16,141 research outputs found
Future dynamics in f(R) theories
The gravity theories provide an alternative way to explain the current
cosmic acceleration without invoking dark energy matter component. However, the
freedom in the choice of the functional forms of gives rise to the
problem of how to constrain and break the degeneracy among these gravity
theories on theoretical and/or observational grounds. In this paper to proceed
further with the investigation on the potentialities, difficulties and
limitations of gravity, we examine the question as to whether the future
dynamics can be used to break the degeneracy between gravity theories by
investigating the future dynamics of spatially homogeneous and isotropic dust
flat models in two gravity theories, namely the well known gravity and another by A. Aviles et al., whose motivation comes
from the cosmographic approach to gravity. To this end we perform a
detailed numerical study of the future dynamic of these flat model in these
theories taking into account the recent constraints on the cosmological
parameters made by the Planck team. We show that besides being powerful for
discriminating between gravity theories, the future dynamics technique
can also be used to determine the fate of the Universe in the framework of
these gravity theories. Moreover, there emerges from our numerical
analysis that if we do not invoke a dark energy component with
equation-of-state parameter one still has dust flat FLRW solution
with a big rip, if gravity deviates from general relativity via . We also show that FLRW dust solutions with do not
necessarily lead to singularity.Comment: 12 pages, 8 figures. V2: Generality and implications of the results
are emphasized, connection with the recent literature improved, typos
corrected, references adde
Memory effects on the statistics of fragmentation
We investigate through extensive molecular dynamics simulations the
fragmentation process of two-dimensional Lennard-Jones systems. After
thermalization, the fragmentation is initiated by a sudden increment to the
radial component of the particles' velocities. We study the effect of
temperature of the thermalized system as well as the influence of the impact
energy of the ``explosion'' event on the statistics of mass fragments. Our
results indicate that the cumulative distribution of fragments follows the
scaling ansatz , where is
the mass, and are cutoff parameters, and is a scaling
exponent that is dependent on the temperature. More precisely, we show clear
evidence that there is a characteristic scaling exponent for each
macroscopic phase of the thermalized system, i.e., that the non-universal
behavior of the fragmentation process is dictated by the state of the system
before it breaks down.Comment: 5 pages, 8 figure
Travel of studies: cities of João Pessoa, Maceio, Natal and Recife: a look on the urban space and brazilian architectural production
Ponencia presentada a Session 8: Dimensiones psicosociales de la arquitectura y el urbanismo / Psycological dimensions of architecture and planningThis article aims to present the methodology and the final results of the elective course “Travel of Studies” which belongs to the new pedagogical project from the Architecture and Urbanism course at the University Federal of Pernambuco. It was offered for the first time in 2013.The discipline was organized to occur in four long weekends through visits of four capitals of the Northeast of Brazil: Recife, João Pessoa, Natal and Maceió. The purpose was to allow the students to apprehend the cities through four axis: intervention in historical center (axis 1), production of urban space (axis 2), production of coastline space (axis 3) and contemporary architecture (axis 4). After the four visits were complete, we prepared a poster with the comparison of the cities based on the identification of the similarities and differences of each axis we have studied
SAMplus: adaptive optics at optical wavelengths for SOAR
Adaptive Optics (AO) is an innovative technique that substantially improves
the optical performance of ground-based telescopes. The SOAR Adaptive Module
(SAM) is a laser-assisted AO instrument, designed to compensate ground-layer
atmospheric turbulence in near-IR and visible wavelengths over a large Field of
View. Here we detail our proposal to upgrade SAM, dubbed SAMplus, that is
focused on enhancing its performance in visible wavelengths and increasing the
instrument reliability. As an illustration, for a seeing of 0.62 arcsec at 500
nm and a typical turbulence profile, current SAM improves the PSF FWHM to 0.40
arcsec, and with the upgrade we expect to deliver images with a FWHM of
arcsec -- up to 0.23 arcsec FWHM PSF under good seeing
conditions. Such capabilities will be fully integrated with the latest SAM
instruments, putting SOAR in an unique position as observatory facility.Comment: To appear in Proc. SPIE 10703 (Ground-based and Airborne
Instrumentation for Astronomy VII; SPIEastro18
Rotation intrinsic spin coupling--the parallelism description
For the Dirac particle in the rotational system, the rotation induced inertia
effect is analogously treated as the modification of the "spin connection" on
the Dirac equation in the flat spacetime, which is determined by the equivalent
tetrad. From the point of view of parallelism description of spacetime, the
obtained torsion axial-vector is just the rotational angular velocity, which is
included in the "spin connection". Furthermore the axial-vector spin coupling
induced spin precession is just the rotation-spin(1/2) interaction predicted by
Mashhoon. Our derivation treatment is straightforward and simplified in the
geometrical meaning and physical conception, however the obtained conclusions
are consistent with that of the other previous work.Comment: 10 pages, no figur
On bare masses in time-symmetric initial-value solutions for two black holes
The Brill-Lindquist time-symmetric initial-value solution for two uncharged
black holes is rederived using the Hamiltonian constraint equation with Dirac
delta distributions as a source for the binary black-hole field. The bare
masses of the Brill-Lindquist black holes are introduced in a way which is
applied, after straightforward modification, to the Misner-Linquist binary
black-hole solution.Comment: LaTeX, 4 page
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