780 research outputs found
Cosmological Constraints from Hubble Parameter versus Redshift Data
We use the Simon, Verde, & Jimenez (2005) determination of the redshift
dependence of the Hubble parameter to constrain cosmological parameters in
three dark energy cosmological models. We consider the standard CDM
model, the XCDM parameterization of the dark energy equation of state, and a
slowly rolling dark energy scalar field with an inverse power-law potential.
The constraints are restrictive, consistent with those derived from Type Ia
supernova redshift-magnitude data, and complement those from galaxy cluster gas
mass fraction versus redshift data.Comment: Minor changes, including an estimate for H_0. ApJL, in pres
COBE-DMR-Normalized Dark Energy Cosmogony
Likelihood analyses of the COBE-DMR sky maps are used to determine the
normalization of the inverse-power-law-potential scalar field dark energy
model. Predictions of the DMR-normalized model are compared to various
observations to constrain the allowed range of model parameters. Although the
derived constraints are restrictive, evolving dark energy density scalar field
models remain an observationally-viable alternative to the constant
cosmological constant model.Comment: 26 pages, 10 figures, ApJ accepte
Scalar Non-Luminous Matter in Galaxies
As a candidate for dark matter in galaxies, we study an SU(3) triplet of
complex scalar fields which are non-minimally coupled to gravity. In the
spherically symmetric static spacetime where the flat rotational velocity
curves of stars in galaxies can be explained, we find simple solutions of
scalar fields with SU(3) global symmetry broken to U(1) X U(1), in an
exponential scalar potential, which will be useful in a quintessence model of
the late-time acceleration of the Universe.Comment: 6 pages, no figure, LaTex. Submitted to IJMP
Environmental Dependence of Masses and Coupling Constants
We construct a class of scalar field models coupled to matter that lead to
the dependence of masses and coupling constants on the ambient matter density.
Such models predict a deviation of couplings measured on the Earth from values
determined in low-density astrophysical environments, but do not necessarily
require the evolution of coupling constants with the redshift in the recent
cosmological past. Additional laboratory and astrophysical tests of \Delta
\alpha and \Delta(m_p/m_e) as functions of the ambient matter density are
warranted.Comment: 20 pages, no figures, references added, minor editorial change
Scalar Field, Four Dimensional Spacetime Volume and the Holographic Dark Energy
We explore the cosmic evolution of a scalar field which is identified with
the four dimensional spacetime volume. Given a specific form for the Lagrangian
of the scalar field, a new holographic dark energy model is present. The energy
density of dark energy is reversely proportional to the square of the radius of
the cosmic null hypersurface which is present as a new infrared cutoff for the
Universe. We find this holographic dark energy belongs to the phantom dark
energy for some appropriate parameters in order to interpret the current
acceleration of the Universe.Comment: 7 pages, 5 figures. References added and some typos correcte
Accelerating Universes with Scaling Dark Matter
Friedmann-Robertson-Walker universes with a presently large fraction of the
energy density stored in an -component with , are considered. We
find all the critical points of the system for constant equations of state in
that range. We consider further several background quantities that can
distinguish the models with different values. Using a simple toy model
with a varying equation of state, we show that even a large variation of
at small redshifts is very difficult to observe with measurements up
to . Therefore, it will require accurate measurements in the range
and independent accurate knowledge of (and/or
) in order to resolve a variable from a constant .Comment: submitted to IJMPD (uses Latex, 12 pages, 6 Figures) Minor
corrections, Figures 4, 6 revised. Conclusions unchange
Statefinder Parameters for Tachyon Dark Energy Model
In this paper we study the statefinder parameters for the tachyon dark energy
model. There are two kinds of stable attractor solutions in this model. The
statefinder diagrams characterize the properties of the tachyon dark energy
model. Our results show that the evolving trajectories of the attractor
solutions lie in the total region and pass through the LCDM fixed point, which
is different from other dark energy model.Comment: 5 pages, 5 figures, accepted by MPL
Python I, II, and III CMB Anisotropy Measurement Constraints on Open and Flat-Lambda CDM Cosmogonies
We use Python I, II, and III cosmic microwave background anisotropy data to
constrain cosmogonies. We account for the Python beamwidth and calibration
uncertainties. We consider open and spatially-flat-Lambda cold dark matter
cosmogonies, with nonrelativistic-mass density parameter Omega_0 in the range
0.1--1, baryonic-mass density parameter Omega_B in the range (0.005--0.029)
h^{-2}, and age of the universe t_0 in the range (10--20) Gyr. Marginalizing
over all parameters but Omega_0, the combined Python data favors an open
(spatially-flat-Lambda) model with Omega_0 simeq 0.2 (0.1). At the 2 sigma
confidence level model normalizations deduced from the combined Python data are
mostly consistent with those drawn from the DMR, UCSB South Pole 1994, ARGO,
MAX 4 and 5, White Dish, and SuZIE data sets.Comment: 20 pages, 7 figures, accepted by Ap
Cosmological Parameter Determination from Counts of Galaxies
We study constraints that anticipated DEEP survey galaxy counts versus
redshift data will place on cosmological model parameters in models with and
without a constant or time-variable cosmological constant . This data
will result in fairly tight constraints on these parameters. For example, if
all other parameters of a spatially-flat model with a constant are
known, the galaxy counts data should constrain the nonrelativistic matter
density parameter to about 5% (10%, 1.5%) at 1 with neutral
(worst case, best case) assumptions about data quality.Comment: 15 pages, 6 figure
Median Statistics, H_0, and the Accelerating Universe
(Abridged) We develop median statistics that provide powerful alternatives to
chi-squared likelihood methods and require fewer assumptions about the data.
Applying median statistics to Huchra's compilation of nearly all estimates of
the Hubble constant, we find a median value H_0=67 km/s/Mpc. Median statistics
assume only that the measurements are independent and free of systematic
errors. This estimate is arguably the best summary of current knowledge because
it uses all available data and, unlike other estimates, makes no assumption
about the distribution of measurement errors. The 95% range of purely
statistical errors is +/- 2 km/s/Mpc. The statistical precision of this result
leads us to analyze the range of possible systematic errors in the median,
which we estimate to be roughly +/- 5 km/s/Mpc (95% limits), dominating over
the statistical errors. A Bayesian median statistics treatment of high-redshift
Type Ia supernovae (SNe Ia) apparent magnitude versus redshift data from Riess
et al. yields a posterior probability that the cosmological constant Lambda > 0
of 70 or 89%, depending on the prior information used. The posterior
probability of an open universe is about 47%. Analysis of the Perlmutter et al.
high-redshift SNe Ia data show the best-fit flat-Lambda model favored over the
best-fit Lambda = 0 open model by odds of 366:1; corresponding Riess et al.
odds are 3:1 (assuming prior odds of 1:1).Median statistics analyses of the SNe
Ia data do not rule out a time-variable Lambda model, and may even favor it
over a time-independent Lambda and a Lambda = 0 open model.Comment: Significant revisions include discussion of systematic errors in the
median of H_0. Accepted for publication in The Astrophysical Journal, v548,
February 20, 2001 issue. 47 pages incl. figures and table
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