127 research outputs found
Influence of synthesis conditions and processing on the cathode performance of 5V-spinel materials. Titanium doping of LiNi₀̣₅Mn₁̣₅O₄ spinel
Cosmological Models and Latest Observational Data
In this note, we consider the observational constraints on some cosmological
models by using the 307 Union type Ia supernovae (SNIa), the 32 calibrated
Gamma-Ray Bursts (GRBs) at , the updated shift parameter from WMAP
5-year data (WMAP5), and the distance parameter of the measurement of the
baryon acoustic oscillation (BAO) peak in the distribution of SDSS luminous red
galaxies with the updated scalar spectral index from WMAP5. The tighter
constraints obtained here update the ones obtained previously in the
literature.Comment: 10 pages, 5 figures, 1 table, revtex4; v2: discussions added,
accepted by Eur. Phys. J. C; v3: published versio
Constraints on early dark energy from CMB lensing and weak lensing tomography
Dark energy can be studied by its influence on the expansion of the Universe
as well as on the growth history of the large-scale structure. In this paper,
we follow the growth of the cosmic density field in early dark energy
cosmologies by combining observations of the primary CMB temperature and
polarisation power spectra at high redshift, of the CMB lensing deflection
field at intermediate redshift and of weak cosmic shear at low redshifts for
constraining the allowed amount of early dark energy. We present these
forecasts using the Fisher-matrix formalism and consider the combination of
Planck-data with the weak lensing survey of Euclid. We find that combining
these data sets gives powerful constraints on early dark energy and is able to
break degeneracies in the parameter set inherent to the various observational
channels. The derived statistical 1-sigma-bound on the early dark energy
density parameter is sigma(Omega_d^e)=0.0022 which suggests that early dark
energy models can be well examined in our approach. In addition, we derive the
dark energy figure of merit for the considered dark energy parameterisation and
comment on the applicability of the growth index to early dark energy
cosmologies.Comment: 25 pages, 14 figures, 3 tables; v2: very minor additions, updated to
match version to be published in JCA
Reliability of fluctuation-induced transport in a Maxwell-demon-type engine
We study the transport properties of an overdamped Brownian particle which is
simultaneously in contact with two thermal baths. The first bath is modeled by
an additive thermal noise at temperature . The second bath is associated
with a multiplicative thermal noise at temperature . The analytical
expressions for the particle velocity and diffusion constant are derived for
this system, and the reliability or coherence of transport is analyzed by means
of their ratio in terms of a dimensionless P\'{e}clet number. We find that the
transport is not very coherent, though one can get significantly higher
currents.Comment: 14 pages, 5 figure
The Crossing Statistic: Dealing with Unknown Errors in the Dispersion of Type Ia Supernovae
We propose a new statistic that has been designed to be used in situations
where the intrinsic dispersion of a data set is not well known: The Crossing
Statistic. This statistic is in general less sensitive than `chi^2' to the
intrinsic dispersion of the data, and hence allows us to make progress in
distinguishing between different models using goodness of fit to the data even
when the errors involved are poorly understood. The proposed statistic makes
use of the shape and trends of a model's predictions in a quantifiable manner.
It is applicable to a variety of circumstances, although we consider it to be
especially well suited to the task of distinguishing between different
cosmological models using type Ia supernovae. We show that this statistic can
easily distinguish between different models in cases where the `chi^2'
statistic fails. We also show that the last mode of the Crossing Statistic is
identical to `chi^2', so that it can be considered as a generalization of
`chi^2'.Comment: 14 pages, 5 figures. Paper restructured and extended and new
interpretation of the method presented. New results concerning model
selection. Treatment and error-analysis made fully model independent.
References added. Accepted for publication in JCA
QCD ghost f(T)-gravity model
Within the framework of modified teleparallel gravity, we reconstruct a f(T)
model corresponding to the QCD ghost dark energy scenario. For a spatially flat
FRW universe containing only the pressureless matter, we obtain the time
evolution of the torsion scalar T (or the Hubble parameter). Then, we calculate
the effective torsion equation of state parameter of the QCD ghost f(T)-gravity
model as well as the deceleration parameter of the universe. Furthermore, we
fit the model parameters by using the latest observational data including
SNeIa, CMB and BAO data. We also check the viability of our model using a
cosmographic analysis approach. Moreover, we investigate the validity of the
generalized second law (GSL) of gravitational thermodynamics for our model.
Finally, we point out the growth rate of matter density perturbation. We
conclude that in QCD ghost f(T)-gravity model, the universe begins a matter
dominated phase and approaches a de Sitter regime at late times, as expected.
Also this model is consistent with current data, passes the cosmographic test,
satisfies the GSL and fits the data of the growth factor well as the LCDM
model.Comment: 19 pages, 9 figures, 2 tables. arXiv admin note: substantial text
overlap with arXiv:1111.726
Observational constraint on generalized Chaplygin gas model
We investigate observational constraints on the generalized Chaplygin gas
(GCG) model as the unification of dark matter and dark energy from the latest
observational data: the Union SNe Ia data, the observational Hubble data, the
SDSS baryon acoustic peak and the five-year WMAP shift parameter. It is
obtained that the best fit values of the GCG model parameters with their
confidence level are ()
, ()
. Furthermore in this model, we can see that the
evolution of equation of state (EOS) for dark energy is similar to quiessence,
and its current best-fit value is with the confidence
level .Comment: 9 pages, 5 figure
Running coupling: Does the coupling between dark energy and dark matter change sign during the cosmological evolution?
In this paper we put forward a running coupling scenario for describing the
interaction between dark energy and dark matter. The dark sector interaction in
our scenario is free of the assumption that the interaction term is
proportional to the Hubble expansion rate and the energy densities of dark
sectors. We only use a time-variable coupling (with the scale factor
of the universe) to characterize the interaction . We propose a
parametrization form for the running coupling in which the
early-time coupling is given by a constant , while today the coupling is
given by another constant, . For investigating the feature of the running
coupling, we employ three dark energy models, namely, the cosmological constant
model (), the constant model (), and the time-dependent
model (). We constrain the models with the current
observational data, including the type Ia supernova, the baryon acoustic
oscillation, the cosmic microwave background, the Hubble expansion rate, and
the X-ray gas mass fraction data. The fitting results indicate that a
time-varying vacuum scenario is favored, in which the coupling crosses
the noninteracting line () during the cosmological evolution and the sign
changes from negative to positive. The crossing of the noninteracting line
happens at around , and the crossing behavior is favored at about
1 confidence level. Our work implies that we should pay more attention
to the time-varying vacuum model and seriously consider the phenomenological
construction of a sign-changeable or oscillatory interaction between dark
sectors.Comment: 8 pages, 5 figures; refs added; to appear in EPJ
Forecasting Cosmic Doomsday from CMB/LSS Cross-Correlations
A broad class of dark energy models, which have been proposed in attempts at
solving the cosmological constant problems, predict a late time variation of
the equation of state with redshift. The variation occurs as a scalar field
picks up speed on its way to negative values of the potential. The negative
potential energy eventually turns the expansion into contraction and the local
universe undergoes a big crunch. In this paper we show that cross-correlations
of the CMB anisotropy and matter distribution, in combination with other
cosmological data, can be used to forecast the imminence of such cosmic
doomsday.Comment: 12 pages, 12 figure
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