7,177 research outputs found
Energy management of three-dimensional minimum-time intercept
A real-time computer algorithm to control and optimize aircraft flight profiles is described and applied to a three-dimensional minimum-time intercept mission
Energy conditions in f(R) gravity and Brans-Dicke theories
The equivalence between f(R) gravity and scalar-tensor theories is invoked to
study the null, strong, weak and dominant energy conditions in Brans-Dicke
theory. We consider the validity of the energy conditions in Brans-Dicke theory
by invoking the energy conditions derived from a generic f(R) theory. The
parameters involved are shown to be consistent with an accelerated expanding
universe.Comment: 9 pages, 1 figure, to appear in IJMP
Global Modeling of Spur Formation in Spiral Galaxies
We investigate the formation of substructure in spiral galaxies using global
MHD simulations, including gas self-gravity. Our models extend previous local
models by Kim and Ostriker (2002) by including the full effects of curvilinear
coordinates, a realistic log-spiral perturbation, self-gravitational
contribution from 5 radial wavelengths of the spiral shock, and variation of
density and epicyclic frequency with radius. We show that with realistic Toomre
Q values, self-gravity and galactic differential rotation produce filamentary
gaseous structures with kpc-scale separations, regardless of the strength -- or
even presence -- of a stellar spiral potential. However, the growth of sheared
features distinctly associated with the spiral arms, described as spurs or
feathers in optical and IR observations of many spiral galaxies, requires a
sufficiently strong spiral potential in self gravitating models. Unlike
independently-growing ''background'' filaments, the orientation of arm spurs
depends on galactic location. Inside corotation, spurs emanate outward, on the
convex side of the arm; outside corotation, spurs grow inward, on the concave
side of the arm. Based on spacing, orientation, and the relation to arm clumps,
it is possible to distinguish ''true spurs'' that originate as instabilities in
the spiral arms from independently growing ''background'' filaments. Our models
also suggest that magnetic fields are important in preserving grand design
spiral structure when gas in the arms fragments via self-gravity into GMCs.Comment: 36 pages, 17 figures, Accepted for publication in ApJ. PDF version
with high resolution figures available at
http://www.astro.umd.edu/~shetty/Research
Lovelock Thin-Shell Wormholes
We construct the asymptotically flat charged thin-shell wormholes of Lovelock
gravity in seven dimensions by cut-and-paste technique, and apply the
generalized junction conditions in order to calculate the energy-momentum
tensor of these wormholes on the shell. We find that for negative second order
and positive third order Lovelock coefficients, there are thin-shell wormholes
that respect the weak energy condition. In this case, the amount of normal
matter decreases as the third order Lovelock coefficient increases. For
positive second and third order Lovelock coefficients, the weak energy
condition is violated and the amount of exotic matter decreases as the charge
increases. Finally, we perform a linear stability analysis against a symmetry
preserving perturbation, and find that the wormholes are stable provided the
derivative of surface pressure density with respect to surface energy density
is negative and the throat radius is chosen suitable.Comment: 13 pages, 6 figure
A Lemaitre-Tolman-Bondi cosmological wormhole
We present a new analytical solution of the Einstein field equations
describing a wormhole shell of zero thickness joining two
Lema{\i}tre-Tolman-Bondi universes, with no radial accretion. The material on
the shell satisfies the energy conditions and, at late times, the shell becomes
comoving with the dust-dominated cosmic substratum.Comment: 5 pages, latex, no figures, to appear in Phys. Rev.
Acoustic horizons for axially and spherically symmetric fluid flow
We investigate the formation of acoustic horizons for an inviscid fluid
moving in a pipe in the case of stationary and axi-symmetric flow. We show
that, differently from what is generally believed, the acoustic horizon forms
in correspondence of either a local minimum or maximum of the flux tube
cross-section. Similarly, the external potential is required to have either a
maximum or a minimum at the horizon, so that the external force has to vanish
there. Choosing a power-law equation of state for the fluid, , we solve the equations of the fluid dynamics and show that the two
possibilities are realized respectively for and . These results
are extended also to the case of spherically symmetric flow.Comment: 6 pages, 3 figure
2-(Ammoniomethyl)pyridinium sulfate monohydrate
In the crystal of the title hydrated molecular salt, C6H10N2
2+·SO4
2−·H2O, N—H⋯O and O—H⋯O hydrogen bonds link the molecules into layers parallel to the ab plane. C—H⋯O hydrogen bonds are observed both within these layers and between molecules and ions in adjacent layers
van Vleck determinants: traversable wormhole spacetimes
Calculating the van Vleck determinant in traversable wormhole spacetimes is
an important ingredient in understanding the physical basis behind Hawking's
chronology protection conjecture. This paper presents extensive computations of
this object --- at least in the short--throat flat--space approximation. An
important technical trick is to use an extension of the usual junction
condition formalism to probe the full Riemann tensor associated with a thin
shell of matter. Implications with regard to Hawking's chronology protection
conjecture are discussed. Indeed, any attempt to transform a single isolated
wormhole into a time machine results in large vacuum polarization effects
sufficient to disrupt the internal structure of the wormhole before the onset
of Planck scale physics, and before the onset of time travel. On the other
hand, it is possible to set up a putative time machine built out of two or more
wormholes, each of which taken in isolation is not itself a time machine. Such
``Roman configurations'' are much more subtle to analyse. For some particularly
bizarre configurations (not traversable by humans) the vacuum polarization
effects can be arranged to be arbitrarily small at the onset of Planck scale
physics. This indicates that the disruption scale has been pushed down into the
Planck slop. Ultimately, for these configurations, questions regarding the
truth or falsity of Hawking's chronology protection can only be addressed by
entering the uncharted wastelands of full fledged quantum gravity.Comment: 42 pages, ReV_TeX 3.
From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture
The recent interest in ``time machines'' has been largely fueled by the
apparent ease with which such systems may be formed in general relativity,
given relatively benign initial conditions such as the existence of traversable
wormholes or of infinite cosmic strings. This rather disturbing state of
affairs has led Hawking to formulate his Chronology Protection Conjecture,
whereby the formation of ``time machines'' is forbidden. This paper will use
several simple examples to argue that the universe appears to exhibit a
``defense in depth'' strategy in this regard. For appropriate parameter regimes
Casimir effects, wormhole disruption effects, and gravitational back reaction
effects all contribute to the fight against time travel. Particular attention
is paid to the role of the quantum gravity cutoff. For the class of model
problems considered it is shown that the gravitational back reaction becomes
large before the Planck scale quantum gravity cutoff is reached, thus
supporting Hawking's conjecture.Comment: 43 pages,ReV_TeX,major revision
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