49 research outputs found
Coupled quintessence and vacuum decay
We discuss observational consequences of a class of cosmological models
characterized by the dilution of pressureless matter attenuated with respect to
the usual scaling due to the decay of vacuum energy. We carry out a
joint statistical analysis of observational data from the new \emph{gold}
sample of 182 SNe Ia, recent estimates of the CMB shift parameter, and BAO
measurements from the SDSS to show that such models favor the decay of vacuum
only into the dark matter sector, and that the separately conserved baryons
cannot be neglected. In order to explore ways to more fundamentally motivated
models, we also derive a coupled scalar field version for this general class of
vacuum decay scenarios.Comment: 6 pages, 3 figures, LaTe
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
Cosmology in a String-Dominated Universe
The string-dominated universe locally resembles an open universe, and fits
dynamical measures of power spectra, cluster abundances, redshift distortions,
lensing constraints, luminosity and angular diameter distance relations and
microwave background observations. We show examples of networks which might
give rise to recent string-domination without requiring any fine-tuned
parameters. We discuss how future observations can distinguish this model from
other cosmologies.Comment: 17 pages including 4 figures, of which one is in colo
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
Black Hole Formation with an Interacting Vacuum Energy Density
We discuss the gravitational collapse of a spherically symmetric massive core
of a star in which the fluid component is interacting with a growing vacuum
energy density. The influence of the variable vacuum in the collapsing core is
quantified by a phenomenological \beta-parameter as predicted by dimensional
arguments and the renormalization group approach. For all reasonable values of
this free parameter, we find that the vacuum energy density increases the
collapsing time but it cannot prevent the formation of a singular point.
However, the nature of the singularity depends on the values of \beta. In the
radiation case, a trapped surface is formed for \beta<1/2 whereas for
\beta>1/2, a naked singularity is developed. In general, the critical value is
\beta=1-2/3(1+\omega), where the \omega-parameter describes the equation of
state of the fluid component.Comment: 9 pages, 8 figure
Cosmic Concordance and Quintessence
We present a comprehensive study of the observational constraints on
spatially flat cosmological models containing a mixture of matter and
quintessence --- a time varying, spatially inhomogeneous component of the
energy density of the universe with negative pressure. Our study also includes
the limiting case of a cosmological constant. Low red shift constraints include
the Hubble parameter, baryon fraction, cluster abundance, age of the universe,
bulk velocity and shape of the mass power spectrum; intermediate red shift
constraints are due to type 1a supernovae, gravitational lensing, the Ly-a
forest, and the evolution of large scale structure; high red shift constraints
are based on cosmic microwave background temperature anisotropy. Mindful of
systematic errors, we adopt a conservative approach in applying these
constraints. We determine that quintessence models in which the matter density
parameter is 0.2 \ls \Omega_m \ls 0.5 and the effective, density-averaged
equation of state is -1 \le w \ls -0.2, are consistent with the most
reliable, current low red shift and CMB observations at the level.
Factoring in the constraint due to type 1a SNe, the range for the equation of
state is reduced to -1 \le w \ls -0.4, where this range represents models
consistent with each observational constraint at the 2 level or better
(concordance analysis). A combined maximum likelihood analysis suggests a
smaller range, -1 \le w \ls -0.6. We find that the best-fit and
best-motivated quintessence models lie near , , and spectral index , with an effective equation of state for ``tracker'' quintessence and for ``creeper''
quintessence. (abstract shortened)Comment: revised to match ApJ version; 33 pages; 20 figures, 4 in color; uses
emulateapj.st
Perturbation evolution in cosmologies with a decaying cosmological constant
Structure formation models with a cosmological constant are successful in
explaining large-scale structure data, but are threatened by the
magnitude-redshift relation for Type Ia supernovae. This has led to discussion
of models where the cosmological `constant' decays with time, which might
anyway be better motivated in a particle physics context. The simplest such
models are based on scalar fields, and general covariance demands that a
time-evolving scalar field also supports spatial perturbations. We consider the
effect of such perturbations on the growth of adiabatic energy density
perturbations in a cold dark matter component. We study two types of model, one
based on an exponential potential for the scalar field and the other on a
pseudo-Nambu Goldstone boson. For each potential, we study two different
scenarios, one where the scalar field presently behaves as a decaying
cosmological constant and one where it behaves as dust. The initial scalar
field perturbations are fixed by the adiabatic condition, as expected from the
inflationary cosmology, though in fact we show that the choice of initial
condition is of little importance. Calculations are carried out in both the
zero-shear (conformal newtonian) and uniform-curvature gauges. We find that
both potentials allow models which can provide a successful alternative to
cosmological constant models.Comment: 14 pages RevTeX file with three figures incorporated (uses RevTeX and
epsf). Also available by e-mailing ARL, or by WWW at
http://star-www.maps.susx.ac.uk/papers/lsstru_papers.html Revised version
corrects an error in Eq10; results unchange
Properties of cosmologies with dynamical pseudo Nambu-Goldstone bosons
We study observational constraints on cosmological models with a quintessence
field in the form of a dynamical pseudo Nambu-Goldstone boson. After reviewing
the properties of the solutions, from a dynamical systems phase space analysis,
we consider the constraints on parameter values imposed by luminosity distances
from the 60 Type Ia supernovae published by Perlmutter et al., and also from
gravitational lensing statistics of distant quasars. In the case of the Type Ia
supernovae we explicitly allow for the possibility of evolution of the peak
luminosities of the supernovae sources, using simple empirical models which
have been recently discussed in the literature. We find weak evidence to
suggest that the models with supernovae evolution fit the data better in the
context of the quintessence models in question. If source evolution is a
reality then the greatest challenge facing these models is the tension between
current value of the expansion age, H_0 t_0, and the fraction of the critical
energy density, Omega_{phi0}, corresponding to the scalar field. Nonetheless
there are ranges of the free parameters which fit all available cosmological
data.Comment: 22 pages, RevTeX, 13 figures, epsf. v3: References added, plus a few
sentences to clarify some small points; v4: Typos fixe
Cosmological constraints from lensing statistics and supernovae on the cosmic equation of state
We investigate observational constraints from lensing statistics and high-z
type Ia supernovae on flat cosmological models with nonrelativistic matter and
an exotic fluid with equation of state, . We show that
agreement with both tests at the 68% confidence level is possible if the
parameter is low () and with lower values of corresponding to higher .
We find that a conventional cosmological constant model with is the best fit model of the combined likelihood.Comment: 7 pages, 4 postscript figures, revtex, submitted to Phys. Rev.
New Constraints from High Redshift Supernovae and Lensing Statistics upon Scalar Field Cosmologies
We explore the implications of gravitationally lensed QSOs and high-redshift
SNe Ia observations for spatially flat cosmological models in which a
classically evolving scalar field currently dominates the energy density of the
Universe. We consider two representative scalar field potentials that give rise
to effective decaying (``quintessence'') models:
pseudo-Nambu-Goldstone bosons () and an inverse
power-law potential (). We show that a
large region of parameter space is consistent with current data if . On the other hand, a higher lower bound for the matter density
parameter suggested by large-scale galaxy flows, ,
considerably reduces the allowed parameter space, forcing the scalar field
behavior to approach that of a cosmological constant.Comment: 6 pages, 2 figures, submitted to PR