17,463 research outputs found
Wormholes in the accelerating universe
We discuss different arguments that have been raised against the viability of
the big trip process, reaching the conclusions that this process can actually
occur by accretion of phantom energy onto the wormholes and that it is stable
and might occur in the global context of a multiverse model. We finally argue
that the big trip does not contradict any holographic bounds on entropy and
information.Comment: 2 pages, LaTex, to appear in the Proceedings of the 11th Marcel
Grossmann Conference, 200
The silicate absorption profile in the ISM towards the heavily obscured nucleus of NGC 4418
The 9.7-micron silicate absorption profile in the interstellar medium
provides important information on the physical and chemical composition of
interstellar dust grains. Measurements in the Milky Way have shown that the
profile in the diffuse interstellar medium is very similar to the amorphous
silicate profiles found in circumstellar dust shells around late M stars, and
narrower than the silicate profile in denser star-forming regions. Here, we
investigate the silicate absorption profile towards the very heavily obscured
nucleus of NGC 4418, the galaxy with the deepest known silicate absorption
feature, and compare it to the profiles seen in the Milky Way. Comparison
between the 8-13 micron spectrum obtained with TReCS on Gemini and the larger
aperture spectrum obtained from the Spitzer archive indicates that the former
isolates the nuclear emission, while Spitzer detects low surface brightness
circumnuclear diffuse emission in addition. The silicate absorption profile
towards the nucleus is very similar to that in the diffuse ISM in the Milky Way
with no evidence of spectral structure from crystalline silicates or silicon
carbide grains.Comment: 7 Pages, 3 figures. MNRAS in pres
Worse than a big rip?
We show that a generalised phantom Chaplygin gas can present a future
singularity in a finite future cosmic time. Unlike the big rip singularity,
this singularity happens for a finite scale factor, but like the big rip
singularity, it would also take place at a finite future cosmic time. In
addition, we define a dual of the generalised phantom Chaplygin gas which
satisfies the null energy condition. Then, in a Randall-Sundrum 1 brane-world
scenario, we show that the same kind of singularity at a finite scale factor
arises for a brane filled with a dual of the generalised phantom Chaplygin gas.Comment: 6 pages, 4 figures, RevTeX 4. Discussion expanded and references
added. Version to appear in PL
Nanowires: A route to efficient thermoelectric devices
Miniaturization of electronic devices aims at manufacturing ever smaller
products, from mesoscopic to nanoscopic sizes. This trend is challenging
because the increased levels of dissipated power demands a better understanding
of heat transport in small volumes. A significant amount of the consumed energy
is transformed into heat and dissipated to the environment. Thermoelectric
materials offer the possibility to harness dissipated energy and make devices
less energy-demanding. Heat-to-electricity conversion requires materials with a
strongly suppressed thermal conductivity but still high electronic conduction.
Nanowires can meet nicely these two requirements because enhanced phonon
scattering at the surface and defects reduces the lattice thermal conductivity
while electric conductivity is not deteriorated, leading to an overall
remarkable thermoelectric efficiency. Therefore, nanowires are regarded as a
promising route to achieving valuable thermoelectric materials at the
nanoscale. In this paper, we present an overview of key experimental and
theoretical results concerning the thermoelectric properties of nanowires. The
focus of this review is put on the physical mechanisms by which the efficiency
of nanowires can be improved. Phonon scattering at surfaces and interfaces,
enhancement of the power factor by quantum effects and topological protection
of electron states to prevent the degradation of electrical conductivity in
nanowires are thoroughly discussed
Entanglement of two qubits mediated by one-dimensional plasmonic waveguides
We investigate qubit-qubit entanglement mediated by plasmons supported by
one-dimensional waveguides. We explore both the situation of spontaneous
formation of entanglement from an unentangled state and the emergence of driven
steady-state entanglement under continuous pumping. In both cases, we show that
large values for the concurrence are attainable for qubit-qubit distances
larger than the operating wavelength by using plasmonic waveguides that are
currently available.Comment: 4 pages, 4 figures. Minor Changes. Journal Reference added.
Highlighted in Physic
Dark Energy Accretion onto black holes in a cosmic scenario
In this paper we study the accretion of dark energy onto a black hole in the
cases that dark energy is equipped with a positive cosmological constant and
when the space-time is described by a Schwarzschild-de Sitter metric. It is
shown that, if confronted with current observational data, the results derived
when no cosmological constant is present are once again obtained in both cases.Comment: 7 pages, 3 figure
Influence of ruminal degradable intake protein restriction on characteristics of digestion and growth performance of feedlot cattle during the late finishing phase.
Two trials were conducted to evaluate the influence of supplemental urea withdrawal on characteristics of digestion (Trial 1) and growth performance (Trial 2) of feedlot cattle during the last 40 days on feed. Treatments consisted of a steam-flaked corn-based finishing diet supplemented with urea to provide urea fermentation potential (UFP) of 0, 0.6, and 1.2%. In Trial 1, six Holstein steers (160 ± 10 kg) with cannulas in the rumen and proximal duodenum were used in a replicated 3 × 3 Latin square experiment. Decreasing supplemental urea decreased (linear effect, P ≤ 0.05) ruminal OM digestion. This effect was mediated by decreases (linear effect, P ≤ 0.05) in ruminal digestibility of NDF and N. Passage of non-ammonia and microbial N (MN) to the small intestine decreased (linear effect, P = 0.04) with decreasing dietary urea level. Total tract digestion of OM (linear effect, P = 0.06), NDF (linear effect, P = 0.07), N (linear effect, P = 0.04) and dietary DE (linear effect, P = 0.05) decreased with decreasing urea level. Treatment effects on total tract starch digestion, although numerically small, likewise tended (linear effect, P = 0.11) to decrease with decreasing urea level. Decreased fiber digestion accounted for 51% of the variation in OM digestion. Ruminal pH was not affected by treatments averaging 5.82. Decreasing urea level decreased (linear effect, P ≤ 0.05) ruminal N-NH and blood urea nitrogen. In Trial 2, 90 crossbred steers (468 kg ± 8), were used in a 40 d feeding trial (5 steers/pen, 6 pens/ treatment) to evaluate treatment effects on final-phase growth performance. Decreasing urea level did not affect DMI, but decreased (linear effect, P ≤ 0.03) ADG, gain efficiency, and dietary NE. It is concluded that in addition to effects on metabolizable amino acid flow to the small intestine, depriving cattle of otherwise ruminally degradable N (RDP) during the late finishing phase may negatively impact site and extent of digestion of OM, depressing ADG, gain efficiency, and dietary NE
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