10,982 research outputs found
Germinação de sementes do híbrido interespecífico do caiaué com dendezeiro: cultivar BRS Manicoré.
O objetivo desse estudo foi aperfeiçoar a metodologia utilizada, adequando o período de superação de dormência e o teor de água ideal para germinação das sementes
Evaluation of the susceptibility of Portuguese Autochtonous Flora do the pine wilt nematode (Bursaphelenchus xylophilus)
Cosmological Inhomogeneities with Bose-Einstein Condensate Dark Matter
We consider the growth of cosmological perturbations to the energy density of
dark matter during matter domination when dark matter is a scalar field that
has undergone Bose-Einstein condensation. We study these inhomogeneities within
the framework of both Newtonian gravity, where the calculation and results are
more transparent, and General Relativity. The direction we take is to derive
analytical expressions, which can be obtained in the small pressure limit.
Throughout we compare our results to those of the standard cosmology, where
dark matter is assumed pressureless, using our analytical expressions to
showcase precise differences. We find, compared to the standard cosmology, that
Bose-Einstein condensate dark matter leads to a scale factor, gravitational
potential and density contrast that increase at faster rates.Comment: 17 pages, 2 figures; typos corrected, references adde
Accelerating Cold Dark Matter Cosmology ()
A new kind of accelerating flat model with no dark energy that is fully
dominated by cold dark matter (CDM) is investigated. The number of CDM
particles is not conserved and the present accelerating stage is a consequence
of the negative pressure describing the irreversible process of gravitational
particle creation. A related work involving accelerating CDM cosmology has been
discussed before the SNe observations [Lima, Abramo & Germano, Phys. Rev. D53,
4287 (1996)]. However, in order to have a transition from a decelerating to an
accelerating regime at low redshifts, the matter creation rate proposed here
includes a constant term of the order of the Hubble parameter. In this case,
does not need to be small in order to solve the age problem and the
transition happens even if the matter creation is negligible during the
radiation and part of the matter dominated phase. Therefore, instead of the
vacuum dominance at redshifts of the order of a few, the present accelerating
stage in this sort of Einstein-de Sitter CDM cosmology is a consequence of the
gravitational particle creation process. As an extra bonus, in the present
scenario does not exist the coincidence problem that plagues models with
dominance of dark energy. The model is able to harmonize a CDM picture with the
present age of the universe, the latest measurements of the Hubble parameter
and the Supernovae observations.Comment: 9 pages, 6 figures, typos corrected, references added, discussion in
Appendix B extende
The role of pressure anisotropy in the turbulent intracluster medium
In low-density plasma environments, such as the intracluster medium (ICM),
the Larmour frequency is much larger than the ion-ion collision frequency. In
such a case, the thermal pressure becomes anisotropic with respect to the
magnetic field orientation and the evolution of the turbulent gas is more
correctly described by a kinetic approach. A possible description of these
collisionless scenarios is given by the so-called kinetic magnetohydrodynamic
(KMHD) formalism, in which particles freely stream along the field lines, while
moving with the field lines in the perpendicular direction. In this way a
fluid-like behavior in the perpendicular plane is restored. In this work, we
study fast growing magnetic fluctuations in the smallest scales which operate
in the collisionless plasma that fills the ICM. In particular, we focus on the
impact of a particular evolution of the pressure anisotropy and its
implications for the turbulent dynamics of observables under the conditions
prevailing in the ICM. We present results from numerical simulations and
compare the results which those obtained using an MHD formalism.Comment: 7 pages, 14 figures, Journal of Physics: Conference Serie
- …