64 research outputs found
Interacting quintessence and the coincidence problem
We investigate the role of a possible coupling of dark matter and dark
energy. In particular, we explore the consequences of such an interaction for
the coincidence problem, i.e., for the question, why the energy densities of
dark matter and dark energy are of the same order just at the present epoch. We
demonstrate, that, with the help of a suitable coupling, it is possible to
reproduce any scaling solution , where is the
scale factor of the Robertson-Walker metric and is a constant parameter.
and are the densities of dark energy and dark matter,
respectively. Furthermore, we show that an interaction between dark matter and
dark energy can drive the transition from an early matter dominated era to a
phase of accelerated expansion with a stable, stationary ratio of the energy
densities of both components.Comment: 3 pages, contribution to the Tenth Marcel Grossmann Meeting, Rio de
Janeiro, 20-26 July 200
Stable Inflationary Dissipative Cosmologies
The stability of the de Sitter era of cosmic expansion in spatially curved
homogeneous isotropic universes is studied. The source of the gravitational
field is an imperfect fluid such that the parameters that characterize it may
change with time. In this way we extend our previous analysis for
spatially-flat spaces as well as the work of Barrow.Comment: 13 pages, LaTeX 2.09, 1 figure. To be published in International
Journal of Modern Physics
Transient cosmic acceleration from interacting fluids
Recent investigations seem to favor a cosmological dynamics according to
which the accelerated expansion of the Universe may have already peaked and is
now slowing down again \cite{sastaro}. As a consequence, the cosmic
acceleration may be a transient phenomenon. We investigate a toy model that
reproduces such a background behavior as the result of a time-dependent
coupling in the dark sector which implies a cancelation of the "bare"
cosmological constant. With the help of a statistical analysis of Supernova
Type Ia (SNIa) data we demonstrate that for a certain parameter combination a
transient accelerating phase emerges as a pure interaction effect.Comment: Latex file, 23 pages, 21 figures in eps format. Discussion enlarged,
new subsection on scalar field dynamics included, accepted for publication in
JCAP
Holographic Dark Energy with Curvature
In this paper we consider an holographic model of dark energy, where the
length scale is the Hubble radius, in a non flat geometry. The model contains
the possibility to alleviate the cosmic coincidence problem, and also
incorporate a mechanism to obtain the transition from decelerated to an
accelerated expansion regime. We derive an analytic form for the Hubble
parameter in a non flat universe, and using it, we perform a Bayesian analysis
of this model using SNIa, BAO and CMB data. We find from this analysis that the
data favored a small value for , however high enough to still produce
cosmological consequences.Comment: 6 pages, 2 figure
Interacting holographic dark energy model and generalized second law of thermodynamics in non-flat universe
In the present paper we consider the interacting holographic model of dark
energy to investigate the validity of the generalized second laws of
thermodynamics in non-flat (closed) universe enclosed by the event horizon
measured from the sphere of the horizon named . We show that for as the
system's IR cut-off the generalized second law is respected for the special
range of the deceleration parameter.Comment: 11 pages, no figure
The Holographic Model of Dark Energy and Thermodynamics of Non-Flat Accelerated Expanding Universe
Motivated by recent results on non-vanishing spatial curvature \cite{curve}
we employ the holographic model of dark energy to investigate the validity of
first and second laws of thermodynamics in non-flat (closed) universe enclosed
by apparent horizon and the event horizon measured from the sphere of
horizon named . We show that for the apparent horizon the first law is
roughly respected for different epochs while the second laws of thermodynamics
is respected while for as the system's IR cut-off first law is broken down
and second law is respected for special range of deceleration parameter. It is
also shown that at late-time universe is equal to and the
thermodynamic laws are hold, when the universe has non-vanishing curvature.
Defining the fluid temperature to be proportional to horizon temperature the
range for coefficient of proportionality is obtained provided that the
generalized second law of thermodynamics is hold.Comment: 12 pages, no figure, abstract and text extended, references added,
accepted for publication in JCA
Interacting Kasner-type cosmologies
It is well known that Kasner-type cosmologies provide a useful framework for
analyzing the three-dimensional anisotropic expansion because of the
simplification of the anisotropic dynamics. In this paper relativistic
multi-fluid Kasner-type scenarios are studied. We first consider the general
case of a superposition of two ideal cosmic fluids, as well as the particular
cases of non-interacting and interacting ones, by introducing a
phenomenological coupling function . For two-fluid cosmological scenarios
there exist only cosmological scaling solutions, while for three-fluid
configurations there exist not only cosmological scaling ones, but also more
general solutions. In the case of triply interacting cosmic fluids we can have
energy transfer from two fluids to a third one, or energy transfer from one
cosmic fluid to the other two. It is shown that by requiring the positivity of
energy densities there always is a matter component which violates the dominant
energy condition in this kind of anisotropic cosmological scenarios.Comment: Accepted for publication in Astrophysics &Space Science, 8 page
Quintessence dissipative superattractor cosmology
We investigate the simplest quintessence dissipative dark matter attractor
cosmology characterized by a constant quintessence baryotropic index and a
power--law expansion. We show a class of accelerated coincidence--solving
attractor solutions converging to this asymptotic behavior. Despite its
simplicity, such a ``superattractor'' regime provides a model of the recent
universe that also exhibits an excellent fit to supernovae luminosity
observations and no age conflict. Our best fit gives for
the power-law exponent. We calculate for this regime the evolution of density
and entropy perturbations.Comment: 15 pages, Revtex, 2 figures. v3: Minor typos correcte
Interacting Ghost Dark Energy in Non-Flat Universe
A new dark energy model called "ghost dark energy" was recently suggested to
explain the observed accelerating expansion of the universe. This model
originates from the Veneziano ghost of QCD. The dark energy density is
proportional to Hubble parameter, , where is a
constant of order and is
QCD mass scale. In this paper, we extend the ghost dark energy model to the
universe with spatial curvature in the presence of interaction between dark
matter and dark energy. We study cosmological implications of this model in
detail. In the absence of interaction the equation of state parameter of ghost
dark energy is always and mimics a cosmological constant in the
late time, while it is possible to have provided the interaction is
taken into account. When , all previous results of ghost dark energy in
flat universe are recovered. To check the observational consistency, we use
Supernova type Ia (SNIa) Gold sample, shift parameter of Cosmic Microwave
Background radiation (CMB) and the Baryonic Acoustic Oscillation peak from
Sloan Digital Sky Survey (SDSS). The best fit values of free parameter at
confidence interval are: ,
and . Consequently
the total energy density of universe at present time in this model at 68% level
equates to .Comment: 19 pages, 9 figures. V2: Added comments, observational consequences,
references, figures and major corrections. Accepted for publication in
General Relativity and Gravitatio
Fitting Type Ia supernovae with coupled dark energy
We discuss the possible consistency of the recently discovered Type Ia
supernovae at z>1 with models in which dark energy is strongly coupled to a
significant fraction of dark matter, and in which an (asymptotic) accelerated
phase exists where dark matter and dark energy scale in the same way. Such a
coupling has been suggested for a possible solution of the coincidence problem,
and is also motivated by string cosmology models of "late time" dilaton
interactions. Our analysis shows that, for coupled dark energy models, the
recent data are still consistent with acceleration starting as early as at
(to within 90% c.l.), although at the price of a large "non-universality"
of the dark energy coupling to different matter fields. Also, as opposed to
uncoupled models which seem to prefer a ``phantom'' dark energy, we find that a
large amount of coupled dark matter is compatible with present data only if the
dark energy field has a conventional equation of state w>-1.Comment: 13 pages, 6 figures. Final version, accepted for publication in JCA
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