19,068 research outputs found
Cooling slope casting to obtain thixotropic feedstock
Thixoforming, and related semi-solid processing routes for metallic alloys, require feedstock with a non-dendritic microstructure in the semi-solid state. The material then behaves in a thixotropic way in that, when it is sheared it flows and can be forced to fill a die and, when it is allowed to stand it thickens again. The New Rheocasting (the NRC process) is a recently developed semi-solid processing route. There are two versions of this route. In one, molten alloy is poured directly into a tilted mould and, through careful temperature control during cooling, a spheroidal semi-solid microstructure is achieved. The material in the mould is then upended into a shot sleeve and hence forced into a die. Alternatively, the molten alloy is poured onto a cooling slope and thence into a mould before processing. The aim of the work described in this paper was to develop understanding of the microstructural development during the initial stages of this process. The results for pouring A356 aluminium alloy via a cooling slope into a mould are presented
Supermassive black holes or boson stars? Hair counting with gravitational wave detectors
The evidence for supermassive Kerr black holes in galactic centers is strong
and growing, but only the detection of gravitational waves will convincingly
rule out other possibilities to explain the observations. The Kerr spacetime is
completely specified by the first two multipole moments: mass and angular
momentum. This is usually referred to as the ``no-hair theorem'', but it is
really a ``two-hair'' theorem. If general relativity is the correct theory of
gravity, the most plausible alternative to a supermassive Kerr black hole is a
rotating boson star. Numerical calculations indicate that the spacetime of
rotating boson stars is determined by the first three multipole moments
(``three-hair theorem''). LISA could accurately measure the oscillation
frequencies of these supermassive objects. We propose to use these measurements
to ``count their hair'', unambiguously determining their nature and properties.Comment: 8 pages. This essay received an honorable mention in the Gravity
Research Foundation Essay Competition, 200
Plasma and testicular testosterone levels, volume density and number of Leydig cells and spermatogenic efficiency of rabbits
Plasma and tissue testosterone concentrations were determined by radioimmunoassay in 12 eight-month-old sexually mature New Zealand White rabbits and evaluated for possible associations with spermatogenic efficiency as well as with volume density and number of Leydig cells. Testicular tissue was processed histologically and histometry was performed in order to quantify germ cells, Sertoli cells and Leydig cells. Spermatogenic efficiency, reported as the ratios among germ cells (spermatogonia, primary spermatocytes and round spermatids) and by the ratio of germ cells to Sertoli cells, was not associated with testosterone levels. However, Leydig cell parameters such as number of Leydig cells per gram of testis, total number of Leydig cells per testis and percent cell volume of Leydig cell nuclei were correlated significantly with testosterone levels. The statistically significant correlation (r = 0.82, P\u3c0.05) observed between testosterone levels and the number of Leydig cells per gram of testis suggests that, in the rabbit, the latter parameter can serve as a criterion for monitoring testosterone levels in this species under normal conditions
Testing strong gravity with gravitational waves and Love numbers
The LIGO observation of GW150914 has inaugurated the gravitational-wave astronomy era and the possibility of testing gravity in extreme regimes. While distorted black holes are the most convincing sources of gravitational waves, similar signals might be produced also by other compact objects. In particular, we discuss what the gravitational-wave ringdown could tell us about the nature of the emitting object, and how measurements of the tidal Love numbers could help us in understanding the internal structure of compact dark objects
The holographic RG flow in a field theory on a curved background
As shown by Freedman, Gubser, Pilch and Warner, the RG flow in
super-Yang-Mills theory broken to an theory by the addition of a
mass term can be described in terms of a supersymmetric domain wall solution in
five-dimensional gauged supergravity. The FGPW flow is an example
of a holographic RG flow in a field theory on a flat background. Here we put
the field theory studied by Freedman, Gubser, Pilch and Warner on a curved
background, and we construct the supersymmetric domain wall solution
which describes the RG flow in this field theory. This solution is a curved
(non Ricci flat) domain wall solution. This example demonstrates that
holographic RG flows in supersymmetric field theories on a curved
background can be described in terms of curved supersymmetric domain wall
solutions.Comment: 14 pages, LaTe
Late-Time Tails of Wave Propagation in Higher Dimensional Spacetimes
We study the late-time tails appearing in the propagation of massless fields
(scalar, electromagnetic and gravitational) in the vicinities of a
D-dimensional Schwarzschild black hole. We find that at late times the fields
always exhibit a power-law falloff, but the power-law is highly sensitive to
the dimensionality of the spacetime. Accordingly, for odd D>3 we find that the
field behaves as t^[-(2l+D-2)] at late times, where l is the angular index
determining the angular dependence of the field. This behavior is entirely due
to D being odd, it does not depend on the presence of a black hole in the
spacetime. Indeed this tails is already present in the flat space Green's
function. On the other hand, for even D>4 the field decays as t^[-(2l+3D-8)],
and this time there is no contribution from the flat background. This power-law
is entirely due to the presence of the black hole. The D=4 case is special and
exhibits, as is well known, the t^[-(2l+3)] behavior. In the extra dimensional
scenario for our Universe, our results are strictly correct if the extra
dimensions are infinite, but also give a good description of the late time
behaviour of any field if the large extra dimensions are large enough.Comment: 6 pages, 3 figures, RevTeX4. Version to appear in Rapid
Communications of Physical Review
The past, present, and future of Learning Analytics
Learning Analytics (LA) is a recent research field, in which Business Intelligence and Analytics techniques are applied to learners and their contexts, with the purpose of acquiring a greater insight about the entire learning process (including outcomes). In this talk, we explore the LA landscape, delving into the definitions, techniques, challenges, and lessons learned.info:eu-repo/semantics/acceptedVersio
Entanglement versus mixedness for coupled qubits under a phase damping channel
Quantification of entanglement against mixing is given for a system of
coupled qubits under a phase damping channel. A family of pure initial joint
states is defined, ranging from pure separable states to maximally entangled
state. An ordering of entanglement measures is given for well defined initial
state amount of entanglement.Comment: 9 pages, 2 figures. Replaced with final published versio
Bent-Double Radio Sources as Probes of Intergalactic Gas
As the most common environment in the universe, groups of galaxies are likely
to contain a significant fraction of the missing baryons in the form of
intergalactic gas. The density of this gas is an important factor in whether
ram pressure stripping and strangulation affect the evolution of galaxies in
these systems. We present a method for measuring the density of intergalactic
gas using bent-double radio sources that is independent of temperature, making
it complementary to current absorption line measurements. We use this method to
probe intergalactic gas in two different environments: inside a small group of
galaxies as well as outside of a larger group at a 2 Mpc radius and measure
total gas densities of and per cubic centimeter (random and systematic
errors) respectively. We use X-ray data to place an upper limit of K on the temperature of the intragroup gas in the small group.Comment: 6 pages, 1 figure, accepted for publication in Ap
Ultrarelativistic boost of spinning black rings
We study the D=5 Emparan-Reall spinning black ring under an ultrarelativistic
boost along an arbitrary direction. We analytically determine the resulting
shock pp-wave, in particular for boosts along axes orthogonal and parallel to
the plane of rotation. The solution becomes physically more interesting and
simpler if one enforces equilibrium between the forces on the ring. We also
comment on the ultrarelativistic limit of recently found supersymmetric black
rings with two independent angular momenta. Essential distinct features with
respect to the boosted Myers-Perry black holes are pointed out.Comment: 15 pages, 2 figures. v2: added multipole expansions at spatial
infinity, and a comparison with the boosted Myers-Perry solution in a new
appendix. To appear in JHE
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