65 research outputs found
Power-laws f(R) theories are cosmologically unacceptable
In a recent paper [1] (PRL 98,131302,2007) we have shown that f(R)=R + mu
R^{n} modified gravity dark energy models are not cosmologically viable because
during the matter era that precedes the accelerated stage the cosmic expansion
is given by a sim t^{1/2} rather than a sim t^{2/3}, where a is a scale factor
and t is the cosmic time. A recent work [2] (PLB 639,135-143,2006) by
Capozziello et al. criticised our results presenting some apparent
counter-examples to our claim in f(R)= mu R^{n} models. We show here that those
particular R^{n} models can produce an expansion as a sim t^{2/3} but this does
not connect to a late-time acceleration. Hence, though acceptable f(R) dark
energy models may exist, the R^{n} models presented in Capozziello et al. are
not cosmologically viable, confirming our previous results in Ref. [1].Comment: 4 pages, 1 figure, title change
Slow-roll, acceleration, the Big Rip and WKB approximation in NLS-type formulation of scalar field cosmology
Aspects of non-linear Schr\"{o}dinger-type (NLS) formulation of scalar
(phantom) field cosmology on slow-roll, acceleration, WKB approximation and Big
Rip singularity are presented. Slow-roll parameters for the curvature and
barotropic density terms are introduced. We reexpress all slow-roll parameters,
slow-roll conditions and acceleration condition in NLS form. WKB approximation
in the NLS formulation is also discussed when simplifying to linear case. Most
of the Schr\"{o}dinger potentials in NLS formulation are very slowly-varying,
hence WKB approximation is valid in the ranges. In the NLS form of Big Rip
singularity, two quantities are infinity in stead of three. We also found that
approaching the Big Rip, , which is the
same as effective phantom equation of state in the flat case.Comment: [7 pages, no figure, more reference added, accepted by JCAP
Coupled quintessence and curvature-assisted acceleration
Spatially homogeneous models with a scalar field non-minimally coupled to the
space-time curvature or to the ordinary matter content are analysed with
respect to late-time asymptotic behaviour, in particular to accelerated
expansion and isotropization. It is found that a direct coupling to the
curvature leads to asymptotic de Sitter expansion in arbitrary exponential
potentials, thus yielding a positive cosmological constant although none is
apparent in the potential. This holds true regardless of the steepness of the
potential or the smallness of the coupling constant. For matter-coupled scalar
fields, the asymptotics are obtained for a large class of positive potentials,
generalizing the well-known cosmic no-hair theorems for minimal coupling. In
this case it is observed that the direct coupling to matter does not impact the
late-time dynamics essentially.Comment: 17 pages, no figures. v2: typos correcte
Interacting agegraphic dark energy models in phase space
Agegraphic dark energy, has been recently proposed, based on the so-called
Karolyhazy uncertainty relation, which arises from quantum mechanics together
with general relativity. In the first part of the article we study the original
agegraphic dark energy model by including the interaction between agegraphic
dark energy and pressureless (dark) matter. The phase space analysis was made
and the critical points were found, one of which is the attractor corresponding
to an accelerated expanding Universe.
Recent observations of near supernova show that the acceleration of Universe
decreases. This phenomenon is called the transient acceleration. In the second
part of Article we consider the 3-component Universe composed of a scalar
field, interacting with the dark matter on the agegraphic dark energy
background. We show that the transient acceleration appears in frame of such a
model. The obtained results agree with the observations.Comment: 15 pages, 5 figures, 2 table
Scalar field exact solutions for non-flat FLRW cosmology: A technique from non-linear Schr\"odinger-type formulation
We report a method of solving for canonical scalar field exact solution in a
non-flat FLRW universe with barotropic fluid using non-linear Schr\"{o}dinger
(NLS)-type formulation in comparison to the method in the standard Friedmann
framework. We consider phantom and non-phantom scalar field cases with
exponential and power-law accelerating expansion. Analysis on effective
equation of state to both cases of expansion is also performed. We speculate
and comment on some advantage and disadvantage of using the NLS formulation in
solving for the exact solution.Comment: 12 pages, GERG format, Reference added. accepted by Gen. Relativ. and
Gra
Phantom scalar emission in the Kerr black hole spacetime
We study the absorption probability and Hawking radiation spectra of a
phantom scalar field in the Kerr black hole spacetime. We find that the
presence of the negative kinetic energy terms modifies the standard results in
the greybody factor, super-radiance and Hawking radiation. Comparing with the
usual scalar particle, the phantom scalar emission is enhanced in the black
hole spacetime.Comment: 11 pages, 6 figures, a revised version accepted for publication in
CQ
(m,n)-type holographic dark energy models
We construct -type holographic dark energy models at a
phenomenological level, which can be viewed as a generalization of agegraphic
models with the conformal-like age as the holographic characteristic size. For
some values of the holographic dark energy can automatically evolve
across into a phantom phase even without introducing an interaction
between the dark energy and background matter. Our construction is also
applicable to the holographic dark energy with generalized future event horizon
as the characteristic size. Finally, we address the issue on the stability of
our model and show that they are generally stable under the scalar
perturbation.Comment: 18 pages, no figure, Sec. 5 added, introduction and conclusion
improved, a reference added, some references updated, published in MPL
Cosmological Constraints on the Sign-Changeable Interactions
Recently, Cai and Su [Phys. Rev. D {\bf 81}, 103514 (2010)] found that the
sign of interaction in the dark sector changed in the approximate redshift
range of 0.45\,\lsim\, z\,\lsim\, 0.9, by using a model-independent method to
deal with the observational data. In fact, this result raises a remarkable
problem, since most of the familiar interactions cannot change their signs in
the whole cosmic history. Motivated by the work of Cai and Su, we have proposed
a new type of interaction in a previous work [H. Wei, Nucl. Phys. B {\bf 845},
381 (2011)]. The key ingredient is the deceleration parameter in the
interaction , and hence the interaction can change its sign when our
universe changes from deceleration () to acceleration (). In the
present work, we consider the cosmological constraints on this new type of
sign-changeable interactions, by using the latest observational data. We find
that the cosmological constraints on the model parameters are fairly tight. In
particular, the key parameter can be constrained to a narrow range.Comment: 15 pages, 1 table, 8 figures, revtex4; v2: published versio
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