5,057 research outputs found
Reconstructing the Equation of State for Dark Energy In the Double Complex Symmetric Gravitational Theory
We propose to study the accelerating expansion of the universe in the double
complex symmetric gravitational theory (DCSGT). The universe we live in is
taken as the real part of the whole spacetime which is double
complex. By introducing the spatially flat FRW metric, not only the double
Friedmann Equations but also the two constraint conditions and
are obtained. Furthermore, using parametric ansatz, we reconstruct the
and for dark energy from real observational data. We
find that in the two cases of and , the
corresponding equations of state remain close to -1 at present
() and change from below -1 to above -1. The results illustrate that the
whole spacetime, i.e. the double complex spacetime , may be
either ordinary complex () or hyperbolic complex
(). And the fate of the universe would be Big Rip in the
future.Comment: 5 pages, 5 figures, accepted by Commun. Theor. Phy
Type Ia Supernova Light Curve Inference: Hierarchical Bayesian Analysis in the Near Infrared
We present a comprehensive statistical analysis of the properties of Type Ia
SN light curves in the near infrared using recent data from PAIRITEL and the
literature. We construct a hierarchical Bayesian framework, incorporating
several uncertainties including photometric error, peculiar velocities, dust
extinction and intrinsic variations, for coherent statistical inference. SN Ia
light curve inferences are drawn from the global posterior probability of
parameters describing both individual supernovae and the population conditioned
on the entire SN Ia NIR dataset. The logical structure of the hierarchical
model is represented by a directed acyclic graph. Fully Bayesian analysis of
the model and data is enabled by an efficient MCMC algorithm exploiting the
conditional structure using Gibbs sampling. We apply this framework to the
JHK_s SN Ia light curve data. A new light curve model captures the observed
J-band light curve shape variations. The intrinsic variances in peak absolute
magnitudes are: sigma(M_J) = 0.17 +/- 0.03, sigma(M_H) = 0.11 +/- 0.03, and
sigma(M_Ks) = 0.19 +/- 0.04. We describe the first quantitative evidence for
correlations between the NIR absolute magnitudes and J-band light curve shapes,
and demonstrate their utility for distance estimation. The average residual in
the Hubble diagram for the training set SN at cz > 2000 km/s is 0.10 mag. The
new application of bootstrap cross-validation to SN Ia light curve inference
tests the sensitivity of the model fit to the finite sample and estimates the
prediction error at 0.15 mag. These results demonstrate that SN Ia NIR light
curves are as effective as optical light curves, and, because they are less
vulnerable to dust absorption, they have great potential as precise and
accurate cosmological distance indicators.Comment: 24 pages, 15 figures, 4 tables. Accepted for publication in ApJ.
Corrected typo, added references, minor edit
Dark Energy Accretion onto a Black Hole in an Expanding Universe
By using the solution describing a black hole embedded in the FLRW universe,
we obtain the evolving equation of the black hole mass expressed in terms of
the cosmological parameters. The evolving equation indicates that in the
phantom dark energy universe the black hole mass becomes zero before the Big
Rip is reached.Comment: 7 pages, no figures, errors is correcte
SALT: a Spectral Adaptive Light curve Template for Type Ia Supernovae
We present a new method to parameterize Type Ia Supernovae (SN Ia)
multi-color light curves. The method was developed in order to analyze the
large number of SN Ia multi-color light curves measured in current
high-redshift projects. The technique is based on empirically modeling SN Ia
luminosity variations as a function of phase, wavelength, a shape parameter,
and a color parameter. The model is trained with a sample of well measured
nearby SN Ia and then tested with an independent set of supernovae by building
an optimal luminosity distance estimator combining the supernova rest-frame
luminosity, shape parameter and color reconstructed with the model. The
distances we measure using B- and V-band data show a dispersion around the
Hubble line comparable or lower than obtained with other methods. With this
model, we are able to measure distances using U- and B-band data with a
dispersion around the Hubble line of 0.16 +- 0.05.Comment: Accepted in A&A, June 23, 2005 (printer friendly replacement version,
includes language corrections
Naked Singularity in a Modified Gravity Theory
The cosmological constant induced by quantum fluctuation of the graviton on a
given background is considered as a tool for building a spectrum of different
geometries. In particular, we apply the method to the Schwarzschild background
with positive and negative mass parameter. In this way, we put on the same
level of comparison the related naked singularity (-M) and the positive mass
wormhole. We discuss how to extract information in the context of a f(R)
theory. We use the Wheeler-De Witt equation as a basic equation to perform such
an analysis regarded as a Sturm-Liouville problem . The application of the same
procedure used for the ordinary theory, namely f(R)=R, reveals that to this
approximation level, it is not possible to classify the Schwarzschild and its
naked partner into a geometry spectrum.Comment: 8 Pages. Contribution given to DICE 2008. To appear in the
proceeding
Cosmological parameter extraction and biases from type Ia supernova magnitude evolution
We study different one-parametric models of type Ia Supernova magnitude
evolution on cosmic time scales. Constraints on cosmological and Supernova
evolution parameters are obtained by combined fits on the actual data coming
from Supernovae, the cosmic microwave background, and baryonic acoustic
oscillations. We find that data prefer a magnitude evolution such that
high-redshift Supernova are brighter than would be expected in a standard
cosmos with a dark energy component. Data however are consistent with
non-evolving magnitudes at the one-sigma level, except special cases.
We simulate a future data scenario where SN magnitude evolution is allowed
for, and neglect the possibility of such an evolution in the fit. We find the
fiducial models for which the wrong model assumption of non-evolving SN
magnitude is not detectable, and for which at the same time biases on the
fitted cosmological parameters are introduced. Of the cosmological parameters
the overall mass density has the strongest chances to be biased due to the
wrong model assumption. Whereas early-epoch models with a magnitude offset ~z^2
show up to be not too dangerous when neglected in the fitting procedure, late
epoch models with magnitude offset ~sqrt(z) have high chances to bias the fit
results.Comment: 12 pages, 5 figures, 3 tables. Accepted for publication by A&A.
Revised version: Corrected Typos, reference added to section
Spherically symmetric vacuum solutions of modified gravity theory in higher dimensions
In this paper we investigate spherically symmetric vacuum solutions of
gravity in a higher dimensional spacetime. With this objective we construct a
system of non-linear differential equations, whose solutions depend on the
explicit form assumed for the function . We explicit
show that for specific classes of this function exact solutions from the field
equations are obtained; also we find approximated results for the metric tensor
for more general cases admitting close to the unity.Comment: 14 pages, no figure. New version accepted for publication in EPJ
Geometry and Destiny
The recognition that the cosmological constant may be non-zero forces us to
re-evaluate standard notions about the connection between geometry and the fate
of our Universe. An open Universe can recollapse, and a closed Universe can
expand forever. As a corollary, we point out that there is no set of
cosmological observations we can perform that will unambiguously allow us to
determine what the ultimate destiny of the Universe will be.Comment: 7 pages, Gravity Research Foundation Essa
Crossing of the Phantom Divided Barrier with Lorentz Invariance Violating Fields
We study possible crossing of the phantom divided barrier in a Lorentz
invariance violating dark energy model. Lorentz invariance violation which is
achieved by introducing a vector field in the action, incorporates directly in
the dynamics of the scalar field and equation of state. This interesting
feature allows us to study phantom divided barrier crossing in the context of
Lorentz invariance violation. We show that for suitable choice of parameter
space, equation of state can cross phantom divided barrier just by one scalar
field and Lorentz violating vector field controls this crossing.Comment: 14 pages, 4 figures, Revised and References added, Accepted for
Publication in Europhysics Letter
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