183 research outputs found
Certain aspects of regularity in scalar field cosmological dynamics
We consider dynamics of the FRW Universe with a scalar field. Using
Maupertuis principle we find a curvature of geodesics flow and show that zones
of positive curvature exist for all considered types of scalar field potential.
Usually, phase space of systems with the positive curvature contains islands of
regular motion. We find these islands numerically for shallow scalar field
potentials. It is shown also that beyond the physical domain the islands of
regularity exist for quadratic potentials as well.Comment: 15 pages with 4 figures; typos corrected, final version to appear in
Regular and Chaotic Dynamic
All Static Circularly Symmetric Perfect Fluid Solutions of 2+1 Gravity
Via a straightforward integration of the Einstein equations with cosmological
constant, all static circularly symmetric perfect fluid 2+1 solutions are
derived. The structural functions of the metric depend on the energy density,
which remains in general arbitrary. Spacetimes for fluids fulfilling linear and
polytropic state equations are explicitly derived; they describe, among others,
stiff matter, monatomic and diatomic ideal gases, nonrelativistic degenerate
fermions, incoherent and pure radiation. As a by--product, we demonstrate the
uniqueness of the constant energy density perfect fluid within the studied
class of metrics. A full similarity of the perfect fluid solutions with
constant energy density of the 2+1 and 3+1 gravities is established.Comment: revtex4, 8 page
Fine-Tuning Solution for Hybrid Inflation in Dissipative Chaotic Dynamics
We study the presence of chaotic behavior in phase space in the
pre-inflationary stage of hybrid inflation models. This is closely related to
the problem of initial conditions associated to these inflationary type of
models. We then show how an expected dissipative dynamics of fields just before
the onset of inflation can solve or ease considerably the problem of initial
conditions, driving naturally the system towards inflation. The chaotic
behavior of the corresponding dynamical system is studied by the computation of
the fractal dimension of the boundary, in phase space, separating inflationary
from non-inflationary trajectories. The fractal dimension for this boundary is
determined as a function of the dissipation coefficients appearing in the
effective equations of motion for the fields.Comment: 10 pages, 4 eps figures (uses epsf), Revtex. Replaced with version to
match one in press Physical Review
Cations extraction of sandy-clay soils from Cavado valley, Portugal, using sodium salts solutions
Cases of contamination by metals in the water wells of the Cavado Valley in north-west Portugal can be attributed to the heavy leaching of clay soils due to an excess of nitrogen resulting from the intensive use of fertilisers in agricultural areas. This work focuses on the natural weathering characteristics of soils, particularly the clay material, through the study of samples collected near the River Cavado. Samples taken from various sites, after physico-chemical characterisation, were subjected to clay dissolution tests, using sodium salts of different ionic forces, to detect the relationship between certain physico-chemical parameters of water, such as pH, nitrate, chloride and sulphate content, in the dissolution of clay and the subsequent
extraction of such cations as Al, Fe and K. In acidic sandy clay soils, the mineralogical composition of which was characterised by a predominance of quartz, micas, kaolinite and K-feldspars, decreases of the clay
material/water pH ratio increases dissolution of the micaceous and K-feldspars phases. The presence of nitrates in the aqueous solution apparently advanced the extraction of all three cations Al, Fe and K. The specific surface area of the clay material showed a significant correlation with the main kinetic parameters of cation extraction.Têm ocorrido casos de contaminações de águas de poços, por metais, no vale do Rio Cávado, região noroeste de Portugal. A princípio, poderiam ser explicáveis pela elevada lixiviação dos solos arenoargilosos
da região, quando da prática de adubações intensivas de nitrogênio em áreas agrícolas. Assim, estudaram-se as características do intemperismo natural dos solos, particularmente da fração argila, característica da margem norte do rio Cávado. Coletaram-se amostras de vários locais, que foram submetidas, após caracterização físico-química, a ensaios de dissolução a partir de soluções de sais de sódio com diferentes
forças iônicas. O objetivo foi observar as relações de determinados parâmetros físico-químicos da água, tais
como: pH, nitratos, cloretos e sulfatos na dissolução das argilas e a conseqüente extração de espécies químicas tais como Al, K e Fe. Para solos areno-argilosos, ácidos, cuja composição mineralógica se caracteriza
por um predomínio de quartzo, micas, caulinita e feldspato-K, o abaixamento do pH da suspensão solo/água promove a solubilização das fases micáceas e feldspáticas. A presença do nitrato nas soluções aquosas promoveu aparentemente a extração de todos os três cátions: Al, K e Fe. O efeito da área superfícial específica
das partículas dos solos condicionou fortemente vários dos parâmetros cinéticos estudados relativos à extração dos cátions.(undefined
Panspermia, Past and Present: Astrophysical and Biophysical Conditions for the Dissemination of Life in Space
Astronomically, there are viable mechanisms for distributing organic material
throughout the Milky Way. Biologically, the destructive effects of ultraviolet
light and cosmic rays means that the majority of organisms arrive broken and
dead on a new world. The likelihood of conventional forms of panspermia must
therefore be considered low. However, the information content of dam-aged
biological molecules might serve to seed new life (necropanspermia).Comment: Accepted for publication in Space Science Review
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