6,276 research outputs found
On the Space Time of a Galaxy
We present an exact solution of the averaged Einstein's field equations in
the presence of two real scalar fields and a component of dust with spherical
symmetry. We suggest that the space-time found provides the characteristics
required by a galactic model that could explain the supermassive central object
and the dark matter halo at once, since one of the fields constitutes a central
oscillaton surrounded by the dust and the other scalar field distributes far
from the coordinate center and can be interpreted as a halo. We show the
behavior of the rotation curves all along the background. Thus, the solution
could be a first approximation of a ``long exposition photograph'' of a galaxy.Comment: 8 pages REVTeX, 11 eps figure
Quintessence and Scalar Dark Matter in the Universe
Continuing with previous works, we present a cosmological model in which dark
matter and dark energy are modeled by scalar fields and ,
respectively, endowed with the scalar potentials and . This model contains 95% of
scalar field. We obtain that the scalar dark matter mass is The solution obtained allows us to recover the success of the
standard CDM. The implications on the formation of structure are reviewed. We
obtain that the minimal cutoff radio for this model is Comment: 4 pages REVTeX, 3 eps color figures. Minor changes and references
updated. To appear in Classical and Quantum Gravity as a Letter to the
Editor. More information at http://www.fis.cinvestav.mx/~siddh/PHI
A Note on the Local Cosmological Constant and the Dark Energy Coincidence Problem
It has been suggested that the Dark Energy Coincidence Problem could be
interpreted as a possible link between the cosmological constant and a massive
graviton. We show that by using that link and models for the graviton mass a
dark energy density can be obtained that is indeed very close to measurements
by WMAP. As a consequence of the models, the cosmological constant was found to
depend on the density of matter. A brief outline of the cosmological
consequences such as the effect on the black hole solution is given
Vector field and rotational curves in dark galactic halos
We study equations of a non-gauge vector field in a spherically symmetric
static metric. The constant vector field with a scale arrangement of
components: the time component about the Planck mass m_{Pl} and the radial
component about M suppressed with respect to the Planck mass, serves as a
source of metric reproducing flat rotation curves in dark halos of spiral
galaxies, so that the velocity of rotation v_0 is determined by the hierarchy
of scales: \sqrt{2} v_0^2= M/m_{Pl}, and M\sim 10^{12} GeV. A natural estimate
of Milgrom's acceleration about the Hubble rate is obtained.Comment: 17 pages, iopart style, misprint remove
Quantum mechanical counterpart of nonlinear optics
Raman-type laser excitation of a trapped atom allows one to realize the
quantum mechanical counterpart of phenomena of nonlinear optics, such as
Kerr-type nonlinearities, parametric amplification, and multi-mode mixing.
Additionally, huge nonlinearities emerge from the interference of the atomic
wave function with the laser waves. They lead to a partitioning of the phase
space accompanied by a significantly different action of the time evolution in
neighboring phase-space zones. For example, a nonlinearly modified coherent
"displacement" of the motional quantum state may induce strong amplitude
squeezing and quantum interferences.Comment: 6 pages, 4 figures, to be published in Phys. Rev. A 55 (June
5-Arylazo-2,2´-bithiophenes: a novel promising series of NLO chromophores
The synthesis of 5-arylazo- substituted bithiophenes and their UV-visible, solvatochromic and nonlinear optical properties (NLO) are described. In agreement with the solvatochromic data and also with the second-order molecular NLO characterization, the new donor-acceptor systems could find application as suitable solvatochromic probes and also as new NLO materials.Fundação para a Ciência e Tecnologia (FCT
Development of an FIA system with amperometric detection for determination of bentazone in estuarine waters
On the basis of its electrochemical behaviour a
new flow-injection analysis (FIA) method with amperometric
detection has been developed for quantification of
the herbicide bentazone (BTZ) in estuarine waters. Standard
solutions and samples (200 µL) were injected into a
water carrier stream and both pH and ionic strength were
automatically adjusted inside the manifold. Optimization
of critical FIA conditions indicated that the best analytical
results were obtained at an oxidation potential of 1.10 V,
pH 4.5, and an overall flow-rate of 2.4 mL min–1. Analysis
of real samples was performed by means of calibration
curves over the concentration range 2.5x10–6 to
5.0x10–5 mol L–1, and results were compared with those
obtained by use of an independent method (HPLC). The
accuracy of the amperometric determinations was ascertained;
errors relative to the comparison method were below
4% and sampling rates were approximately 100 samples
h–1. The repeatability of the proposed method was
calculated by assessing the relative standard deviation (%)
of ten consecutive determinations of one sample; the value
obtained was 2.1%
ISePorto Robotic Soccer Team: A New Player Generation
Proceedings of the Scientific Meeting of the Portuguese Robotics Open 2004This paper describes the recent modifications in ISePorto MSL robotic
football team and future improvements concerning the development and evolution of
the team. The robot was substantially redesigned in order to achieve high reliability,
allow better control and coordination capabilities and substantial increase in perception.
New mechanical and hardware redesign is presented. Motion control subsystems, new
vision hardware sensor and overall architecture are described. The team redesign is
done for preparation for participating in the Robocup 2004. The main goal is to achieve
not only an important evolution in the team control and coordination but also increased
overall reliability
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