5,725 research outputs found
An alternative theoretical approach to describe planetary systems through a Schrodinger-type diffusion equation
In the present work we show that planetary mean distances can be calculated
with the help of a Schrodinger-type diffusion equation. The obtained results
are shown to agree with the observed orbits of all the planets and of the
asteroid belt in the solar system, with only three empty states. Furthermore,
the equation solutions predict a fundamental orbit at 0.05 AU from solar-type
stars, a result confirmed by recent discoveries. In contrast to other similar
approaches previously presented in the literature, we take into account the
flatness of the solar system, by considering the flat solutions of the
Schrodinger-type equation. The model has just one input parameter, given by the
mean distance of Mercury.Comment: 6 pages. Version accepted for publication in Chaos, Solitons &
Fractal
Variations of the Energy of Free Particles in the pp-Wave Spacetimes
We consider the action of exact plane gravitational waves, or pp-waves, on
free particles. The analysis is carried out by investigating the variations of
the geodesic trajectories of the particles, before and after the passage of the
wave. The initial velocities of the particles are non-vanishing. We evaluate
numerically the Kinetic energy per unit mass of the free particles, and obtain
interesting, quasi-periodic behaviour of the variations of the Kinetic energy
with respect to the width of the gaussian that represents the wave.
The variation of the energy of the free particle is expected to be exactly
minus the variation of the energy of the gravitational field, and therefore
provides an estimation of the local variation of the gravitational energy. The
investigation is carried out in the context of short bursts of gravitational
waves, and of waves described by normalised gaussians, that yield impulsive
waves in a certain limit.Comment: 20 pages, 18 figures, further arguments supporting the localizability
of the gravitational energy are presented, published in Univers
Non-singular inflation with vacuum decay
On the basis of a semi-classical analysis of vacuum energy in an expanding
spacetime, we describe a non-singular cosmological model in which the vacuum
density decays with time, with a concomitant production of matter. During an
infinitely long period we have an empty, inflationary universe, with H \approx
1. This primordial era ends in a fast phase transition, during which H and
\Lambda decrease to nearly zero in a few Planck times, with release of a huge
amount of radiation. The late-time scenario is similar to the standard model,
with the radiation phase followed by a long dust era, which tends
asymptotically to a de Sitter universe, with vacuum dominating again. An
analysis of the redshift-distance relation for supernovas Ia leads to
cosmological parameters in agreement with other current estimations.Comment: Work presented at IRGAC 2006, Barcelona, July 11-15 2006. To appear
in a special issue of Journal of Physics
A Godel-Friedman cosmology?
Based on the mathematical similarity between the Friedman open metric and
Godel's metric in the case of nearby distances, we investigate a new scenario
for the Universe's evolution, where the present Friedman universe originates
from a primordial Godel universe by a phase transition during which the
cosmological constant vanishes. Using Hubble's constant and the present matter
density as input, we show that the radius and density of the primordial Godel
universe are close, in order of magnitude, to the present values, and that the
time of expansion coincides with the age of the Universe in the standard
Friedman model. In addition, the conservation of angular momentum provides, in
this context, a possible origin for the rotation of galaxies, leading to a
relation between the masses and spins corroborated by observational data.Comment: Extended version, accepted for publication in Physical Review
Avaliação do potencial alelopĂĄtico de trĂȘs espĂ©cies de Eucalyptus na germinação de espĂ©cies cultivadas e nativas do cerrado.
Realizaram-se ensaios de germinação com sementes de Lactuca sativa L. (alface) cv. AurĂ©lia, sorgo cv. BR 304, Anadenanthera peregrina (L.) Speg. (angico) e Enterolobium contortisiliquum (Vell.) (tamboril), com o objetivo de avaliar o potencial alelopĂĄtico de extratos aquosos foliares de trĂȘs espĂ©cies de Eucalyptus. Prepararam-se os extratos colocando as folhas senescentes, secas e moĂdas de E. camaldulensis Dehnh (EC), E. grandis W.Hill ex Maien (EG) e E. urophylla S.T.Blake (EU) em ĂĄgua destilada na concentração 5% (p/v). ApĂłs agitados e centrifugados, obtiveram-se extratos a 100% (v/v), com os quais diluiram-se para 75, 50, 25, 12,5, 0% (v/v) e medidos o pH e o potencial osmĂłtico em cada concentração. As sementes foram colocadas para germinar em placas gerbox, forradas com papel de filtro e umedecidas com os extratos, numa amostragem de 100 sementes por tratamento (4x25). As placas foram colocadas em cĂąmara germinadora a 25°C, na presença de luz. A germinação foi verificada a cada 48h, durante 7 dias, sendo o critĂ©rio germinativo a emergĂȘncia radicular. O pH dos extratos das trĂȘs espĂ©cies de Eucalyptus diminuiu com o aumento da concentração, porĂ©m, nĂŁo afetou a germinação das espĂ©cies receptoras testadas. Os extratos aquosos de folhas de EC, EG e EU nĂŁo interferiram na germinação das sementes de angico, tamboril e sorgo, porĂ©m inibiram a germinação das sementes de alface. Eucalyptus camaldulensis; E; grandis; E; urophyll
Consorciação de feijão-caupi e milho em um agroecossistema manejado sob bases ecológicas na Região de Dourados, Mato Grosso do Sul.
bitstream/item/68469/1/034-Consorciacao.-shaline.pdfPublicado também no Cadernos de Agroecologia, v. 7, n. 2, 2012
Equality of opportunity and educational achievement in Portugal
Portugal has one of the highest levels of income inequality in Europe, and low wages and unemployment are concentrated among low skill individuals. Education is an important determinant of inequality. However, there are large differences in the educational attainment of different individuals in the population, and the sources of these differences emerge early in the life-cycle when families play a central role in individual development. We estimate that most of the variance of school achievement at age 15 is explained by family characteristics. Observed school inputs explain very little of adolescent performance. Children from highly educated parents benefit of rich cultural environments in the home and become highly educated adults. Education policy needs to be innovative: (1) it needs to explicitly recognize the fundamental long run role of families on child development; (2) it needs to acknowledge the failure of traditional input based policies
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