16,465 research outputs found
A Note on Gauss-Bonnet Holographic Superconductors
We present an analytic treatment near the phase transition for the critical
temperature of (3+1)-dimensional holographic superconductors in
Einstein-Gauss-Bonnet gravity with backreaction. We find that the backreaction
makes the critical temperature of the superconductor decrease and condensation
harder. This is consistent with previous numerical results.Comment: 6 pages, typos corrected, references added, published versio
Crossing w=-1 in Gauss-Bonnet Brane World with Induced Gravity
Recent type Ia supernovas data seemingly favor a dark energy model whose
equation of state crosses -1 very recently, which is a much more amazing
problem than the acceleration of the universe. In this paper we show that it is
possible to realize such a crossing without introducing any phantom component
in a Gauss-Bonnet brane world with induced gravity, where a four dimensional
curvature scalar on the brane and a five dimensional Gauss-Bonnet term in the
bulk are present. In this realization, the Gauss-Bonnet term and the mass
parameter in the bulk play a crucial role.Comment: Revtex 16 pages including 10 eps files, references added, to appear
in Comm. Theor. Phy
Crossing the phantom divide in an interacting generalized Chaplygin gas
Unified generalized Chaplygin gas models assuming an interaction between dark
energy and dark matter fluids have been previously proposed. Following these
ideas, we consider a particular relation between dark densities, which allows
the possibility of a time varying equation of state for dark energy that
crosses the phantom divide at a recent epoch. Moreover, these densities decay
during all the evolution of the Universe, avoiding a Big Rip. We find also a
scaling solution, i.e. these densities are asymptotically proportional in the
future, which contributes to the solution of the coincidence problem.Comment: Improved version, 10 pages, 4 figures, References adde
Comparison of dark energy models: A perspective from the latest observational data
In this paper, we compare some popular dark energy models under the
assumption of a flat universe by using the latest observational data including
the type Ia supernovae Constitution compilation, the baryon acoustic
oscillation measurement from the Sloan Digital Sky Survey, the cosmic microwave
background measurement given by the seven-year Wilkinson Microwave Anisotropy
Probe observations and the determination of from the Hubble Space
Telescope. Model comparison statistics such as the Bayesian and Akaike
information criteria are applied to assess the worth of the models. These
statistics favor models that give a good fit with fewer parameters. Based on
this analysis, we find that the simplest cosmological constant model that has
only one free parameter is still preferred by the current data. For other
dynamical dark energy models, we find that some of them, such as the
dark energy, constant , generalized Chaplygin gas,
Chevalliear-Polarski-Linder parametrization, and holographic dark energy
models, can provide good fits to the current data, and three of them, namely,
the Ricci dark energy, agegraphic dark energy, and Dvali-Gabadadze-Porrati
models, are clearly disfavored by the data.Comment: 19 pages, 10 figures; new data used, typos fixed; version for
publication in SCIENCE CHINA Physics, Mechanics & Astronom
Super-acceleration on the Brane by Energy Flow from the Bulk
We consider a brane cosmological model with energy exchange between brane and
bulk. Parameterizing the energy exchange term by the scale factor and Hubble
parameter, we are able to exactly solve the modified Friedmann equation on the
brane. In this model, the equation of state for the effective dark energy has a
transition behavior changing from to , while
the equation of state for the dark energy on the brane has . Fitting data
from type Ia supernova, Sloan Digital Sky Survey and Wilkinson Microwave
Anisotropy Probe, our universe is predicted now in the state of
super-acceleration with .Comment: Revtex, 11 pages including 2 figures,v2: tpos fixed, references
added, to appear in JCA
Natural Phantom Dark Energy, Wiggling Hubble Parameter and Direct Data
Recent direct data indicate that the parameter may wiggle with
respect to . On the other hand the luminosity distance data of supernovae
flatten the wiggles of because of integration effect. It is expected
that the fitting results can be very different in a model permitting a wiggling
because the data of supernovae is highly degenerated to such a model. As
an example the natural phantom dark energy is investigated in this paper. The
dynamical property of this model is studied. The model is fitted by the direct
data set and the SNLS data set, respectively. And the results are quite
different, as expected. The quantum stability of this model is also shortly
discussed. We find it is a viable model if we treat it as an effective theory
truncated by an upperbound.Comment: 14 pages, 2 figures, discussions on the stability added, conclusions
not change
Reconstructing quintom from WMAP 5-year observations: Generalized ghost condensate
In the 5-year WMAP data analysis, a new parametrization form for dark energy
equation-of-state was used, and it has been shown that the equation-of-state,
, crosses the cosmological-constant boundary . Based on this
observation, in this paper, we investigate the reconstruction of quintom dark
energy model. 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. Therefore, we reconstruct this
scalar-field quintom dark energy model from the WMAP 5-year observational
results. As a comparison, we also discuss the quintom reconstruction based on
other specific dark energy ansatzs, such as the CPL parametrization and the
holographic dark energy scenarios.Comment: 8 pages, 11 figure
Hessence: A New View of Quintom Dark Energy
Recently a lot of attention has been drawn to build dark energy model in
which the equation-of-state parameter can cross the phantom divide .
One of models to realize crossing the phantom divide is called quintom model,
in which two real scalar fields appears, one is a normal scalar field and the
other is a phantom-type scalar field. In this paper we propose a non-canonical
complex scalar field as the dark energy, which we dub ``hessence'', to
implement crossing the phantom divide, in a similar sense as the quintom dark
energy model. In the hessence model, the dark energy is described by a single
field with an internal degree of freedom rather than two independent real
scalar fields. However, the hessence is different from an ordinary complex
scalar field, we show that the hessence can avoid the difficulty of the Q-balls
formation which gives trouble to the spintessence model (An ordinary complex
scalar field acts as the dark energy). Furthermore, we find that, by choosing a
proper potential, the hessence could correspond to a Chaplygin gas at late
times.Comment: Latex2e, 12 pages, no figure; v2: discussions and references added,
14 pages, 3 eps figures; v3: published versio
Interacting model of new agegraphic dark energy: observational constraints and age problem
Many dark energy models fail to pass the cosmic age test because of the old
quasar APM 08279+5255 at redshift , the CDM model and
holographic dark energy models being no exception. In this paper, we focus on
the topic of age problem in the new agegraphic dark energy (NADE) model. We
determine the age of the universe in the NADE model by fitting the
observational data, including type Ia supernovae (SNIa), baryon acoustic
oscillations (BAO) and the cosmic microwave background (CMB). We find that the
NADE model also faces the challenge of the age problem caused by the old quasar
APM 08279+5255. In order to overcome such a difficulty, we consider the
possible interaction between dark energy and dark matter. We show that this
quasar can be successfully accommodated in the interacting new agegraphic dark
energy (INADE) model at the level under the current observational
constraints.Comment: 12 pages, 5 figures; typos corrected; version for publication in
SCIENCE CHINA Physics, Mechanics & Astronom
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