.Comment: 3 pages, 4 figures. To appear in Brazilian Journal of Physic
Irreversible processes and the accelerated-decelerated phases of the Universe
A model for the Universe is proposed where it is considered as a mixture of
scalar and matter fields. The particle production is due to an irreversible
transfer of energy from the gravitational field to the matter field and
represented by a non-equilibrium pressure. This model can simulate three
distinct periods of the Universe: (a) an accelerated epoch where the energy
density of the scalar field prevails over the matter field, (b) a past
decelerated period where the energy density of the matter field becomes more
predominant than the scalar energy density, and (c) a present acceleration
phase where the scalar energy density overcomes the energy density of the
matter field.Comment: 6 pages, 2 figures, to be published in Brazilian Journal of Physic
Accelerated expansion in bosonic and fermionic 2D cosmologies with quantum effects
In this work we analyze the effects produced by bosonic and fermionic
constituents, including quantum corrections, in two-dimensional (2D)
cosmological models. We focus on a gravitational theory related to the
Callan-Giddings-Harvey-Strominger model, to simulate the dynamics of a young,
spatially-lineal, universe. The cosmic substratum is formed by an {\it
inflaton} field plus a matter component, sources of the 2D gravitational field;
the degrees of freedom also include the presence of a dilaton field. We show
that this combination permits, among other scenarios, the simulation of a
period of inflation, that would be followed by a (bosonic/fermionic) matter
dominated era. We also analyse how quantum effects contribute to the destiny of
the expansion, given the fact that in 2D we have a consistent (renormalizable)
quantum theory of gravity. The dynamical behavior of the system follows from
the solution of the gravitational field equations, the (Klein-Gordon and Dirac)
equations for the sources and the dilaton field equation. Consistent
(accelerated) regimes are present among the solutions of the 2D equations; the
results depend strongly on the initial conditions used for the dilaton field.
In the particular case where fermions are included as matter fields a
transition to a decelerated expansion is possible, something that does not
happen in the exclusively bosonic case.Comment: 6 pages, 5 figures, to appear in EP
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