12,733 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
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
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
Reconstructing the properties of dark energy from recent observations
We explore the properties of dark energy from recent observational data,
including the Gold Sne Ia, the baryonic acoustic oscillation peak from SDSS,
the CMB shift parameter from WMAP3, the X-ray gas mass fraction in cluster and
the Hubble parameter versus redshift. The model with curvature
and two parameterized dark energy models are studied. For the
model, we find that the flat universe is consistent with observations at the
confidence level and a closed universe is slightly favored by these
data. For two parameterized dark energy models, with the prior given on the
present matter density, , with ,
and , our result seems to suggest that the
trend of dependence for an evolving dark energy from a
combination of the observational data sets is model-dependent.Comment: 16 pages, 15 figures, To appear in JCA
The generalized second law of thermodynamics in generalized gravity theories
We investigate the generalized second law of thermodynamics (GSL) in
generalized theories of gravity. We examine the total entropy evolution with
time including the horizon entropy, the non-equilibrium entropy production, and
the entropy of all matter, field and energy components. We derive a universal
condition to protect the generalized second law and study its validity in
different gravity theories. In Einstein gravity, (even in the phantom-dominated
universe with a Schwarzschild black hole), Lovelock gravity, and braneworld
gravity, we show that the condition to keep the GSL can always be satisfied. In
gravity and scalar-tensor gravity, the condition to protect the GSL can
also hold because the gravity is always attractive and the effective Newton
constant should be approximate constant satisfying the experimental bounds.Comment: 19 pages, no figure, mistakes corrected, references added, to appear
in Class. Quantum Gra
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
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
Is Noncommutative Eternal Inflation Possible?
We investigate the condition for eternal inflation to take place in the
noncommutative spacetime. We find that the possibility for eternal inflation's
happening is greatly suppressed in this case. If eternal inflation cannot
happen in the low energy region where the noncommutativity is very weak (the UV
region), it will never happen during the whole inflationary history. Based on
these conclusions, we argue that an initial condition for eternal inflation is
available from the property of spacetime noncommutativity.Comment: 14 pages, 2 figures, accepted by JCA
- …