1,006 research outputs found
The evolution of density perturbations in two quintessence models
In this work we investigate the evolution of matter density perturbations for
two different quintessence models. One of them is based on the Einstein theory
of gravity, while the other is based on the Brans-Dicke scalar tensor theory.
We show that it is possible to constrain the parameter space of the models
using the determinations for the growth rate of perturbations derived from data
of the 2-degree Field Galaxy Redshift Survey.Comment: 11 pages, 2 figure
New exact cosmological solutions to Einstein's gravity minimally coupled to a Quintessence field
A linear relationship between the Hubble expansion parameter and the time
derivative of the scalar field is assumed in order to derive exact analytic
cosmological solutions to Einstein's gravity with two fluids: a barotropic
perfect fluid of ordinary matter, together with a self-interacting scalar field
fluid accounting for the dark energy in the universe. A priori assumptions
about the functional form of the self-interaction potential or about the scale
factor behavior are not neccessary. These are obtained as outputs of the
assumed linear relationship between the Hubble expansion parameter and the time
derivative of the scalar field. As a consequence only a class of exponential
potentials and their combinations can be treated. The relevance of the
solutions found for the description of the cosmic evolution are discussed in
some detail. The possibility to have superaccelerated expansion within the
context of normal quintessence models is also discussed.Comment: 10 pages, Revtex, 3 eps figures. Some misprints corrected. New
references added. The full space of solution investigated in detail and the
possibility to accomodate superaccelerated expansion (superquintessence)
within the frame of normal quintessence models is also discussed. One figure
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Comment on "Extended Born-Infeld theory and the bouncing magnetic universe"
In a recent paper [Phys. Rev. D{\bf 85}, 023528 (2012)] the authors proposed
a generalized Born-Infeld electrodynamics coupled to general relativity which
produces a nonsingular bouncing universe. For a magnetic universe the resulting
cosmic evolution inevitably interpolates between asymptotic de Sitter states.
Here we shall show that (i) the above theory does not have the standard weak
field Maxwell limit, (ii) a sudden curvature singularity -- not better than the
big bang -- arises, (iii) the speed of sound squared is a negative quantity
signaling instability against small perturbations of the background energy
density, and that (iv) the conclusion about the inevitability of the asymptotic
vacuum regime in a magnetic universe is wrong.Comment: 6 pages, 6 eps figures, here we have shown that Born-Infeld magnetic
universes either are singular or are unstable against small perturbations of
the background energy density or bot
Phase Space Dynamics of Non-Gravitational Interactions between Dark Matter and Dark Energy: The Case of Ghost Dark Energy
We study the phase space asymptotics of the so called Veneziano ghost dark
energy models. Models where the ghost field's energy density: i)
, and ii) , are investigated.
Both, cases with and without additional non-gravitational interaction between
cold dark matter and ghost dark energy, are subject to scrutiny. We pay special
attention to the choice of phase space variables leading to bounded and compact
phase space so that no critical point of physical interest is missing. A rich
asymptotic structure is revealed: depending on the kind of non-minimal coupling
critical points associated with radiation dominance, matter dominance, cold
dark matter/ghost dark energy scaling, and ghost dark energy dominance, are
found. Past and future attractors, as well as saddle equilibrium points, are
identified in the corresponding phase spaces.Comment: 19 pages, 2 eps figures. A remark about the relationship between
ghost dark energy models and models of dark energy with a generalized
equation of state added. New bibliographic reference
No compelling cosmological models come out of magnetic universes which are based in nonlinear electrodynamics
Here we investigate the cosmic dynamics of Friedmann-Robertson-Walker
universes -- flat spatial sections -- which are driven by nonlinear
electrodynamics (NLED) Lagrangians. We pay special attention to the check of
the sign of the square sound speed since, whenever the latter quantity is
negative, the corresponding cosmological model is classically unstable against
small perturbations of the background energy density. Besides, based on
causality arguments, one has to require that the mentioned small perturbations
of the background should propagate at most at the local speed of light. We also
look for the occurrence of curvature singularities. Our results indicate that
several cosmological models which are based in known NLED Lagrangians, either
are plagued by curvature singularities of the sudden and/or big rip type, or
are violently unstable against small perturbations of the cosmological
background -- due to negative sign of the square sound speed -- or both. In
addition, causality issues associated with superluminal propagation of the
background perturbations may also arise.Comment: 15 pages, 15 figures. arXiv admin note: text overlap with
arXiv:1310.3021. Version that matches the one accepted by PR
Dirac-Born-Infeld Field Trapped in the Braneworld
We apply the dynamical systems tools to study the (linear) cosmic dynamics of
a Dirac-Born-Infeld-type field trapped in the braneworld. We focus,exclusively,
in Randall-Sundrum and in Dvali-Gabadadze-Porrati brane models. We analyze the
existence and stability of asymptotic solutions for the AdS throat and the
quadratic potential and a particular choice of the warp factor and of the
potential for the DBI field (). It is demonstrated, in
particular, that in the ultra-relativistic approximation matter-scaling and
scalar field-dominated solutions always arise. In the first scenario the empty
universe is the past attractor, while in the second model the past attractor is
the matter-dominated phase.Comment: 14 pages, 2 figure
Interacting Phantom Energy and Avoidance of the Big Rip Singularity
Models of the universe with arbitrary (non gravitational) interaction between
the components of the cosmic fluid: the phantom energy and the background, are
investigated. A general form of the interaction that is inspired in
scalar-tensor theories of gravity is considered. No specific model for the
phantom fluid is assumed. We concentrate our investigation on solutions that
are free of the coincidence problem. We found a wide region in the parameter
space where the solutions are free of the big rip singularity also. Physical
arguments, together with arguments based on the analysis of the observational
evidence, suggest that phantom models without big rip singularity might be
preferred by Nature.Comment: 10 pages (Latex format), 6 eps figures. Some arguments rewritten and
commets added. 3 eps figures removed. References updated. Section on
Supernovae Observations removed. An author adde
Exact Scaling Solutions in Normal and Brans-Dicke Models of Dark Energy
A linear relationship between the Hubble expansion parameter and the time
derivative of the scalar field is explored in order to derive exact
cosmological, attractor-like solutions, both in Einstein's theory and in
Brans-Dicke gravity with two fluids: a background fluid of ordinary matter,
together with a self-interacting scalar field accounting for the dark energy in
the universe. A priori assumptions about the functional form of the
self-interaction potential or about the scale factor behavior are not
necessary. These are obtained as outputs of the assumed relationship between
the Hubble parameter and the time derivative of the scalar field. A parametric
class of scaling quintessence models given by a self-interaction potential of a
peculiar form: a combination of exponentials with dependence on the barotropic
index of the background fluid, arises. Both normal quintessence described by a
self-interacting scalar field minimally coupled to gravity and Brans-Dicke
quintessence given by a non-minimally coupled scalar field are then analyzed
and the relevance of these models for the description of the cosmic evolution
are discussed in some detail. The stability of these solutions is also briefly
commented on.Comment: 17 pages, 5 eps figures. This is a published version that comprises
(and extends) the material in papers gr-qc/0212006 and gr-qc/021009
Modelling dark energy with quintessence and a cosmological constant
In this talk we present a model of the universe in which dark energy is
modelled explicitely with both a dynamical quintessence field and a
cosmological constant. Our results confirm the possibility of a collapsing
universe (for a given region of the parameter space), which is advantageous for
an adequate formulation of both perturbative quantum field and string theories.
We have also reproduced the measurements of modulus distance from supernovae
with good accuracy.Comment: Talk presented by R Cardenas in UK Cosmology Meeting at the
University of Wales Swansea, September 2nd-4th, 200
Dynamics of FRW Universes Sourced by Non-Linear Electrodynamics
We apply the dynamical systems tools to study the (linear) dynamics of
Friedmann-Robertson-Walker universes that are fuelled by non-linear
electrodynamics. We focus, mainly, in two particular models. In the first model
the cosmic evolution is fuelled by cold dark matter, a cosmological constant
and a non-linear electrodynamics field. In the second case non-singular
cosmology and late-time accelerated expansion are unified in a model where the
Einstein's field equations are sourced only by cold dark matter and a
non-linear electrodynamics field. It is shown that, in contrast to previous
claims, the cosmological effects coming from the non-linear electrodynamics
field are not as generic as though. In fact, critical points in the phase space
that could be associated with non-linear electrodynamic effects are not found.Comment: 8 pages, 10 figures, pdf file, tex fil
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