237 research outputs found
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.
Cosmic anti-friction and accelerated expansion
We explain an accelerated expansion of the present universe, suggested from
observations of supernovae of type Ia at high redshift, by introducing an
anti-frictional force that is self-consistently exerted on the particles of the
cosmic substratum. Cosmic anti-friction, which is intimately related to
``particle production'', is shown to give rise to an effective negative
pressure of the cosmic medium. While other explanations for an accelerated
expansion (cosmological constant, quintessence) introduce a component of dark
energy besides ``standard'' cold dark matter (CDM) we resort to a
phenomenological one-component model of CDM with internal self-interactions. We
demonstrate how the dynamics of the LambdaCDM model may be recovered as a
special case of cosmic anti-friction. We discuss the connection with
two-component models and obtain an attractor behavior for the ratio of the
energy densities of both components which provides a possible phenomenological
solution to the coincidence problem.Comment: 19 pages, 7 (3 new) figures, new derivation of kinetic equation with
force term, accepted by Physical Review
Inhomogeneous models of interacting dark matter and dark energy
We derive and analyze a class of spherically symmetric cosmological models
whose source is an interactive mixture of inhomogeneous cold dark matter (DM)
and a generic homogeneous dark energy (DE) fluid. If the DE fluid corresponds
to a quintessense scalar field, the interaction term can be associated with a
well motivated non--minimal coupling to the DM component. By constructing a
suitable volume average of the DM component we obtain a Friedman evolution
equation relating this average density with an average Hubble scalar, with the
DE component playing the role of a repulsive and time-dependent term.
Once we select an ``equation of state'' linking the energy density () and
pressure () of the DE fluid, as well as a free function governing the radial
dependence, the models become fully determinate and can be applied to known
specific DE sources, such as quintessense scalar fields or tachyonic fluids.
Considering the simple equation of state with , we show that the free parameters and boundary conditions can be selected
for an adequate description of a local DM overdensity evolving in a suitable
cosmic background that accurately fits current observational data. While a DE
dominated scenario emerges in the asymptotic future, with total and
tending respectively to 1 and -1/2 for all cosmic observers, the effects of
inhomogeneity and anisotropy yield different local behavior and evolution rates
for these parameters in the local overdense region. We suggest that the models
presented can be directly applied to explore the effects of various DE
formalisms on local DM cosmological inhomogeneities.Comment: 15 pages, revtex4, 10 eps figure
Inhomogeneous vacuum energy
Vacuum energy remains the simplest model of dark energy which could drive the
accelerated expansion of the Universe without necessarily introducing any new
degrees of freedom. Inhomogeneous vacuum energy is necessarily interacting in
general relativity. Although the four-velocity of vacuum energy is undefined,
an interacting vacuum has an energy transfer and the vacuum energy defines a
particular foliation of spacetime with spatially homogeneous vacuum energy in
cosmological solutions. It is possible to give a consistent description of
vacuum dynamics and in particular the relativistic equations of motion for
inhomogeneous perturbations given a covariant prescription for the vacuum
energy, or equivalently the energy transfer four-vector, and we construct
gauge-invariant vacuum perturbations. We show that any dark energy cosmology
can be decomposed into an interacting vacuum+matter cosmology whose
inhomogeneous perturbations obey simple first-order equations.Comment: 8 pages; v2 clarified discussion of Chaplygin gas model, references
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Curvature force and dark energy
A curvature self-interaction of the cosmic gas is shown to mimic a
cosmological constant or other forms of dark energy, such as a rolling tachyon
condensate or a Chaplygin gas. Any given Hubble rate and deceleration parameter
can be traced back to the action of an effective curvature force on the gas
particles. This force self-consistently reacts back on the cosmological
dynamics. The links between an imperfect fluid description, a kinetic
description with effective antifriction forces, and curvature forces, which
represent a non-minimal coupling of gravity to matter, are established.Comment: 14 pages; references added, to appear in New Journal of Physics (v3
Observational constraints on Rastall's cosmology
Rastall's theory is a modification of General Relativity, based on the
non-conservation of the stress-energy tensor. The latter is encoded in a
parameter such that restores the usual law. We test Rastall's theory in cosmology, on a flat
Robertson-Walker metric, investigating a two-fluid model and using the type Ia
supernovae Constitution dataset. One of the fluids is pressureless and obeys
the usual conservation law, whereas the other is described by an equation of
state , with constant. The Bayesian analysis of the
Constitution set does not strictly constrain the parameter and prefers
values of close to -1. We then address the evolution of small
perturbations and show that they are dramatically unstable if and
, i.e. General Relativity is the favored configuration. The only
alternative is , for which the dynamics becomes independent from
.Comment: Latex file, 14 pages, 6 figures in eps format. Substantial
modifications performed, main conclusions change
Bulk viscosity driving the acceleration of the Universe
The possibility that the present acceleration of the universe is driven by a
kind of viscous fluid is exploited. At background level this model is similar
to the generalized Chaplygin gas model (GCGM). But, at perturbative level, the
viscous fluid exhibits interesting properties. In particular the oscillations
in the power spectrum that plagues the GCGM are not present. Possible
fundamental descriptions for this viscous dark energy are discussed.Comment: Latex file, 8 pages, 3 eps figure
Thermodynamics of viscous dark energy in an RSII braneworld
We show that for an RSII braneworld filled with interacting viscous dark
energy and dark matter, one can always rewrite the Friedmann equation in the
form of the first law of thermodynamics, , at apparent horizon.
In addition, the generalized second law of thermodynamics can fulfilled in a
region enclosed by the apparent horizon on the brane for both constant and time
variable 5-dynamical Newton's constant . These results hold regardless of
the specific form of the dark energy. Our study further support that in an
accelerating universe with spatial curvature, the apparent horizon is a
physical boundary from the thermodynamical point of view.Comment: 11 page
Plasma waves driven by gravitational waves in an expanding universe
In a Friedmann-Robertson-Walker (FRW) cosmological model with zero spatial
curvature, we consider the interaction of the gravitational waves with the
plasma in the presence of a weak magnetic field. Using the relativistic
hydromagnetic equations it is verified that large amplitude magnetosonic waves
are excited, assuming that both, the gravitational field and the weak magnetic
field do not break the homogeneity and isotropy of the considered FRW
spacetime.Comment: 14 page
On Internal Consistency of Holographic Dark Energy Models
Holographic dark energy (HDE) models, underlain by an effective quantum field
theory (QFT) with a manifest UV/IR connection, have become a convincing
candidate for the dark energy in the universe. On the other hand, the maximum
number of quantum states a conventional QFT in the box of size is capable
to describe, refer to those boxes which are on the brink of experiencing a
sudden collapse to a black hole. Another restriction on the underlying QFT is
that the UV cutoff, which cannot be chosen independently of the IR cutoff and
therefore becomes a function of time in a cosmological setting, should stay the
largest energy scale even in the standard cosmological epochs preceding a dark
energy dominated one. We show that, irrespective of whether one deals with the
saturated form of HDE or takes a certain degree of non-saturation in the past,
the above restrictions cannot be met in a radiation-dominated universe, an
epoch in the history of the universe which is expected to be perfectly
describable within conventional QFT.Comment: 8 pages, version to appear in JCA
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