594 research outputs found
Some FRW Models of Accelerating Universe with Dark Energy
The paper deals with a spatially homogeneous and isotropic FRW space-time
filled with perfect fluid and dark energy components. The two sources are
assumed to interact minimally, and therefore their energy momentum tensors are
conserved separately. A special law of variation for the Hubble parameter
proposed by Berman (1983) has been utilized to solve the field equations. The
Berman's law yields two explicit forms of the scale factor governing the FRW
space-time and constant values of deceleration parameter. The role of dark
energy with variable equation of state parameter has been studied in detail in
the evolution of FRW universe. It has been found that dark energy dominates the
universe at the present epoch, which is consistent with the observations. The
physical behavior of the universe is discussed in detail.Comment: 10 pages, 5 figure
A Phantom Menace? Cosmological consequences of a dark energy component with super-negative equation of state
It is extraordinary that a number of observations indicate that we live in a
spatially flat, low matter density Universe, which is currently undergoing a
period of accelerating expansion. The effort to explain this current state has
focused attention on cosmological models in which the dominant component of the
cosmic energy density has negative pressure, with an equation of state . Remarking that most observations are consistent with models right up to
the or cosmological constant () limit, it is natural to ask
what lies on the other side, at . In this regard, we construct a toy
model of a ``phantom'' energy component which possesses an equation of state
. Such a component is found to be compatible with most classical tests of
cosmology based on current data, including the recent type 1a SNe data as well
as the cosmic microwave background anisotropy and mass power spectrum. If the
future observations continue to allow , then barring unanticipated
systematic effects, the dominant component of the cosmic energy density may be
stranger than anything expected.Comment: update of original version, includes new material, matches version
appearing in Phys. Lett. B, (17 pages, 7 eps figures
Phantom Wormholes in (2+1)-dimensions
In this paper, we have constructed a (2+1)-dimensional wormhole using
inhomogeneous and anisotropic distribution of phantom energy. We have
determined the exact form of the equation of state of phantom energy that
supports the wormhole structure. Interestingly, this equation of state is
linear but variable one and is dependent only on the radial parameter of the
model.Comment: 10 pages, 5 figure
The Big Trip and Wheeler-DeWitt equation
Of all the possible ways to describe the behavior of the universe that has
undergone a big trip the Wheeler-DeWitt equation should be the most accurate --
provided, of course, that we employ the correct formulation. In this article we
start by discussing the standard formulation introduced by Gonz\'alez-D\'iaz
and Jimenez-Madrid, and show that it allows for a simple yet efficient method
of the solution's generation, which is based on the Moutard transformation.
Next, by shedding the unnecessary restrictions, imposed on aforementioned
standard formulation we introduce a more general form of the Wheeler-DeWitt
equation. One immediate prediction of this new formula is that for the universe
the probability to emerge right after the big trip in a state with will
be maximal if and only if .Comment: accepted in Astrophysics and Space Scienc
Attractor Solution of Phantom Field
In light of recent study on the dark energy models that manifest an equation
of state , we investigate the cosmological evolution of phantom field in
a specific potential, exponential potential in this paper. The phase plane
analysis show that the there is a late time attractor solution in this model,
which address the similar issues as that of fine tuning problems in
conventional quintessence models. The equation of state is determined by
the attractor solution which is dependent on the parameter in the
potential. We also show that this model is stable for our present observable
universe.Comment: 9 pages, 3 ps figures; typos corrected, references updated, this is
the final version to match the published versio
Stable phantom-divide crossing in two scalar models with matter
We construct cosmological models with two scalar fields, which has the
structure as in the ghost condensation model or k-essence model. The models can
describe the stable phantom crossing, which should be contrasted with one
scalar tensor models, where the infinite instability occurs at the crossing the
phantom divide. We give a general formulation of the reconstruction in terms of
the e-foldings N by including the matter although in the previous two scalar
models, which are extensions of the scalar tensor model, it was difficult to
give a formulation of the reconstruction when we include matters. In the
formulation of the reconstruction, we start with a model with some arbitrary
functions, and find the functions which generates the history in the expansion
of the universe. We also give general arguments for the stabilities of the
models and the reconstructed solution. The viability of a model is also
investigated by comparing the observational data.Comment: 12 pages, 1 figur
Quantum driven Bounce of the future Universe
It is demonstrated that due to back-reaction of quantum effects, expansion of
the universe stops at its maximum and takes a turnaround. Later on, it
contracts to a very small size in finite future time. This phenomenon is
followed by a " bounce" with re-birth of an exponentially expanding
non-singular universe
Phantom Field with O(N) Symmetry in Exponential Potential
In this paper, we study the phase space of phantom model with O(\emph{N})
symmetry in exponential potential. Different from the model without O(\emph{N})
symmetry, the introduction of the symmetry leads to a lower bound on the
equation of state for the existence of stable phantom dominated attractor
phase. The reconstruction relation between the potential of O(\textit{N})
phantom system and red shift has been derived.Comment: 5 pages, 3 figures, replaced with the version to appear on Phys. Rev.
Asymptotic behavior of w in general quintom model
For the quintom models with arbitrary potential , the
asymptotic value of equation of state parameter w is obtained by a new method.
In this method, w of stable attractors are calculated by using the ratio (d ln
V)/(d ln a) in asymptotic region. All the known results, have been obtained by
other methods, are reproduced by this method as specific examples.Comment: 8 pages, one example is added, accepted for publication in Gen. Rel.
Gra
Horava-Lifshitz Dark Energy
We formulate Horava-Lifshitz cosmology with an additional scalar field that
leads to an effective dark energy sector. We find that, due to the inherited
features from the gravitational background, Horava-Lifshitz dark energy
naturally presents very interesting behaviors, possessing a varying
equation-of-state parameter, exhibiting phantom behavior and allowing for a
realization of the phantom divide crossing. In addition, Horava-Lifshitz dark
energy guarantees for a bounce at small scale factors and it may trigger the
turnaround at large scale factors, leading naturally to cyclic cosmology.Comment: 17 pages, no figures, version published at EJP
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