25,302 research outputs found
Graviton propagator from background-independent quantum gravity
We study the graviton propagator in euclidean loop quantum gravity, using the
spinfoam formalism. We use boundary-amplitude and group-field-theory
techniques, and compute one component of the propagator to first order, under a
number of approximations, obtaining the correct spacetime dependence. In the
large distance limit, the only term of the vertex amplitude that contributes is
the exponential of the Regge action: the other terms, that have raised doubts
on the physical viability of the model, are suppressed by the phase of the
vacuum state, which is determined by the extrinsic geometry of the boundary.Comment: 6 pages. Substantially revised second version. Improved boundary
state ansat
Muon Detection of TeV Gamma Rays from Gamma Ray Bursts
Because of the limited size of the satellite-borne instruments, it has not
been possible to observe the flux of gamma ray bursts (GRB) beyond GeV energy.
We here show that it is possible to detect the GRB radiation of TeV energy and
above, by detecting the muon secondaries produced when the gamma rays shower in
the Earth's atmosphere. Observation is made possible by the recent
commissioning of underground detectors (AMANDA, the Lake Baikal detector and
MILAGRO) which combine a low muon threshold of a few hundred GeV or less, with
a large effective area of 10^3 m^2 or more. Observations will not only provide
new insights in the origin and characteristics of GRB, they also provide
quantitative information on the diffuse infrared background.Comment: Revtex, 12 pages, 3 postscript figures, uses epsfig.st
Analysis of process variables via CFD to evaluate the performance of a FCC riser
Feedstock conversion and yield products are studied through a 3D model simulating the main reactor of the fluid catalytic cracking (FCC) process. Computational fluid dynamic (CFD) is used with Eulerian-Eulerian approach to predict the fluid catalytic cracking behavior. The model considers 12 lumps with catalyst deactivation by coke and poisoning by alkaline nitrides and polycyclic aromatic adsorption to estimate the kinetic behavior which, starting from a given feedstock, produces several cracking products. Different feedstock compositions are considered. The model is compared with sampling data at industrial operation conditions. The simulation model is able to represent accurately the products behavior for the different operating conditions considered. All the conditions considered were solved using a solver ANSYS CFX 14.0. The different operation process variables and hydrodynamic effects of the industrial riser of a fluid catalytic cracking (FCC) are evaluated. Predictions from the model are shown and comparison with experimental conversion and yields products are presented; recommendations are drawn to establish the conditions to obtain higher product yields in the industrial process
Detection of a Moving Rigid Solid in a Perfect Fluid
In this paper, we consider a moving rigid solid immersed in a potential
fluid. The fluid-solid system fills the whole two dimensional space and the
fluid is assumed to be at rest at infinity. Our aim is to study the inverse
problem, initially introduced in [3], that consists in recovering the position
and the velocity of the solid assuming that the potential function is known at
a given time. We show that this problem is in general ill-posed by providing
counterexamples for which the same potential corresponds to different positions
and velocities of a same solid. However, it is also possible to find solids
having a specific shape, like ellipses for instance, for which the problem of
detection admits a unique solution. Using complex analysis, we prove that the
well-posedness of the inverse problem is equivalent to the solvability of an
infinite set of nonlinear equations. This result allows us to show that when
the solid enjoys some symmetry properties, it can be partially detected.
Besides, for any solid, the velocity can always be recovered when both the
potential function and the position are supposed to be known. Finally, we prove
that by performing continuous measurements of the fluid potential over a time
interval, we can always track the position of the solid.Comment: 19 pages, 14 figure
Monitoring luminous yellow massive stars in M33: new yellow hypergiant candidates
The evolution of massive stars surviving the red supergiant (RSG) stage
remains unexplored due to the rarity of such objects. The yellow hypergiants
(YHGs) appear to be the warm counterparts of post-RSG classes located near the
Humphreys-Davidson upper luminosity limit, which are characterized by
atmospheric instability and high mass-loss rates. We aim to increase the number
of YHGs in M33 and thus to contribute to a better understanding of the
pre-supernova evolution of massive stars. Optical spectroscopy of five
dust-enshrouded YSGs selected from mid-IR criteria was obtained with the goal
of detecting evidence of extensive atmospheres. We also analyzed BVI photometry
for 21 of the most luminous YSGs in M33 to identify changes in the spectral
type. To explore the properties of circumstellar dust, we performed SED-fitting
of multi-band photometry of the 21 YSGs. We find three luminous YSGs in our
sample to be YHG candidates, as they are surrounded by hot dust and are
enshrouded within extended, cold dusty envelopes. Our spectroscopy of star 2
shows emission of more than one H component, as well as emission of
CaII, implying an extended atmospheric structure. In addition, the long-term
monitoring of the star reveals a dimming in the visual light curve of amplitude
larger than 0.5 mag that caused an apparent drop in the temperature that
exceeded 500 K. We suggest the observed variability to be analogous to that of
the Galactic YHG Cas. Five less luminous YSGs are suggested as post-RSG
candidates showing evidence of hot or/and cool dust emission. We demonstrate
that mid-IR photometry, combined with optical spectroscopy and time-series
photometry, provide a robust method for identifying candidate YHGs. Future
discovery of YHGs in Local Group galaxies is critical for the study of the late
evolution of intermediate-mass massive stars.Comment: 24 pages, 12 figures, 7 Tables. A&A in pres
Atomic Interferometer with Amplitude Gratings of Light and its Applications to Atom Based Tests of the Equivalence Principle
We have developed a matter wave interferometer based on the diffraction of
atoms from effective absorption gratings of light. In a setup with cold
rubidium atoms in an atomic fountain the interferometer has been used to carry
out tests of the equivalence principle on an atomic basis. The gravitational
acceleration of the two isotopes 85Rb and 87Rb was compared, yielding a
difference Dg/g =(1.2 +-1.7)x10^{-7}. We also perform a differential free fall
measurement of atoms in two different hyperfine states, and obtained a result
of Dg/g =(0.4 +-1.2)x10^{-7}.Comment: 4 Pages, 4 figures, accepted for Physical Review Letter
Prima Facie Questions in Quantum Gravity
The long history of the study of quantum gravity has thrown up a complex web
of ideas and approaches. The aim of this article is to unravel this web a
little by analysing some of the {\em prima facie\/} questions that can be asked
of almost any approach to quantum gravity and whose answers assist in
classifying the different schemes. Particular emphasis is placed on (i) the
role of background conceptual and technical structure; (ii) the role of
spacetime diffeomorphisms; and (iii) the problem of time.Comment: 20,IC/TP/0
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