6,187 research outputs found
Inflation is the generic feature of phantom field-not the big-rip
A class of solutions for phantom field corresponding to a generalized
k-essence lagrangian has been presented, employing a simple method which
provides the scope to explore many such. All the solutions having dynamical
state parameter are found to touch the magic line w = -1, asymptotically. The
solutions with constant equation of state can represent phantom, quitessence or
an ordinary scalar field cosmologies depending on the choice of a couple of
parameters of the theory. For w approximately equal to -1, quintessence and
phantom models are indistinguishable through the Hubble parameter. Finally,
inflation rather than big-rip has been found to be the generic feature of
phantom cosmology.Comment: 8 pages, 3 figure
A model-independent dark energy reconstruction scheme using the geometrical form of the luminosity-distance relation
We put forward a new model-independent reconstruction scheme for dark energy
which utilises the expected geometrical features of the luminosity-distance
relation. The important advantage of this scheme is that it does not assume
explicit ansatzes for cosmological parameters but only some very general
cosmological properties via the geometrical features of the reconstructed
luminosity-distance relation. Using the recently released supernovae data by
the Supernova Legacy Survey together with a phase space representation, we show
that the reconstructed luminosity-distance curves best fitting the data
correspond to a slightly varying dark energy density with the Universe
expanding slightly slower than the Lambda CDM model. However, the Lambda CDM
model fits the data at 1 sigma significance level and the fact that our best
fitting luminosity-distance curve is lower than that of the corresponding
Lambda CDM model could be due to systematics. The transition from an
accelerating to a decelerating expansion occurs at a redshift larger than
z=0.35. Interpreting the dark energy as a minimally coupled scalar field we
also reconstruct the scalar field and its potential. We constrain
using the baryon acoustic oscillation peak in the SDSS luminous
red galaxy sample and find that the best fit is obtained with
, in agreement with the CMB data.Comment: 10 pages, 18 figure
Networks of cosmological histories, crossing of the phantom divide line and potentials with cusps
We discuss the phenomenon of the smooth dynamical gravity induced crossing of
the phantom divide line in a framework of simple cosmological models where it
appears to occur rather naturally, provided the potential of the unique scalar
field has some kind of cusp. The behavior of cosmological trajectories in the
vicinity of the cusp is studied in some detail and a simple mechanical analogy
is presented. The phenomenon of certain complementarity between the smoothness
of the spacetime geometry and matter equations of motion is elucidated. We
introduce a network of cosmological histories and qualitatively describe some
of its properties
Variable Modified Chaplygin Gas and Accelerating Universe
In this letter, I have proposed a model of variable modified Chaplygin gas
and shown its role in accelerating phase of the universe. I have shown that the
equation of state of this model is valid from the radiation era to quiessence
model. The graphical representations of statefinder parameters characterize
different phase of evolution of the universe. All results presented in the
letter concerns the case .Comment: 7 Latex pages, 5 figures, revtex styl
Reconstructing the potentials for the quintessence and tachyon dark energy, from the holographic principle
We propose an holographic quintessence and tachyon models of dark energy. The
correspondence between the quintessence and tachyon energy densities with the
holographic density, allows the reconstruction of the potentials and the
dynamics for the quintessence and tachyon fields, in flat FRW background. The
proposed infrared cut-off for the holographic energy density works for two
cases of the constant : for we reconstructed the holographic
quintessence model in the region before the crossing for the EoS
parameter. The cosmological dynamics for was also reconstructed for
the holographic quintessence and tachyon models.Comment: 21 pages, 18 figures, 2 table
Constraints on holographic dark energy models using the differential ages of passively evolving galaxies
Using the absolute ages of passively evolving galaxies observed at different
redshifts, one can obtain the differential ages, the derivative of redshift
with respect to the cosmic time (i.e. ). Thus, the
Hubble parameter can be measured through the relation . By comparing the measured Hubble parameter at different
redshifts with the theoretical one containing free cosmological parameters, one
can constrain current cosmological models. In this paper, we use this method to
present the constraint on a spatially flat Friedman-Robert-Walker Universe with
a matter component and a holographic dark energy component, in which the
parameter plays a significant role in this dark energy model. Firstly we
consider three fixed values of =0.6, 1.0 and 1.4 in the fitting of data. If
we set free, the best fitting values are , ,
. It is shown that the holographic dark energy behaves like a
quintom-type at the level. This result is consistent with some other
independent cosmological constrains, which imply that is favored. We
also test the results derived from the differential ages using another
independent method based on the lookback time to galaxy clusters and the age of
the universe. It shows that our results are reliable.Comment: 18 pages including 7 figures and 1 tables. Final version for
publication in Modern Physics Letters A (MPLA)[minor revision to match the
appear version
Accelerating Universe as Window for Extra Dimensions
Homogeneous cosmological solutions are obtained in five dimensional space
time assuming equations of state and where p
is the isotropic 3 - pressure and , that for the fifth dimension. Using
different values for the constants k and many known solutions are
rediscovered. Further the current acceleration of the universe has led us to
investigate higher dimensional gravity theory, which is able to explain
acceleration from a theoretical view point without the need of introducing dark
energy by hand. We argue that the terms containing higher dimensional metric
coefficients produce an extra negative pressure that apparently drives an
acceleration of the 3D space, tempting us to suggest that the accelerating
universe seems to act as a window to the existence of extra spatial dimensions.
Interestingly the 5D matter field remains regular while the \emph{effective}
negative pressure is responsible for the inflation. Relaxing the assumptions of
two equations of state we also present a class of solutions which provide early
deceleration followed by a late acceleration in a unified manner. Interesting
to point out that in this case our cosmology apparently mimics the well known
quintessence scenario fuelled by a generalised Chaplygin-type of fluid where a
smooth transition from a dust dominated model to a de Sitter like one takes
place.Comment: 20 pages,3 figure
Phantom universe from CPT symmetric QFT
Inspired by the generalization of quantum theory for the case of
non-Hermitian Hamiltonians with CPT symmetry, we construct a simple classical
cosmological scalar field based model describing a smooth transition from
ordinary dark energy to the phantom one
Interacting holographic tachyon model of dark energy
We propose a holographic tachyon model of dark energy with interaction
between the components of the dark sector. The correspondence between the
tachyon field and the holographic dark energy densities allows the
reconstruction of the potential and the dynamics of the tachyon scalar field in
a flat Friedmann-Robertson-Walker universe. We show that this model can
describe the observed accelerated expansion of our universe with a parameter
space given by the most recent observational results.Comment: 7 pages, 8 figures, accepted for publication in IJMP
Reconstructing generalized ghost condensate model with dynamical dark energy parametrizations and observational datasets
Observations of high-redshift supernovae indicate that the universe is
accelerating at the present stage, and we refer to the cause for this cosmic
acceleration as ``dark energy''. In particular, the analysis of current data of
type Ia supernovae (SNIa), cosmic large-scale structure (LSS), and the cosmic
microwave background (CMB) anisotropy implies that, with some possibility, the
equation-of-state parameter of dark energy may cross the cosmological-constant
boundary () during the recent evolution stage. The model of ``quintom''
has been proposed to describe this crossing behavior for dark energy. As
a single-real-scalar-field model of dark energy, the generalized ghost
condensate model provides us with a successful mechanism for realizing the
quintom-like behavior. In this paper, we reconstruct the generalized ghost
condensate model in the light of three forms of parametrization for dynamical
dark energy, with the best-fit results of up-to-date observational data.Comment: 8 pages, 3 figures; references added; accepted for publication in
Mod. Phys. Lett.
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