4,270 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
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
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
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
Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae
The property of dark energy and the physical reason for acceleration of the
present universe are two of the most difficult problems in modern cosmology.
The dark energy contributes about two-thirds of the critical density of the
present universe from the observations of type-Ia supernova (SNe Ia) and
anisotropy of cosmic microwave background (CMB).The SN Ia observations also
suggest that the universe expanded from a deceleration to an acceleration phase
at some redshift, implying the existence of a nearly uniform component of dark
energy with negative pressure. We use the ``gold'' sample containing 157 SNe Ia
and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the
properties of dark energy and the transition redshift. For a flat universe with
the cosmological constant, we measure , which
is consistent with Riess et al. The transition redshift is
. We also discuss several dark energy models that
define the of the parameterized equation of state of dark energy
including one parameter and two parameters ( being the ratio of the
pressure to energy density). Our calculations show that the accurately
calculated transition redshift varies from to
across these models. We also calculate the minimum
redshift at which the current observations need the universe to
accelerate.Comment: 16 pages, 5 figures, 1 tabl
Carmeli's accelerating universe is spatially flat without dark matter
Carmeli's 5D brane cosmology has been applied to the expanding accelerating
universe and it has been found that the distance redshift relation will fit the
data of the high-z supernova teams without the need for dark matter. Also the
vacuum energy contribution to gravity indicates that the universe is
asymptotically expanding towards a spatially flat state, where the total
mass/energy density tends to unity.Comment: 4 pages, 5 figures, accepted for publication in Int. J. Theor.
Physics, this paper is based on an invited talk at FFP6, Udine, Italy, Sept
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