13,904 research outputs found
On the collapse in fourth order gravities
The gravitational collapse in fourth order theories of gravity defined by an
arbitrary action of the scalar curvature shows significant deviations with
General Relativity. The presence of a new scalar mode produces a higher initial
contraction that favors the reduction of the collapsing time. However,
depending on the particular model, there are fundamental differences when the
modifications to the General Relativity collapse leave the linear regime. These
analyses can be used to exclude an important region of the parameter space
associated with alternative gravitational models.Comment: 4 pages, 2 figures, Contribution to the Proceedings of Spanish
Relativity Meeting ERE2011, Madrid 201
Opposite polarity field with convective downflow and its relation to magnetic spines in a sunspot penumbra
We discuss NICOLE inversions of Fe I 630.15 nm and 630.25 nm Stokes spectra
from a sunspot penumbra recorded with the CRISP imaging spectropolarimeter on
the Swedish 1-m Solar Telescope at a spatial resolution close to 0.15". We
report on narrow radially extended lanes of opposite polarity field, located at
the boundaries between areas of relatively horizontal magnetic field (the
intra-spines) and much more vertical field (the spines). These lanes harbor
convective downflows of about 1 km/s. The locations of these downflows close to
the spines agree with predictions from the convective gap model (the "gappy
penumbra") proposed six years ago, and more recent 3D MHD simulations. We also
confirm the existence of strong convective flows throughout the entire
penumbra, showing the expected correlation between temperature and vertical
velocity, and having vertical RMS velocities of about 1.2 km/s.Comment: Accepted for publication in A&A (06-March-2013). Minor corrections
made in this version
A Simple Kinetic Model Describes the Processivity of Myosin-V
Myosin-V is a motor protein responsible for organelle and vesicle transport
in cells. Recent single-molecule experiments have shown that it is an efficient
processive motor that walks along actin filaments taking steps of mean size
close to 36 nm. A theoretical study of myosin-V motility is presented following
an approach used successfully to analyze the dynamics of conventional kinesin
but also taking some account of step-size variations. Much of the present
experimental data for myosin-V can be well described by a two-state chemical
kinetic model with three load-dependent rates. In addition, the analysis
predicts the variation of the mean velocity and of the randomness -- a
quantitative measure of the stochastic deviations from uniform, constant-speed
motion -- with ATP concentration under both resisting and assisting loads, and
indicates a {\it sub}step of size 13-14 nm (from the ATP-binding
site) that appears to accord with independent observations.Comment: 20 pages, 7 figures, to be published in Biophys. J. in 200
Comment on `Strong Vortex Liquid Correlation' from Multiterminal Measurements on Untwinned YBaCuO Single Crystals'
A.Rydh and \"O.Rapp [Phys. Rev. Lett. {\bf 86}, 1873 (2001).] claim that the
vortex liquid in untwinned YBaCuO crystals is correlated
above the melting transition, in striking contrast to previous work [D.L\'opez
{\it et al.}, Phys. Rev. Lett. {\bf 76}, 4034 (1996).]. In this Comment we
present new measurements using the same experimental technique on twinned and
untwinned YBaCuO crystals with similar overall
characteristics as those reported by Rydh and Rapp . The comparison of the
vortex correlation response in both cases indicates that the central conclusion
of their work is not correct. Our results reconfirm the work by L\'opez {\it et
al.} and points on the origin of the misinterpretation in the work of Rydh and
Rapp.Comment: comment on A.Rydh and \"O.Rapp, Phys. Rev. Lett. {\bf 86}, 1873
(2001). accepted in Phys. Rev. Let
Generation of two-photon states with arbitrary degree of entanglement via nonlinear crystal superlattices
We demonstrate a general method of engineering the joint quantum state of
photon pairs produced in spontaneous parametric downconversion (PDC). The
method makes use of a superlattice structure of nonlinear and linear materials,
in conjunction with a broadband pump, to manipulate the group delays of the
signal and idler photons relative to the pump pulse, and realizes a joint
spectral amplitude with arbitrary degree of entanglement for the generated
pairs. This method of group delay engineering has the potential of synthesizing
a broad range of states including factorizable states crucial for quantum
networking and states optimized for Hong-Ou-Mandel interferometry. Experimental
results for the latter case are presented, illustrating the principles of this
approach.Comment: 4 pages, 4 figures, accepted Phys. Rev. Let
Gimnasia rÃtmica deportiva, ¿una práctica excluida o excluyente?
Contiene: 1-Gimnasia RÃtmica Deportiva, su concepción y su lógica; 2- Esbozo de reseña histórica en el mundo y en Argentina; 3-Su lenguaje a partir de su reglamentación y código de puntuación; 4- Su práctica a partir de la lógica gÃmnica; 5- El cuerpo desde la mirada del alto rendimiento; 6-La construcción de cierta feminidadPanel: "Discursos y Practicas Corporales"Departamento de Educación FÃsic
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