610 research outputs found
Generalized Chaplygin Gas in a modified gravity approach
We study the generalized Chaplygin gas (GCG) scenario in a modified gravity
approach. That is, we impose that our universe has a pure dust configuration,
and allow for a modification of gravity that yields a GCG specific scale factor
evolution. Moreover, assuming that this new hypothetical gravity theory obeys a
generalization of Birkhoff's law, we determine the Schwarzschild-like metric in
this new modified gravity. We also study the large scale structure formation in
this model. Both the linear and non-linear growth are studied together with the
growth of the velocity fluctuation in the linear perturbation theory. We
compare our results with those corresponding to the CDM model and
discuss possible distinguishable features.Comment: 13 pages and 4 figures. Final version to appear in PR
Open inflationary universes in a brane world cosmology
In this paper, we study a type of one-field model for open inflationary
universe models in the context of the brane world models. In the scenario of a
one-bubble universe model, we determine and characterize the existence of the
Coleman-De Lucia instanton, together with the period of inflation after
tunneling has occurred. Our results are compared to those found in the Einstein
theory of Relativistic Models.Comment: 8 pages, 4 Figures, accepted in Physical Review
Qualitative analysis of a scalar-tensor theory with exponential potential
A qualitative analysis of a scalar-tensor cosmological model, with an
exponential potential for the scalar field, is performed. The phase diagram for
the flat case is constructed. It is shown that solutions with an initial and
final inflationary behaviour appear. The conditions for which the scenario
favored by supernova type Ia observations becomes an attractor in the space of
the solutions are established.Comment: Latex file, 9 pages, 1 figur
Effects of Foreground Contamination on the Cosmic Microwave Background Anisotropy Measured by MAP
We study the effects of diffuse Galactic, far-infrared extragalactic source,
and radio point source emission on the cosmic microwave background (CMB)
anisotropy data anticipated from the MAP experiment. We focus on the
correlation function and genus statistics measured from mock MAP
foreground-contaminated CMB anisotropy maps generated in a spatially-flat
cosmological constant dominated cosmological model. Analyses of the simulated
MAP data at 90 GHz (0.3 deg FWHM resolution smoothed) show that foreground
effects on the correlation function are small compared with cosmic variance.
However, the Galactic emission, even just from the region with |b| > 20 deg,
significantly affects the topology of CMB anisotropy, causing a negative genus
shift non-Gaussianity signal. Given the expected level of cosmic variance, this
effect can be effectively reduced by subtracting existing Galactic foreground
emission models from the observed data. IRAS and DIRBE far-infrared
extragalactic sources have little effect on the CMB anisotropy. Radio point
sources raise the amplitude of the correlation function considerably on scales
below 0.5 deg. Removal of bright radio sources above a 5 \sigma detection limit
effectively eliminates this effect. Radio sources also result in a positive
genus curve asymmetry (significant at 2 \sigma) on 0.5 deg scales. Accurate
radio point source data is essential for an unambiguous detection of CMB
anisotropy non-Gaussianity on these scales. Non-Gaussianity of cosmological
origin can be detected from the foreground-subtracted CMB anisotropy map at the
2 \sigma level if the measured genus shift parameter |\Delta\nu| >= 0.02 (0.04)
or if the measured genus asymmetry parameter |\Delta g| >= 0.03 (0.08) on a 0.3
(1.0) deg FWHM scale.Comment: 26 pages, 7 figures, Accepted for Publication in Astrophysical
Journal (Some sentences and figures modified
Is Cosmology Solved?
We have fossil evidence from the thermal background radiation that our
universe expanded from a considerably hotter denser state. We have a well
defined and testable description of the expansion, the relativistic
Friedmann-Lemaitre model. Its observational successes are impressive but I
think hardly enough for a convincing scientific case. The lists of
observational constraints and free hypotheses within the model have similar
lengths. The scorecard on the search for concordant measures of the mass
density parameter and the cosmological constant shows that the high density
Einstein-de Sitter model is challenged, but that we cannot choose between low
density models with and without a cosmological constant. That is, the
relativistic model is not strongly overconstrained, the usual test of a mature
theory. Work in progress will greatly improve the situation and may at last
yield a compelling test. If so, and the relativistic model survives, it will
close one line of research in cosmology: we will know the outlines of what
happened as our universe expanded and cooled from high density. It will not end
research: some of us will occupy ourselves with the details of how galaxies and
other large-scale structures came to be the way they are, others with the issue
of what our universe was doing before it was expanding. The former is being
driven by rapid observational advances. The latter is being driven mainly by
theory, but there are hints of observational guidance.Comment: 13 pages, 3 figures. To be published in PASP as part of the
proceedings of the Smithsonian debate, Is Cosmology Solved
Dynamics of a Generalized Cosmological Scalar-Tensor Theory
A generalized scalar-tensor theory is investigated whose cosmological term
depends on both a scalar field and its time derivative. A correspondence with
solutions of five-dimensional Space-Time-Matter theory is noted. Analytic
solutions are found for the scale factor, scalar field and cosmological term.
Models with free parameters of order unity are consistent with recent
observational data and could be relevant to both the dark-matter and
cosmological-"constant" problems.Comment: 13 page
Supernovae Ia Constraints on a Time-Variable Cosmological "Constant"
The energy density of a scalar field with potential , , behaves like a time-variable cosmological
constant that could contribute significantly to the present energy density.
Predictions of this spatially-flat model are compared to recent Type Ia
supernovae apparent magnitude versus redshift data. A large region of model
parameter space is consistent with current observations. (These constraints are
based on the exact scalar field model equations of motion, not on the widely
used time-independent equation of state fluid approximation equations of
motion.) We examine the consequences of also incorporating constraints from
recent measurements of the Hubble parameter and the age of the universe in the
constant and time-variable cosmological constant models. We also study the
effect of using a non-informative prior for the density parameter.Comment: Accepted for publication in Ap
Quintessence duality
We join quintessence cosmological scenarios with the duality simmetry
existing in string dilaton cosmologies. Actually, we consider the tracker
potential type and show that duality is only
established if .Comment: 6 LaTex Pages, submitted to Physics Letters A; completely revised
version: majior changes in the last par
Can Strong Gravitational Lensing Constrain Dark Energy?
We discuss the ratio of the angular diameter distances from the source to the
lens, , and to the observer at present, , for various dark
energy models. It is well known that the difference of s between the
models is apparent and this quantity is used for the analysis of Type Ia
supernovae. However we investigate the difference between the ratio of the
angular diameter distances for a cosmological constant,
and that for other dark energy models,
in this paper. It has been known that there is
lens model degeneracy in using strong gravitational lensing. Thus, we
investigate the model independent observable quantity, Einstein radius
(), which is proportional to both and velocity
dispersion squared, . values depend on the parameters
of each dark energy model individually. However, for the various dark energy models, is well within
the error of for most of the parameter spaces of the dark energy
models. Thus, a single strong gravitational lensing by use of the Einstein
radius may not be a proper method to investigate the property of dark energy.
However, better understanding to the mass profile of clusters in the future or
other methods related to arc statistics rather than the distances may be used
for constraints on dark energy.Comment: 15 pages, 13 figures, Accepted in PR
Vacuum defects without a vacuum
Topological defects can arise in symmetry breaking models where the scalar
field potential has no minima and is a monotonically decreasing
function of . The properties of such vacuumless defects are quite
different from those of the ``usual'' strings and monopoles. In some models
such defects can serve as seeds for structure formation, or produce an
appreciable density of mini-black holes.Comment: 11 pages, REVTeX, 1 Postscript figure. Minor changes. Final version,
to appear in Phys. Rev.
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