28,337 research outputs found
Generalized virial theorem in Palatini gravity
We use the collision-free Boltzmann equation in Palatini
gravity to derive the virial theorem within the context of the Palatini
approach. It is shown that the virial mass is proportional to certain
geometrical terms appearing in the Einstein field equations which contribute to
gravitational energy and that such geometric mass can be attributed to the
virial mass discrepancy in cluster of galaxies. We then derive the velocity
dispersion relation for clusters followed by the metric tensor components
inside the cluster as well as the lagrangian in terms of the
observational parameters. Since these quantities may also be obtained
experimentally, the virial theorem is a convenient tool to
test the viability of theories in different models. Finally,
we discuss the limitations of our approach in the light of the cosmological
averaging used and questions that have been raised in the literature against
such averaging procedures in the context of the present work.Comment: 16 pages, to appear in PR
STOL Simulation Requirements for Development of Integrated Flight/propulsion Control Systems
The role and use of simulation as a design tool in developing integrated systems where design criteria is largely unavailable is well known. This paper addresses additional simulation needs for the development of Integrated Flight/Propulsion Control Systems (IFPCS) which will improve the probability of properly interpreting simulation results. These needs are based on recent experience with power approach flying qualities evaluations of an advanced fighter configuration which incorporated Short Takeoff and Landing (STOL) technologies and earlier experiences with power approach flying qualities evaluations on the AFTI/F-16 program. The use of motion base platforms with axial and normal degrees of freedom will help in evaluating pilot coupling and workload in the presence of high frequency low amplitude axial accelerations produced by high bandwidth airspeed controllers in a gusty environment
Investigating the cores of fossil systems with Chandra
We investigate the cores of fossil galaxy groups and clusters (`fossil
systems') using archival Chandra data for a sample of 17 fossil systems. We
determined the cool-core fraction for fossils via three observable diagnostics,
the central cooling time, cuspiness, and concentration parameter. We quantified
the dynamical state of the fossils by the X-ray peak/brightest cluster galaxy
(BCG), and the X-ray peak/emission weighted centre separations. We studied the
X-ray emission coincident with the BCG to detect the presence of potential
thermal coronae. A deprojection analysis was performed for z < 0.05 fossils to
obtain cooling time and entropy profiles, and to resolve subtle temperature
structures. We investigated the Lx-T relation for fossils from the 400d
catalogue to see if the scaling relation deviates from that of other groups.
Most fossils are identified as cool-core objects via at least two cool-core
diagnostics. All fossils have their dominant elliptical galaxy within 50 kpc of
the X-ray peak, and most also have the emission weighted centre within that
distance. We do not see clear indications of a X-ray corona associated with the
BCG unlike that has been observed for some other objects. Fossils do not have
universal temperature profiles, with some low-temperature objects lacking
features that are expected for ostensibly relaxed objects with a cool-core. The
entropy profiles of the z < 0.05 fossil systems can be well-described by a
power law model, albeit with indices smaller than 1. The 400d fossils Lx-T
relation shows indications of an elevated normalisation with respect to other
groups, which seems to persist even after factoring in selection effects.Comment: Accepted for publication in Astronomy and Astrophysic
Is dark matter an extra-dimensional effect?
We investigate the possibility that the observed behavior of test particles
outside galaxies, which is usually explained by assuming the presence of dark
matter, is the result of the dynamical evolution of particles in higher
dimensional space-times. Hence, dark matter may be a direct consequence of the
presence of an extra force, generated by the presence of extra-dimensions,
which modifies the dynamic law of motion, but does not change the intrinsic
properties of the particles, like, for example, the mass (inertia). We discuss
in some detail several possible particular forms for the extra force, and the
acceleration law of the particles is derived. Therefore, the constancy of the
galactic rotation curves may be considered as an empirical evidence for the
existence of the extra dimensions.Comment: 11 pages, no figures, accepted for publication in MPLA; references
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Silicon nitride-aluminum oxide solid solution (SiAION) formation and densification by pressure sintering
Stirred-ball-mill-blended Si3N4 and Al2O3 powders were pressure sintered in order to investigate the mechanism of solid solution formation and densification in the Si3N4-Al2O3 system. Powder blends with Si3N4:Al2O3 mole ratios of 4:1, 3:2, and 2:3 were pressure sintered at 27.6-MN/sq m pressure at temperatures to 17000 C (3090 F). The compaction behavior of the powder blends during pressure sintering was determined by observing the density of the powder compact as a function of temperature and time starting from room temperature. This information, combined with the results of X-ray diffraction and metallographic analyses regarding solutioning and phase transformation phenomena in the Si3N4-Al2O3 system, was used to describe the densification behavior
Brane- gravity and dark matter
The collision-free Boltzmann equation is used in the context of brane-
gravity to derive the virial theorem. It is shown that the virial mass is
proportional to certain geometrical terms appearing in the Einstein field
equations and contributes to gravitational energy and that such a geometric
mass can be attributed to the virial mass discrepancy in a cluster of galaxies.
In addition, the galaxy rotation curves are studied by utilizing the concept of
conformal symmetry and notion of conformal Killing symmetry. The field
equations may then be obtained in an exact parametric form in terms of the
parameter representing the conformal factor. This provides the possibility of
studying the behavior of the angular velocity of a test particle moving in a
stable circular orbit. The tangential velocity can be derived as a function of
the conformal factor and integration constants, resulting in a constant value
at large radial distances. Relevant phenomenon such as the deflection of light
passing through a region where the rotation curves are flat and the radar echo
delay are also studied.Comment: 12 pages, 2 figures, to appear in PR
Using permeable membranes to produce hydrogen and oxygen from water
Concept may make it profitable to obtain hydrogen fuel from water. Laboratory tests have demonstrated that method enables decomposition of water several orders of magnitude beyond equilibrium state where only small amounts of free hydrogen are present
The virial theorem and the dynamics of clusters of galaxies in the brane world models
A version of the virial theorem, which takes into account the effects of the
non-compact extra-dimensions, is derived in the framework of the brane world
models. In the braneworld scenario, the four dimensional effective Einstein
equation has some extra terms, called dark radiation and dark pressure,
respectively, which arise from the embedding of the 3-brane in the bulk. To
derive the generalized virial theorem we use a method based on the
collisionless Boltzmann equation. The dark radiation term generates an
equivalent mass term (the dark mass), which gives an effective contribution to
the gravitational energy. This term may account for the well-known virial
theorem mass discrepancy in actual clusters of galaxies. An approximate
solution of the vacuum field equations on the brane, corresponding to weak
gravitational fields, is also obtained, and the expressions for the dark
radiation and dark mass are derived. The qualitative behavior of the dark mass
is similar to that of the observed virial mass in clusters of galaxies. We
compare our model with the observational data for galaxy clusters, and we
express all the physical parameters of the model in terms of observable
quantities. In particular, we predict that the dark mass must extend far beyond
the presently considered virial radius. The behavior of the galaxy cluster
velocity dispersion in brane world models is also considered. Therefore the
study of the matter distribution and velocity dispersion at the extragalactic
scales could provide an efficient method for testing the multi-dimensional
physical models.Comment: 29 pages, no figures, accepted for publication in PR
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