82 research outputs found
A new view of k-essence
K-essence models, relying on scalar fields with non-canonical kinetic terms,
have been proposed as an alternative to quintessence in explaining the observed
acceleration of the Universe. We consider the use of field redefinitions to
cast k-essence in a more familiar form. While k-essence models cannot in
general be rewritten in the form of quintessence models, we show that in
certain dynamical regimes an equivalence can be made, which in particular can
shed light on the tracking behaviour of k-essence. In several cases, k-essence
cannot be observationally distinguished from quintessence using the homogeneous
evolution, though there may be small effects on the perturbation spectrum. We
make a detailed analysis of two k-essence models from the literature and
comment on the nature of the fine tuning arising in the models.Comment: 7 pages RevTeX4 file with four figures incorporate
K-essence and the coincidence problem
K-essence has been proposed as a possible means of explaining the coincidence
problem of the Universe beginning to accelerate only at the present epoch. We
carry out a comprehensive dynamical systems analysis of the k-essence models
given so far in the literature. We numerically study the basin of attraction of
the tracker solutions and we highlight the behaviour of the field close to
sound speed divergences. We find that, when written in terms of parameters with
a simple dynamical interpretation, the basins of attraction represent only a
small region of the phase space.Comment: 5 pages RevTeX4 file with two figures incorporated. Minor changes to
match PRD accepted versio
Prospects and Problems of Tachyon Matter Cosmology
We consider the evolution of FRW cosmological models and linear perturbations
of tachyon matter rolling towards a minimum of its potential. The tachyon
coupled to gravity is described by an effective 4d field theory of string
theory tachyon. In the model where a tachyon potential has a quadratic
minimum at finite value of the tachyon field and , the tachyon
condensate oscillates around its minimum with a decreasing amplitude. It is
shown that its effective equation of state is . However, linear
inhomogeneous tachyon fluctuations coupled to the oscillating background
condensate are exponentially unstable due to the effect of parametric
resonance. In another interesting model, where tachyon potential exponentially
approaches zero at infinity of , rolling tachyon condensate in an expanding
universe behaves as pressureless fluid. Its linear fluctuations coupled with
small metric perturbations evolve similar to these in the pressureless fluid.
However, this linear stage changes to a strongly non-linear one very early, so
that the usual quasi-linear stage observed at sufficiently large scales in the
present Universe may not be realized in the absence of the usual particle-like
cold dark matter.Comment: 12 pages, 3 figure
K-essential Phantom Energy: Doomsday around the Corner? Revisited
We generalize some of those results reported by Gonz\'{a}lez-D\'{i}az by
further tuning the parameter () which is closely related to the
canonical kinetic term in -essence formalism. The scale factor could
be negative and decreasing within a specific range of (, : the equation-of-state parameter) during the initial
evolutional period.Comment: 1 Figure, 6 page
New holographic scalar field models of dark energy in non-flat universe
Motivated by the work of Granda and Oliveros [L.N. Granda, A. Oliveros, Phys.
Lett. B {\bf 671}, 199 (2009)], we generalize their work to the non-flat case.
We study the correspondence between the quintessence, tachyon, K-essence and
dilaton scalar field models with the new holographic dark energy model in the
non-flat FRW universe. We reconstruct the potentials and the dynamics for these
scalar field models, which describe accelerated expansion of the universe. In
the limiting case of a flat universe, i.e. , all results given in [L.N.
Granda, A. Oliveros, Phys. Lett. B {\bf 671}, 199 (2009)] are obtained.Comment: 11 page
Dark matter to dark energy transition in k-essence cosmologies
We implement the transition from dark matter to dark energy in k-essence
cosmologies for a very large set of kinetic functions , in a way alternative
to recent proposals which use generalized Chaplygin gas and transient models.
Here we require that the pressure admits a power-law expansion around some
value of the kinetic energy where the pressure vanishes. In addition, for
suitable values of the parameters of the model, the speed of sound of the dark
matter will be low. We first present the discussion in fairly general terms,
and later consider for illustration two examples.Comment: 5 pages, revte
The generalized second law in irreversible thermodynamics for the interacting dark energy in a non-flat FRW universe enclosed by the apparent horizon
We investigate the validity of the generalized second law in irreversible
thermodynamics in a non-flat FRW universe containing the interacting dark
energy with cold dark matter. The boundary of the universe is assumed to be
enclosed by the dynamical apparent horizon. We show that for the present time,
the generalized second law in nonequilibrium thermodynamics is satisfied for
the special range of the energy transfer constants.Comment: 10 page
Interacting polytropic gas model of phantom dark energy in non-flat universe
By introducing the polytropic gas model of interacting dark energy, we obtain
the equation of state for the polytropic gas energy density in a non-flat
universe. We show that for even polytropic index by choosing
, one can obtain , which
corresponds to a universe dominated by phantom dark energy.Comment: 7 page
Infrared cut-off proposal for the Holographic density
We propose an infrared cut-off for the holographic the dark-energy, which
besides the square of the Hubble scale also contains the time derivative of the
Hubble scale. This avoids the problem of causality which appears using the
event horizon area as the cut-off, and solves the coincidence problem.Comment: 9 pages, 2 figures, to appear in Phys. Lett.
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