489 research outputs found
Galactic metric, dark radiation, dark pressure and gravitational lensing in brane world models
In the braneworld scenario, the four dimensional effective Einstein equation
has extra terms which arise from the embedding of the 3-brane in the bulk.
These non-local effects, generated by the free gravitational field of the bulk,
may provide an explanation for the dynamics of the neutral hydrogen clouds at
large distances from the galactic center, which is usually explained by
postulating the existence of the dark matter. We obtain the exact galactic
metric, the dark radiation and the dark pressure in the flat rotation curves
region in the brane world scenario. Due to the presence of the bulk effects,
the flat rotation curves could extend several hundred kpc. The limiting radius
for which bulk effects are important is estimated and compared with the
numerical values of the truncation parameter of the dark matter halos, obtained
from weak lensing observations. There is a relatively good agreement between
the predictions of the model and observations. The deflection of photons is
also considered and the bending angle of light is computed. The bending angle
predicted by the brane world models is much larger than that predicted by
standard general relativistic and dark matter models. The angular radii of the
Einstein rings are obtained in the small angles approximation. The predictions
of the brane world model for the tangential shear are compared with the
observational data obtained in the weak lensing of galaxies in the Red-Sequence
Cluster Survey. Therefore the study of the light deflection by galaxies and the
gravitational lensing could discriminate between the different dynamical laws
proposed to model the motion of particles at the galactic level and the
standard dark matter models.Comment: 33 pages, 3 figures, accepted for publication in Ap
Studying Migrant Assimilation Through Facebook Interests
Migrants' assimilation is a major challenge for European societies, in part
because of the sudden surge of refugees in recent years and in part because of
long-term demographic trends. In this paper, we use Facebook's data for
advertisers to study the levels of assimilation of Arabic-speaking migrants in
Germany, as seen through the interests they express online. Our results
indicate a gradient of assimilation along demographic lines, language spoken
and country of origin. Given the difficulty to collect timely migration data,
in particular for traits related to cultural assimilation, the methods that we
develop and the results that we provide open new lines of research that
computational social scientists are well-positioned to address.Comment: Accepted as a short paper at Social Informatics 2018
(https://socinfo2018.hse.ru/). Please cite the SocInfo versio
The origin and propagation of VVH primary cosmic ray particles
Several source spectra were constructed from combinations of 4- and s-process nuclei to match the observed charge spectrum of VVH particles. Their propagation was then followed, allowing for interactions and decay, and comparisons were made between the calculated near-earth spectra and those observed during high altitude balloon flights. None of the models gave good agreement with observations
Boson Stars as Gravitational Lenses
We discuss boson stars as possible gravitational lenses and study the lensing
effect by these objects made of scalar particles. The mass and the size of a
boson star may vary from an individual Newtonian object similar to the Sun to
the general relativistic size and mass of a galaxy close to its Schwarzschild
radius. We assume boson stars to be transparent which allows the light to pass
through them though the light is gravitationally deflected. We assume boson
stars of the mass to be on non-cosmological distance from
the observer. We discuss the lens equation for these stars as well as the
details of magnification. We find that there are typically three images of a
star but the deflection angles may vary from arcseconds to even degrees. There
is one tangential critical curve (Einstein ring) and one radial critical curve
for tangential and radial magnification, respectively. Moreover, the deflection
angles for the light passing in the gravitational field of boson stars can be
very large (even of the order of degrees) which reflects the fact they are very
strong relativistic objects. We also propose a suitable formula for the lens
equation for such large deflection angles, and with the reservation that large
deflection angle images are highly demagnified but in the area of the
tangential critical curve, their existence may help in observational detection
of suitable lenses possessing characteristic features of boson stars which
could also serve as a direct evidence for scalar fields in the universe.Comment: accepted by Astrophys. J., 31 pages, AASTeX, 6 figure
Primary cosmic ray particles with z 35 (VVH particles)
Large areas of nuclear emulsions and plastic detectors were exposed to the primary cosmic radiation during high altitude balloon flights. From the analysis of 141 particle tracks recorded during a total exposure of 1.3 x 10 to the 7th power sq m ster.sec., a charge spectrum of the VVH particles has been derived
Cosmic ray diffusive acceleration at shock waves with finite upstream and downstream escape boundaries
In the present paper we discuss the modifications introduced into the
first-order Fermi shock acceleration process due to a finite extent of
diffusive regions near the shock or due to boundary conditions leading to an
increased particle escape upstream and/or downstream the shock. In the
considered simple example of the planar shock wave we idealize the escape
phenomenon by imposing a particle escape boundary at some distance from the
shock. Presence of such a boundary (or boundaries) leads to coupled steepening
of the accelerated particle spectrum and decreasing of the acceleration time
scale. It allows for a semi-quantitative evaluation and, in some specific
cases, also for modelling of the observed steep particle spectra as a result of
the first-order Fermi shock acceleration. We also note that the particles close
to the upper energy cut-off are younger than the estimate based on the
respective acceleration time scale. In Appendix A we present a new
time-dependent solution for infinite diffusive regions near the shock allowing
for different constant diffusion coefficients upstream and downstream the
shock.Comment: LaTeX, 14 pages, 4 postscript figures; Solar Physics (accepted
The solar maximum satellite capture cell: Impact features and orbital debris and micrometeoritic projectile materials
The physical properties of impact features observed in the Solar Max main electronics box (MEB) thermal blanket generally suggest an origin by hypervelocity impact. The chemistry of micrometeorite material suggests that a wide variety of projectile materials have survived impact with retention of varying degrees of pristinity. Impact features that contain only spacecraft paint particles are on average smaller than impact features caused by micrometeorite impacts. In case both types of materials co-occur, it is belevied that the impact feature, generally a penetration hole, was caused by a micrometeorite projectile. The typically smaller paint particles were able to penetrate though the hole in the first layer and deposit in the spray pattern on the second layer. It is suggested that paint particles have arrived with a wide range of velocities relative to the Solar Max satellite. Orbiting paint particles are an important fraction of materials in the near-Earth environment. In general, the data from the Solar Max studies are a good calibration for the design of capture cells to be flown in space and on board Space Station. The data also suggest that development of multiple layer capture cells in which the projectile may retain a large degree of pristinity is a feasible goal
Spacetime perspective of Schwarzschild lensing
We propose a definition of an exact lens equation without reference to a
background spacetime, and construct the exact lens equation explicitly in the
case of Schwarzschild spacetime. For the Schwarzschild case, we give exact
expressions for the angular-diameter distance to the sources as well as for the
magnification factor and time of arrival of the images. We compare the exact
lens equation with the standard lens equation, derived under the
thin-lens-weak-field assumption (where the light rays are geodesics of the
background with sharp bending in the lens plane, and the gravitational field is
weak), and verify the fact that the standard weak-field thin-lens equation is
inadequate at small impact parameter. We show that the second-order correction
to the weak-field thin-lens equation is inaccurate as well. Finally, we compare
the exact lens equation with the recently proposed strong-field thin-lens
equation, obtained under the assumption of straight paths but without the small
angle approximation, i.e., with allowed large bending angles. We show that the
strong-field thin-lens equation is remarkably accurate, even for lightrays that
take several turns around the lens before reaching the observer.Comment: 22 pages, 6 figures, to appear in Phys. Rev.
Image distortion in non perturbative gravitational lensing
We introduce the idea of {\it shape parameters} to describe the shape of the
pencil of rays connecting an observer with a source lying on his past
lightcone. On the basis of these shape parameters, we discuss a setting of
image distortion in a generic (exact) spacetime, in the form of three {\it
distortion parameters}. The fundamental tool in our discussion is the use of
geodesic deviation fields along a null geodesic to study how source shapes are
propagated and distorted on the path to an observer. We illustrate this
non-perturbative treatment of image distortion in the case of lensing by a
Schwarzschild black hole. We conclude by showing that there is a
non-perturbative generalization of the use of Fermat's principle in lensing in
the thin-lens approximation.Comment: 22 pages, 6 figures, to appear in Phys. Rev. D (January 2001
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