7 research outputs found
High-redshift objects and the generalized Chaplygin gas
Motivated by recent developments in particle physics and cosmology, there has
been growing interest in an unified description of dark matter and dark energy
scenarios. In this paper we explore observational constraints from age
estimates of high- objects on cosmological models dominated by an exotic
fluid with equation of state (the so-called generalized
Chaplygin gas) which has the interesting feature of interpolating between
non-relativistic matter and negative-pressure dark energy regimes. As a general
result we find that, if the age estimates of these objects are correct, they
impose very restrictive limits on some of these scenarios.Comment: 5 pages, 3 figures, to appear in Phys. Rev.
Brane World Cosmologies and Statistical Properties of Gravitational Lenses
Brane world cosmologies seem to provide an alternative explanation for the
present accelerated stage of the Universe with no need to invoke either a
cosmological constant or an exotic \emph{quintessence} component. In this paper
we investigate statistical properties of gravitational lenses for some
particular scenarios based on this large scale modification of gravity. We show
that a large class of such models are compatible with the current lensing data
for values of the matter density parameter
(). If one fixes to be , as suggested by
most of the dynamical estimates of the quantity of matter in the Universe, the
predicted number of lensed quasars requires a slightly open universe with a
crossover distance between the 4 and 5-dimensional gravities of the order of
.Comment: 6 pages, 3 figures, revte
Constraining the dark energy with galaxy clusters X-ray data
The equation of state characterizing the dark energy component is constrained
by combining Chandra observations of the X-ray luminosity of galaxy clusters
with independent measurements of the baryonic matter density and the latest
measurements of the Hubble parameter as given by the HST key project. By
assuming a spatially flat scenario driven by a "quintessence" component with an
equation of state we place the following limits on the
cosmological parameters and : (i) and (1) if the
equation of state of the dark energy is restricted to the interval (\emph{usual} quintessence) and (ii) and
() if violates the null energy condition and assume values (\emph{extended} quintessence or ``phantom'' energy). These results are in
good agreement with independent studies based on supernovae observations,
large-scale structure and the anisotropies of the cosmic background radiation.Comment: 6 pages, 4 figures, LaTe
Cosmological consequences of a Chaplygin gas dark energy
A combination of recent observational results has given rise to what is
currently known as the dark energy problem. Although several possible
candidates have been extensively discussed in the literature to date the nature
of this dark energy component is not well understood at present. In this paper
we investigate some cosmological implications of another dark energy candidate:
an exotic fluid known as the Chaplygin gas, which is characterized by an
equation of state , where is a positive constant. By assuming
a flat scenario driven by non-relativistic matter plus a Chaplygin gas dark
energy we study the influence of such a component on the statistical properties
of gravitational lenses. A comparison between the predicted age of the universe
and the latest age estimates of globular clusters is also included and the
results briefly discussed. In general, we find that the behavior of this class
of models may be interpreted as an intermediary case between the standard and
CDM scenarios.Comment: 7 pages, 5 figures, to appear in Phys. Rev.
Testing dark energy beyond the cosmological constant barrier
Although well motivated from theoretical arguments, the cosmological constant
\emph{barrier}, i.e., the imposition that the equation-of-state parameter of
dark energy () is , seems to introduce
bias in the parameter determination from statistical analyses of observational
data. In this regard, \emph{phantom} dark energy or \emph{superquintessence}
has been proposed in which the usual imposition is relaxed.
Here, we study possible observational limits to the \emph{phantom} behavior of
the dark energy from recent distance estimates of galaxy clusters obtained from
interferometric measurements of the Sunyaev-Zel'dovich effect/X-ray
observations, Type Ia supernova data and CMB measurements. We find that there
is much \emph{observationally} acceptable parameter space beyond the
\emph{barrier}, thus opening the possibility of existence of more exotic forms
of energy in the Universe.Comment: 5 pages, 5 figures, to appear in Phys. Rev.
Some Observational Consequences of Brane World Cosmologies
The presence of dark energy in the Universe is inferred directly and
indirectly from a large body of observational evidence. The simplest and most
theoretically appealing possibility is the vacuum energy density (cosmological
constant). However, although in agreement with current observations, such a
possibility exacerbates the well known cosmological constant problem, requiring
a natural explanation for its small, but nonzero, value. In this paper we focus
our attention on another dark energy candidate, one arising from gravitational
\emph{leakage} into extra dimensions. We investigate observational constraints
from current measurements of angular size of high- compact radio-sources on
accelerated models based on this large scale modification of gravity. The
predicted age of the Universe in the context of these models is briefly
discussed. We argue that future observations will enable a more accurate test
of these cosmologies and, possibly, show that such models constitute a viable
possibility for the dark energy problem.Comment: 6 pages, 4 figures, to appear in Phys. Rev. D (minor revisions