122 research outputs found
Constraining slow-roll inflation with WMAP and 2dF
We constrain slow-roll inflationary models using the recent WMAP data
combined with data from the VSA, CBI, ACBAR and 2dF experiments. We find the
slow-roll parameters to be and . For inflation models
we find that at the 2 and levels,
indicating that the model is under very strong pressure from
observations. We define a convergence criterion to judge the necessity of
introducing further power spectrum parameters such as the spectral index and
running of the spectral index. This criterion is typically violated by models
with large negative running that fit the data, indicating that the running
cannot be reliably measured with present data.Comment: 8 pages RevTeX4 file with six figures incorporate
Revisiting Cardassian Model and Cosmic Constraint
In this paper, we revisit the Cardassian model in which the radiation energy
component is included. It is important for early epoch when the radiation
cannot be neglected because the equation of state (EoS) of the effective dark
energy becomes time variable. Therefore, it is not equivalent to the
quintessence model with a constant EoS anymore. This situation was almost
overlooked in the literature. By using the recent released Union2 557 of type
Ia supernovae (SN Ia), the baryon acoustic oscillation (BAO) from Sloan Digital
Sky Survey and the WiggleZ data points, the full information of cosmic
microwave background (CMB) measurement given by the seven-year Wilkinson
Microwave Anisotropy Probe observation, we constrain the Cardassian model via
the Markov Chain Monte Carlo (MCMC) method. A tight constraint is obtained: in regions. The
deviation of Cardassian model from quintessence model is shown in CMB
anisotropic power spectra at high l's parts due to the evolution of EoS. But it
is about the order of 0.1% which cannot be discriminated by current data sets.
The Cardassian model is consistent with current cosmic observational data sets.Comment: 6 pages, 5 figures, match the published versio
WMAP constraints on inflationary models with global defects
We use the cosmic microwave background angular power spectra to place upper
limits on the degree to which global defects may have aided cosmic structure
formation. We explore this under the inflationary paradigm, but with the
addition of textures resulting from the breaking of a global O(4) symmetry
during the early stages of the Universe. As a measure of their contribution, we
use the fraction of the temperature power spectrum that is attributed to the
defects at a multipole of 10. However, we find a parameter degeneracy enabling
a fit to the first-year WMAP data to be made even with a significant defect
fraction. This degeneracy involves the baryon fraction and the Hubble constant,
plus the normalization and tilt of the primordial power spectrum. Hence,
constraints on these cosmological parameters are weakened. Combining the WMAP
data with a constraint on the physical baryon fraction from big bang
nucleosynthesis calculations and high-redshift deuterium abundance, limits the
extent of the degeneracy and gives an upper bound on the defect fraction of
0.13 (95% confidence).Comment: 10pp LaTeX/RevTeX, 6 eps figs; matches accepted versio
Parametrization of Born-Infeld Type Phantom Dark Energy Model
Applying the parametrization of dark energy density, we can construct
directly independent-model potentials. In Born-Infeld type phantom dark energy
model, we consider four special parametrization equation of state parameter.
The evolutive behavior of dark energy density with respect to red-shift ,
potentials with respect to and are shown mathematically. Moreover,
we investigate the effect of parameter upon the evolution of the
constructed potential with respect to . These results show that the
evolutive behavior of constructed Born-Infeld type dark energy model is quite
different from those of the other models.Comment: 5 pages, 4 figures, Accepted for publication in Astrophysics & Space
Scienc
Non-vacuum Solutions of Bianchi Type VI_0 Universe in f(R) Gravity
In this paper, we solve the field equations in metric f(R) gravity for
Bianchi type VI_0 spacetime and discuss evolution of the expanding universe. We
find two types of non-vacuum solutions by taking isotropic and anisotropic
fluids as the source of matter and dark energy. The physical behavior of these
solutions is analyzed and compared in the future evolution with the help of
some physical and geometrical parameters. It is concluded that in the presence
of isotropic fluid, the model has singularity at and represents
continuously expanding shearing universe currently entering into phantom phase.
In anisotropic fluid, the model has no initial singularity and exhibits the
uniform accelerating expansion. However, the spacetime does not achieve
isotropy as in both of these solutions.Comment: 20 pages, 5 figures, accepted for publication in Astrophys. Space Sc
The -essence scalar field in the context of Supernova Ia Observations
A -essence scalar field model having (non canonical) Lagrangian of the
form where
with constant is shown to be consistent with luminosity
distance-redshift data observed for type Ia Supernova. For constant ,
satisfies a scaling relation which is used to set up a differential
equation involving the Hubble parameter , the scale factor and the
-essence field . and are extracted from SNe Ia data and using
the differential equation the time dependence of the field is found to
be: . The constants
have been determined. The time dependence is similar to that of the
quintessence scalar field (having canonical kinetic energy) responsible for
homogeneous inflation. Furthermore, the scaling relation and the obtained time
dependence of the field is used to determine the -dependence of the
function .Comment: 8 pages, 5 figures, Late
Revisiting Generalized Chaplygin Gas as a Unified Dark Matter and Dark Energy Model
In this paper, we revisit generalized Chaplygin gas (GCG) model as a unified
dark matter and dark energy model. The energy density of GCG model is given as
,
where and are two model parameters which will be constrained by
type Ia supernova as standard candles, baryon acoustic oscillation as standard
rulers and the seventh year full WMAP data points. In this paper, we will not
separate GCG into dark matter and dark energy parts any more as adopted in the
literatures. By using Markov Chain Monte Carlo method, we find the result:
and .Comment: 6 pages, 4 figure
Collinear helium under periodic driving: stabilization of the asymmetric stretch orbit
The collinear eZe configuration of helium, with the electrons on opposite
sides of the nucleus, is studied in the presence of an external electromagnetic
(laser or microwave) field. We show that the classically unstable "asymmetric
stretch" orbit, on which doubly excited intrashell states of helium with
maximum interelectronic angle are anchored, can be stabilized by means of a
resonant driving where the frequency of the electromagnetic field equals the
frequency of Kepler-like oscillations along the orbit. A static magnetic field,
oriented parallel to the oscillating electric field of the driving, can be used
to enforce the stability of the configuration with respect to deviations from
collinearity. Quantum Floquet calculations within a collinear model of the
driven two-electron atom reveal the existence of nondispersive wave packets
localized on the stabilized asymmetric stretch orbit, for double excitations
corresponding to principal quantum numbers of the order of N > 10.Comment: 13 pages, 12 figure
Comparison of Recent SnIa datasets
We rank the six latest Type Ia supernova (SnIa) datasets (Constitution (C),
Union (U), ESSENCE (Davis) (E), Gold06 (G), SNLS 1yr (S) and SDSS-II (D)) in
the context of the Chevalier-Polarski-Linder (CPL) parametrization
, according to their Figure of Merit (FoM), their
consistency with the cosmological constant (CDM), their consistency
with standard rulers (Cosmic Microwave Background (CMB) and Baryon Acoustic
Oscillations (BAO)) and their mutual consistency. We find a significant
improvement of the FoM (defined as the inverse area of the 95.4% parameter
contour) with the number of SnIa of these datasets ((C) highest FoM, (U), (G),
(D), (E), (S) lowest FoM). Standard rulers (CMB+BAO) have a better FoM by about
a factor of 3, compared to the highest FoM SnIa dataset (C). We also find that
the ranking sequence based on consistency with CDM is identical with
the corresponding ranking based on consistency with standard rulers ((S) most
consistent, (D), (C), (E), (U), (G) least consistent). The ranking sequence of
the datasets however changes when we consider the consistency with an expansion
history corresponding to evolving dark energy crossing the
phantom divide line (it is practically reversed to (G), (U), (E), (S),
(D), (C)). The SALT2 and MLCS2k2 fitters are also compared and some peculiar
features of the SDSS-II dataset when standardized with the MLCS2k2 fitter are
pointed out. Finally, we construct a statistic to estimate the internal
consistency of a collection of SnIa datasets. We find that even though there is
good consistency among most samples taken from the above datasets, this
consistency decreases significantly when the Gold06 (G) dataset is included in
the sample.Comment: 13 pages, 9 figures. Included recently released SDSS-II dataset.
Improved presentation. Main results unchanged. The mathematica files and
datasets used for the production of the figures may be downloaded from
http://leandros.physics.uoi.gr/datacomp
Stability analysis of agegraphic dark energy in Brans-Dicke cosmology
Stability analysis of agegraphic dark energy in Brans-Dicke theory is
presented in this paper. We constrain the model parameters with the
observational data and thus the results become broadly consistent with those
expected from experiment. Stability analysis of the model without best fitting
shows that universe may begin from an unstable state passing a saddle point and
finally become stable in future. However, with the best fitted model, There is
no saddle intermediate state. The agegraphic dark energy in the model by itself
exhibits a phantom behavior. However, contribution of cold dark matter on the
effective energy density modifies the state of teh universe from phantom phase
to quintessence one. The statefinder diagnosis also indicates that the universe
leaves an unstable state in the past, passes the LCDM state and finally
approaches the sable state in future.Comment: 15 pages, 12 figure
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