709 research outputs found
Scaling attractors for quintessence in flat universe with cosmological term
For evolution of flat universe, we classify late time and future attractors
with scaling behavior of scalar field quintessence in the case of potential,
which, at definite values of its parameters and initial data, corresponds to
exact scaling in the presence of cosmological constant.Comment: 11 pages, 16 eps-figures, revtex4, reference with comment adde
Sterile neutrinos: direct mixing effects versus induced mass matrix of active neutrinos
Mixing of active neutrinos with sterile ones generate ``induced''
contributions to the mass matrix of active neutrinos , where is the Majorana mass of the sterile neutrino
and is the active-sterile mixing angle. We study possible effects
of the induced matrix which can modify substantially the implications of
neutrino oscillation results. We have identified the regions of and
where the induced matrix (i) provides the dominant
structures, (ii) gives the sub-dominant effects and (iii) where its effects can
be neglected. The induced matrix can be responsible for peculiar properties of
the lepton mixing and neutrino mass spectrum, in particular, it can generate
the tri-bimaximal mixing. We update and discuss bounds on the induced masses
from laboratory measurements, astrophysics and cosmology. We find that
substantial impact of the induced matrix is possible if eV and
or MeV and
. The bounds can be relaxed in cosmological
scenarios with low reheating temperature, if sterile neutrinos decay
sufficiently fast, or their masses change with time.Comment: Figures updated, version to be published in Phys. Rev.
Can hyperbolic phase of Brans-Dicke field account for Dark Matter?
We show that the introduction of a hyperbolic phase for Brans-Dicke (BD)
field results in a flat vacuum cosmological solution of Hubble parameter H and
fractional rate of change of BD scalar field, F which asymptotically approach
constant values. At late stages, hyperbolic phase of BD field behaves like dark
matter
Cosmological constraints on unparticle dark matter
In unparticle dark matter (unmatter) models the equation of state of the
unmatter is given by , where is the scaling factor.
Unmatter with such equations of state would have a significant impact on the
expansion history of the universe. Using type Ia supernovae (SNIa), the baryon
acoustic oscillation (BAO) measurements and the shift parameter of the cosmic
microwave background (CMB) to place constraints on such unmatter models we find
that if only the SNIa data is used the constraints are weak. However, with the
BAO and CMB shift parameter data added strong constraints can be obtained. For
the UDM model, in which unmatter is the sole dark matter, we find that
at 95% C.L. For comparison, in most unparticle physics models it is
assumed . For the CUDM model, in which unmatter co-exists with
cold dark matter, we found that the unmatter can at most make up a few percent
of the total cosmic density if , thus it can not be the major component
of dark matter.Comment: Replaced with revised version. BAO data is added to make a tighter
constraint. Version accepted for publication on Euro.Phys.J.
SiFTO: An Empirical Method for Fitting SNe Ia Light Curves
We present SiFTO, a new empirical method for modeling type Ia supernovae (SNe
Ia) light curves by manipulating a spectral template. We make use of
high-redshift SN observations when training the model, allowing us to extend it
bluer than rest frame U. This increases the utility of our high-redshift SN
observations by allowing us to use more of the available data. We find that
when the shape of the light curve is described using a stretch prescription,
applying the same stretch at all wavelengths is not an adequate description.
SiFTO therefore uses a generalization of stretch which applies different
stretch factors as a function of both the wavelength of the observed filter and
the stretch in the rest-frame B band. We compare SiFTO to other published
light-curve models by applying them to the same set of SN photometry, and
demonstrate that SiFTO and SALT2 perform better than the alternatives when
judged by the scatter around the best fit luminosity distance relationship. We
further demonstrate that when SiFTO and SALT2 are trained on the same data set
the cosmological results agree.Comment: Modified to better match published version in Ap
Dark energy constraints and correlations with systematics from CFHTLS weak lensing, SNLS supernovae Ia and WMAP5
We combine measurements of weak gravitational lensing from the CFHTLS-Wide
survey, supernovae Ia from CFHT SNLS and CMB anisotropies from WMAP5 to obtain
joint constraints on cosmological parameters, in particular, the dark energy
equation of state parameter w. We assess the influence of systematics in the
data on the results and look for possible correlations with cosmological
parameters.
We implement an MCMC algorithm to sample the parameter space of a flat CDM
model with a dark-energy component of constant w. Systematics in the data are
parametrised and included in the analysis. We determine the influence of
photometric calibration of SNIa data on cosmological results by calculating the
response of the distance modulus to photometric zero-point variations. The weak
lensing data set is tested for anomalous field-to-field variations and a
systematic shape measurement bias for high-z galaxies.
Ignoring photometric uncertainties for SNLS biases cosmological parameters by
at most 20% of the statistical errors, using supernovae only; the parameter
uncertainties are underestimated by 10%. The weak lensing field-to-field
variance pointings is 5%-15% higher than that predicted from N-body
simulations. We find no bias of the lensing signal at high redshift, within the
framework of a simple model. Assuming a systematic underestimation of the
lensing signal at high redshift, the normalisation sigma_8 increases by up to
8%. Combining all three probes we obtain -0.10<1+w<0.06 at 68% confidence
(-0.18<1+w<0.12 at 95%), including systematic errors. Systematics in the data
increase the error bars by up to 35%; the best-fit values change by less than
0.15sigma. [Abridged]Comment: 14 pages, 10 figures. Revised version, matches the one to be
published in A&A. Modifications have been made corresponding to the referee's
suggestions, including reordering of some section
Probing the Planck Scale with Neutrino Oscillations
Quantum gravity "foam", among its various generic Lorentz non-invariant
effects, would cause neutrino mixing. It is shown here that, if the foam is
manifested as a nonrenormalizable effect at scale M, the oscillation length
generically decreases with energy as (E/M)^(-2). Neutrino observatories and
long-baseline experiments should have therefore already observed foam-induced
oscillations, even if M is as high as the Planck energy scale. The null
results, which can be further strengthened by better analysis of current data
and future experiments, can be taken as experimental evidence that Lorentz
invariance is fully preserved at the Planck scale, as is the case in critical
string theory.Comment: 11 pages, 2 figures. Final version published in PRD. 1 figure,
references, clarifications and explanations added. Results unchange
A combined analysis of short-baseline neutrino experiments in the (3+1) and (3+2) sterile neutrino oscillation hypotheses
We investigate adding two sterile neutrinos to resolve the apparent tension
existing between short-baseline neutrino oscillation results and
CPT-conserving, four-neutrino oscillation models. For both (3+1) and (3+2)
models, the level of statistical compatibility between the combined dataset
from the null short-baseline experiments Bugey, CHOOZ, CCFR84, CDHS, KARMEN,
and NOMAD, on the one hand; and the LSND dataset, on the other, is computed. A
combined analysis of all seven short-baseline experiments, including LSND, is
also performed, to obtain the favored regions in neutrino mass and mixing
parameter space for both models. Finally, four statistical tests to compare the
(3+1) and the (3+2) hypotheses are discussed. All tests show that (3+2) models
fit the existing short-baseline data significantly better than (3+1) models.Comment: 16 pages, 15 figures. Added NOMAD data to the analysis, one
statistical test, and two figures. References and text added. Version
submitted to PR
Constraints on Cosmological Models and Reconstructing the Acceleration History of the Universe with Gamma-Ray Burst Distance Indicators
Gamma-ray bursts (GRBs) have been regarded as standard candles at very high
redshift for cosmology research. We have proposed a new method to calibrate GRB
distance indicators with Type Ia supernova (SNe Ia) data in a completely
cosmology-independent way to avoid the circularity problem that had limited the
direct use of GRBs to probe cosmology [N. Liang, W. K. Xiao, Y. Liu, and S. N.
Zhang, Astrophys. J. 685, 354 (2008).]. In this paper, a simple method is
provided to combine GRB data into the joint observational data analysis to
constrain cosmological models; in this method those SNe Ia data points used for
calibrating the GRB data are not used to avoid any correlation between them. We
find that the CDM model is consistent with the joint data in the
1- confidence region, using the GRB data at high redshift calibrated
with the interpolating method, the Constitution set of SNe Ia, the cosmic
microwave background radiation from Wilkinson Microwave Anisotropy Probe five
year observation, the baryonic acoustic oscillation from the spectroscopic
Sloan Digital Sky Survey Data Release 7 galaxy sample, the x-ray baryon mass
fraction in clusters of galaxies, and the observational Hubble parameter versus
redshift data. Comparing to the joint constraints with GRBs and without GRBs,
we find that the contribution of GRBs to the joint cosmological constraints is
a slight shift in the confidence regions of cosmological parameters to better
enclose the CDM model. Finally, we reconstruct the acceleration
history of the Universe up to with the distance moduli of SNe Ia and GRBs
and find some features that deviate from the CDM model and seem to
favor oscillatory cosmology models; however further investigations are needed
to better understand the situation.Comment: 14 pages, 9 figures, 2 tables; v3: the revised version, fig. 6 and
some discussions added, accepted for for publication in Phys. Rev. D; v4: the
published version (Phys. Rev. D 81, 083518, 2010
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