365,777 research outputs found
Bianchi type I universes with dilaton and magnetic fields
We consider the dynamics of a Bianchi type I spacetime in the presence of
dilaton and magnetic fields. The general solution of the Einstein-Maxwell
dilaton field equations can be obtained in an exact parametric form. Depending
on the numerical values of the parameters of the model there are three distinct
classes of solutions. The time evolution of the mean anisotropy, shear and
deceleration parameter is considered in detail and it is shown that a
magnetic-dilaton anisotropic Bianchi type I geometry does not isotropize, the
initial anisotropy being present in the universe for all times.Comment: 10 pages, 4 figure
Quasi-local energy and the choice of reference
A quasi-local energy for Einstein's general relativity is defined by the
value of the preferred boundary term in the covariant Hamiltonian formalism.
The boundary term depends upon a choice of reference and a time-like
displacement vector field (which can be associated with an observer) on the
boundary of the region. Here we analyze the spherical symmetric cases. For the
obvious analytic choice of reference based on the metric components, we find
that this technique gives the same quasi-local energy values using several
standard coordinate systems and yet can give different values in some other
coordinate systems. For the homogeneous-isotropic cosmologies, the energy can
be non-positive, and one case which is actually flat space has a negative
energy. As an alternative, we introduce a way to determine the choice of both
the reference and displacement by extremizing the energy. This procedure gives
the same value for the energy in different coordinate systems for the
Schwarzschild space, and a non-negative value for the cosmological models, with
zero energy for the dynamic cosmology which is actually Minkowski space. The
timelike displacement vector comes out to be the dual mean curvature vector of
the two-boundary.Comment: 21 pages; revised version to appear in CQ
Study of the Wealth Inequality in the Minority Game
To demonstrate the usefulness of physical approaches for the study of
realistic economic systems, we investigate the inequality of players' wealth in
one of the most extensively studied econophysical models, namely, the minority
game (MG). We gauge the wealth inequality of players in the MG by a well-known
measure in economics known as the modified Gini index. From our numerical
results, we conclude that the wealth inequality in the MG is very severe near
the point of maximum cooperation among players, where the diversity of the
strategy space is approximately equal to the number of strategies at play. In
other words, the optimal cooperation between players comes hand in hand with
severe wealth inequality. We also show that our numerical results in the
asymmetric phase of the MG can be reproduced semi-analytically using a replica
method.Comment: 9 pages in revtex 4 style with 3 figures; minor revision with a
change of title; to appear in PR
Quasi-local energy for cosmological models
First we briefly review our covariant Hamiltonian approach to quasi-local
energy, noting that the Hamiltonian-boundary-term quasi-local energy
expressions depend on the chosen boundary conditions and reference
configuration. Then we present the quasi-local energy values resulting from the
formalism applied to homogeneous Bianchi cosmologies. Finally we consider the
quasi-local energies of the FRW cosmologies. Our results do not agree with
certain widely accepted quasi-local criteria.Comment: Contributed to International Symposium on Cosmology and Particle
Astrophysics (CosPA 2006), Taipei, Taiwan, 15-17 Nov 200
Dirac Leptogenesis with a Non-anomalous Family Symmetry
We propose a model for Dirac leptogenesis based on a non-anomalous
gauged family symmetry. The anomaly cancellation conditions are
satisfied with no new chiral fermions other than the three right-handed
neutrinos, giving rise to stringent constraints among the charges. Realistic
masses and mixing angles are obtained for all fermions. The model predicts
neutrinos of the Dirac type with naturally suppressed masses. Dirac
leptogenesis is achieved through the decay of the flavon fields. The cascade
decays of the vector-like heavy fermions in the Froggatt-Nielsen mechanism play
a crucial role in the separation of the primodial lepton numbers. We find that
a large region of parameter space of the model gives rise to a sufficient
cosmological baryon number asymmetry through Dirac leptogenesis.Comment: 8 pages, 8 figures, version to appear in JHE
Nonlinear analysis of phased-locked loops with rapidly varying phase
The performance of command and telemetry systems, useful in deep-space communications, is frequently affected by the radio-frequency phase error which is introduced at the point of reception by means of the carrier tracking loop. In low data rate communications, this phase error may vary rapidly over the duration of the signaling interval. In this paper such phase variation is characterized by a sinusoidal input phase, K sin (omega sub o t+, pi/6), which models a typical phase variation in communication over turbulent media. Conditions for synchronization stability and the acquisition behavior are examined by detailed computer study of the phase-plane trajectories for the second and third-order loops with perfect integrator. It is demonstrated that for the phase variation considered the third-order loop has no real advantage over the second-order loop. Finally, it is shown that nonzero initial conditions may result in large steady-state phase error
Allowable silicon wafer thickness versus diameter for ingot rotation ID wafering
Inner diameter (ID) wafering of ingot rotation reduce the ID saw blade diameter was investigated. The blade thickness can be reduced, resulting in minimal kerf loss. However, significant breakage of wafers occurs during the rotation wafering as the wafer thickness decreases. Fracture mechanics was used to develop an equation relating wafer thickness, diameter and fracture behavior at the point of fracture by using a model of a wafer, supported by a center column and subjected to a cantilever force. It is indicated that the minimum allowable wafer thickness does not increase appreciably with increasing wafer diameter and that fracture through the thickness rather than through the center supporting column limits the minimum allowable wafer thickness. It is suggested that the minimum allowable wafer thickness can be reduced by using a vacuum chuck on the wafer surface to enhance cleavage fracture of the center core and by using 111 ingots
The Massive Disk Around OH 231.8+4.2
We have obtained 11.7 micron and 17.9 micron images at the Keck I telescope
of the circumstellar dust emission from OH 231.8+4.2, an evolved mass-losing
red giant with a well studied bipolar outflow. We detect both a central
unresolved point source and extended emission which is aligned with the bipolar
outflow seen on larger scales. We find that the unresolved central source can
be explained by an opaque, flared disk with an outer radius near 300 AU and an
outer temperature of about 130 K. One possible model to explain this flaring is
that the material in the disk is orbiting the central star and not simply
undergoing a radial expansion.Comment: ApJ, in pres
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