3,464 research outputs found
WMAP Constraints on the Generalized Chaplygin Gas Model
The generalized Chaplygin gas (GCG) model explains the recent accelerated
expansion of the Universe via an exotic background fluid whose equation of
state is given by p=-A/\rho^\alpha, where A is a positive constant and
0<\alpha\le 1. The model is an interesting alternative to scenarios involving
scalar field potentials, with the ensuing unnatural fine tuning conditions for
the underlying particle physics theories. We derive constraints on the
parameter space of the model from bounds on the location of the first few peaks
and troughs of the the Cosmic Microwave Background Radiation (CMBR) power
spectrum arising from recent WMAP and BOOMERanG data.Comment: 18 pages, 5 figures, version to appear in Phys. Lett.
String Theory and Cosmology
We discuss the main cosmological implications of considering string-loop
effects and a potential for the dilaton in the lowest order string effective
action. Our framework is based on the effective model arising from regarding
homogeneous and isotropic dilaton, metric and Yang-Mills field configurations.
The issues of inflation, entropy crisis and the Polonyi problem as well as the
problem of the cosmological constant are discussed.Comment: 7 pages, plain Tex, no figure
Generalized Chaplygin Gas Model: Dark Energy - Dark Matter Unification and CMBR Constraints
The generalized Chaplygin gas (GCG) model allows for an unified description
of the recent accelerated expansion of the Universe and the evolution of energy
density perturbations. This dark energy - dark matter unification is achieved
through an exotic background fluid whose equation of state is given by , where is a positive constant and .
Stringent constraints on the model parameters can be obtained from recent WMAP
and BOOMERanG bounds on the locations of the first few peaks and troughs of the
Cosmic Microwave Background Radiation (CMBR) power spectrum as well as SNe Ia
data.Comment: 9 pages, 2 figures; essay selected for an honorable mention by the
Gravity Research Foundation, 200
New members of the TW Hydrae Association and two accreting M-dwarfs in Scorpius-Centaurus
We report the serendipitous discovery of several young mid-M stars found
during a search for new members of the 30-40 Myr-old Octans Association. Only
one of the stars may be considered a possible Octans(-Near) member. However,
two stars have proper motions, kinematic distances, radial velocities,
photometry and Li I 6708AA measurements consistent with membership in the 8-10
Myr-old TW Hydrae Association. Another may be an outlying member of TW Hydrae
but has a velocity similar to that predicted by membership in Octans. We also
identify two new lithium-rich members of the neighbouring Scorpius-Centaurus OB
Association (Sco-Cen). Both exhibit large 12 and 22 micron excesses and strong,
variable H-alpha emission which we attribute to accretion from circumstellar
discs. Such stars are thought to be incredibly rare at the ~16 Myr median age
of Sco-Cen and they join only one other confirmed M-type and three higher-mass
accretors outside of Upper Scorpius. The serendipitous discovery of two
accreting stars hosting large quantities of circumstellar material may be
indicative of a sizeable age spread in Sco-Cen, or further evidence that disc
dispersal and planet formation time-scales are longer around lower-mass stars.
To aid future studies of Sco-Cen we also provide a newly-compiled catalogue of
305 early-type Hipparcos members with spectroscopic radial velocities sourced
from the literature.Comment: 12 pages. Accepted for publication in MNRA
Supergravity Inflation on the Brane
We study N=1 Supergravity inflation in the context of the braneworld
scenario. Particular attention is paid to the problem of the onset of inflation
at sub-Planckian field values and the ensued inflationary observables. We find
that the so-called -problem encountered in supergravity inspired
inflationary models can be solved in the context of the braneworld scenario,
for some range of the parameters involved. Furthermore, we obtain an upper
bound on the scale of the fifth dimension, M_5 \lsim 10^{-3} M_P, in case the
inflationary potential is quadratic in the inflaton field, . If the
inflationary potential is cubic in , consistency with observational data
requires that .Comment: 6 pages, 1 figure, to appear in Phys. Rev.
Functional analysis of novel genes differentially expressed genes in heart/hemangioblast precursor cells (H/HPC)
info:eu-repo/semantics/publishedVersio
WMAP Constraints on a Quintessence Model
We use the results from the Wilkinson Microwave Anisotropy Probe (WMAP) for
the locations of peaks and troughs of the Cosmic Microwave Background (CMB)
power spectrum, together with constraints from large-scale structure, to study
a quintessence model in which the pure exponential potential is modified by a
polynomial factor. Our analysis, in the cosmological
parameters space shows that this quintessence model is favoured compared to
CDM for and relatively high values of early
quintessence; for , quintessence and CDM give similar results,
except for high values of early quintessence, in which case CDM is
favoured.Comment: 3 pages. Talk presented by N. M. C. Santos at the Tenth Marcel
Grossmann Meeting on General Relativity, Rio de Janeiro, July 200
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