828 research outputs found
Classical radiation by free-falling charges in de Sitter spacetime
We study the classical radiation emitted by free-falling charges in de Sitter
spacetime coupled to different kinds of fields. Specifically we consider the
cases of the electromagnetic field, linearized gravity and scalar fields with
arbitrary mass and curvature coupling. Given an arbitrary set of such charges,
there is a generic result for sufficiently late times which corresponds to each
charge being surrounded by a field zone with negligible influence from the
other charges. Furthermore, we explicitly find a static solution in the static
patch adapted to a charge (implying no energy loss by the charge) which can be
regularly extended beyond the horizon to the full de Sitter spacetime, and show
that any other solution decays at late times to this one. On the other hand,
for non-conformal scalar fields the inertial observers naturally associated
with spatially flat coordinates will see a non-vanishing flux far from the
horizon, which will fall off more slowly than the inverse square of the
distance for sufficiently light fields (m^2 + \xi R < 5H^2/4) and give rise to
a total integrated flux that grows unboundedly with the radius. This can be
qualitatively interpreted as a consequence of a classical parametric
amplification of the field generated by the charge due to the time-dependent
background spacetime. Most of these results do not hold for massless minimally
coupled scalar fields, whose special behavior is analyzed separately.Comment: 31 pages, REVTeX4, minor changes, one reference added, version to
appear in Phys. Rev.
Quantum Noise and Fluctuations in Gravitation and Cosmology
We give a short update of our research program on nonequilibrium statistical
field theory applied to quantum processes in the early universe and black
holes, as well as the development of stochastic gravity theory as an extension
of semiclassical gravity and an intermediary in the 'bottom-up' approach to
quantum gravity.Comment: 16 pages Latex; small changes in a couple of footnote
Double Bragg diffraction: A tool for atom optics
The use of retro-reflection in light-pulse atom interferometry under
microgravity conditions naturally leads to a double-diffraction scheme. The two
pairs of counterpropagating beams induce simultaneously transitions with
opposite momentum transfer that, when acting on atoms initially at rest, give
rise to symmetric interferometer configurations where the total momentum
transfer is automatically doubled and where a number of noise sources and
systematic effects cancel out. Here we extend earlier implementations for Raman
transitions to the case of Bragg diffraction. In contrast with the
single-diffraction case, the existence of additional off-resonant transitions
between resonantly connected states precludes the use of the adiabatic
elimination technique. Nevertheless, we have been able to obtain analytic
results even beyond the deep Bragg regime by employing the so-called "method of
averaging," which can be applied to more general situations of this kind. Our
results have been validated by comparison to numerical solutions of the basic
equations describing the double-diffraction process.Comment: 26 pages, 20 figures; minor changes to match the published versio
Si3N4 single-crystal nanowires grown from silicon micro and nanoparticles near the threshold of passive oxidation
A simple and most promising oxide-assisted catalyst-free method is used to
prepare silicon nitride nanowires that give rise to high yield in a short time.
After a brief analysis of the state of the art, we reveal the crucial role
played by the oxygen partial pressure: when oxygen partial pressure is slightly
below the threshold of passive oxidation, a high yield inhibiting the formation
of any silica layer covering the nanowires occurs and thanks to the synthesis
temperature one can control nanowire dimensions
Selfconsistent hybridization expansions for static properties of the Anderson impurity model
By means of a projector-operator formalism we derive an approximation based
on a self consistent hybridization expansion to study the ground state
properties of the Anderson Impurity model. We applied the approximation to the
general case of finite Coulomb repulsion , extending previous work with the
same formalism in the infinite- case. The treatment provides a very accurate
calculation of the ground state energy and their related zero temperature
properties in the case in which is large enough, but still finite, as
compared with the rest of energy scales involved in the model. The results for
the valence of the impurity are compared with exact results that we obtain from
equations derived using the Bethe ansatz and with a perturbative approach. The
magnetization and magnetic susceptibility is also compared with Bethe ansatz
results. In order to do this comparison, we also show how to regularize the
Bethe ansatz integral equations necessary to calculate the impurity valence,
for arbitrary values of the parameters.Comment: 8 pages, 5 figure
Feed preference in pigs: Relationship between cereal preference and nutrient composition and digestibility
The present work aimed to identify the nutritional characteristics that best explain cereal feed preference in pigs. A total of 25 cereals of known preference (at 60% of inclusion in complete feed) from a previous study were evaluated. The cereals were analyzed for DM, OM, crude fiber, ether extract, CP, GE, digestible starch, and glycemic index. Additionally, for 12 of the cereals, complete feeds (the same composition as those previously used to measure preference) were prepared, analyzed for DM, OM, CP, and starch, and fed to pigs (33 ± 5.1 kg BW) fitted with ileal T-cannulae to assess the apparent ileal (AID) and total tract digestibility (ATTD) of these nutrients using titanium dioxide as an indigestible marker. The relationships among the different energy and nutrient contents were studied by principal component (PC) analysis, and the correlations between the generated PC scores and cereal preference were analyzed. A correlation between preference and the second PC obtained with data of the 25 cereals was observed (P < 0.01), which indicated that crude fiber (negatively) and digestible starch, OM, and glycemic index (positively) were correlated with feed preference. Statistically significant linear relationships with preference were confirmed for crude fiber, digestible starch, and glycemic index (R = 0.38, 0.36, and 0.23, respectively; P < 0.02). Similarly, the first PC obtained with data of the 12 feeds also correlated with preference (P < 0.01), indicating that the digestible nutrients (positively) and the nondigestible nutrients (negatively) were correlated with preference. Statistically significant relationships with preference were observed for the contents of starch (total, digestible AID, and digestible ATTD: R = 0.62, 0.66, and 0.63, respectively; P < 0.01), AID DM (digestible and nondigestible: R = 0.41 and 0.44, respectively; P < 0.05), ATTD DM (digestible and nondigestible: R = 0.67 and 0.70, respectively; P < 0.01), AID OM (digestible and nondigestible: R = 0.45 and 0.43, respectively; P < 0.05), and ATTD OM (digestible and nondigestible: R = 0.64 and 0.66, respectively;
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