15,451 research outputs found
Superinflation, quintessence, and the avoidance of the initial singularity
We consider the dynamics of a spatially flat universe dominated by a
self-interacting nonminimally coupled scalar field. The structure of the phase
space and complete phase portraits for the conformal coupling case are given.
It is shown that the non-minimal coupling modifies drastically the dynamics of
the universe. New cosmological behaviors are identified, including
superinflation (), avoidance of big bang singularities through
classical birth of the universe from empty Minkowski space, and spontaneous
entry into and exit from inflation. The relevance of this model to the
description of quintessence is discussed.Comment: RevTex, 10 pages, 4 figures, To appear in the proceedings of the 5th
Peyresq meetin
Stability properties and asymptotics for N non-minimally coupled scalar fields cosmology
We consider here the dynamics of some homogeneous and isotropic cosmological
models with interacting classical scalar fields non-minimally coupled to
the spacetime curvature, as an attempt to generalize some recent results
obtained for one and two scalar fields. We show that a Lyapunov function can be
constructed under certain conditions for a large class of models, suggesting
that chaotic behavior is ruled out for them. Typical solutions tend generically
to the empty de Sitter (or Minkowski) fixed points, and the previous asymptotic
results obtained for the one field model remain valid. In particular, we
confirm that, for large times and a vanishing cosmological constant, even in
the presence of the extra scalar fields, the universe tends to an infinite
diluted matter dominated era.Comment: 10 page
The distance to the LMC cluster NGC 1866 and the surrounding field
We use the Main Sequence stars in the LMC cluster NGC 1866 and of Red Clump
stars in the local field to obtain two independent estimates of the LMC
distance. We apply an empirical Main Sequence-fitting technique based on a
large sample of subdwarfs with accurate {\sl Hipparcos} parallaxes in order to
estimate the cluster distance modulus, and the multicolor Red Clump method to
derive distance and reddening of the LMC field. We find that the Main
Sequence-fitting and the Red Clump distance moduli are in significant
disagreement; NGC 1866 distance is equal to 0.08 (consistent with a previous estimate using the same data
and theoretical Main Sequence isochrones), while the field stars provide 0.07. This difference reflects the more general
dichotomy in the LMC distance estimates found in the literature. Various
possible causes for this disagreement are explored, with particular attention
paid to the still uncertain metallicity of the cluster and the star formation
history of the field stars.Comment: 5 pages, incl. 1 figure, uses emulateapj.sty, ApJ accepte
Markov Chain Beam Randomization: a study of the impact of PLANCK beam measurement errors on cosmological parameter estimation
We introduce a new method to propagate uncertainties in the beam shapes used
to measure the cosmic microwave background to cosmological parameters
determined from those measurements. The method, which we call Markov Chain Beam
Randomization, MCBR, randomly samples from a set of templates or functions that
describe the beam uncertainties. The method is much faster than direct
numerical integration over systematic `nuisance' parameters, and is not
restricted to simple, idealized cases as is analytic marginalization. It does
not assume the data are normally distributed, and does not require Gaussian
priors on the specific systematic uncertainties. We show that MCBR properly
accounts for and provides the marginalized errors of the parameters. The method
can be generalized and used to propagate any systematic uncertainties for which
a set of templates is available. We apply the method to the Planck satellite,
and consider future experiments. Beam measurement errors should have a small
effect on cosmological parameters as long as the beam fitting is performed
after removal of 1/f noise.Comment: 17 pages, 23 figures, revised version with improved explanation of
the MCBR and overall wording. Accepted for publication in Astronomy and
Astrophysics (to appear in the Planck pre-launch special issue
Implantação do Banco Ativo de Germoplasma de pinheiro (Annona squamosa L.) na região do Submédio São Francisco.
O presente trabalho tem como objetivos: coletar, identificar, caracterizar, conservar, avaliar e selecionar genótipos promissores que possam ser incorporados em trabalhos de melhoramento genético visando o futuro lançamento de variedades recomendadas para cultivo na regiã
- …