33,136 research outputs found
Kinematic Constraints to the Transition Redshift from SNe Ia Union Data
The kinematic approach to cosmological tests provides a direct evidence to
the present accelerating stage of the universe which does not depend on the
validity of general relativity, as well as on the matter-energy content of the
Universe. In this context, we consider here a linear two-parameter expansion
for the decelerating parameter, , where and are
arbitrary constants to be constrained by the Union supernovae data. By assuming
a flat Universe we find that the best fit to the pair of free parameters is
() = ( whereas the transition redshift is () (). This
kinematic result is in agreement with some independent analyzes and
accommodates more easily many dynamical flat models (like CDM).Comment: 10 pages, 4 figures, 1 tabl
Constraints on Cold Dark Matter Accelerating Cosmologies and Cluster Formation
We discuss the properties of homogeneous and isotropic flat cosmologies in
which the present accelerating stage is powered only by the gravitationally
induced creation of cold dark matter (CCDM) particles (). For
some matter creation rates proposed in the literature, we show that the main
cosmological functions such as the scale factor of the universe, the Hubble
expansion rate, the growth factor and the cluster formation rate are
analytically defined. The best CCDM scenario has only one free parameter and
our joint analysis involving BAO + CMB + SNe Ia data yields
() where
is the observed matter density parameter. In particular, this implies that the
model has no dark energy but the part of the matter that is effectively
clustering is in good agreement with the latest determinations from large scale
structure. The growth of perturbation and the formation of galaxy clusters in
such scenarios are also investigated. Despite the fact that both scenarios may
share the same Hubble expansion, we find that matter creation cosmologies
predict stronger small scale dynamics which implies a faster growth rate of
perturbations with respect to the usual CDM cosmology. Such results
point to the possibility of a crucial observational test confronting CCDM with
CDM scenarios trough a more detailed analysis involving CMB, weak
lensing, as well as the large scale structure.Comment: 12 pages, 3 figures, Accepted for publication by Physical Rev.
Deflationary cosmology: constraints from angular size and ages of globular clusters
Observational constraints to a large class of decaying vacuum cosmologies are
derived using the angular size data of compact radio sources and the latest age
estimates of globular clusters. For this class of deflationary
models, the present value of the vacuum energy density is quantified by a
positive parameter smaller than unity. In the case of milliarcsecond
compact radio-sources, we find that the allowed intervals for and the
matter density parameter are heavily dependent on the value of the
mean projected linear size . For pc, the best
fit occurs for , , and , , respectively. This analysis shows that if
one minimizes for the free parameters , and
, the best fit for these angular size data corresponds to a decaying
with and
pc. Constraints from age estimates of globular clusters and old high redshift
galaxies are not so restrictive, thereby suggesting that there is no age crisis
for this kind of cosmologies.Comment: 6 pages, 3 figures, revised version to appear in Phys. Rev.
- …