40,584 research outputs found
Renormalisation of gravitational self interaction for wiggly strings
It is shown that for any elastic string model with energy density and
tension , the divergent contribution from gravitational self interaction can
be allowed for by an action renormalisation proportional to . This
formula is applied to the important special case of a bare model of the
transonic type (characterised by a constant value of the product ) that
represents the macroscopically averaged effect of shortwavelength wiggles on an
underlying microscopic model of the Nambu-Goto type (characterised by ).Comment: 11 pages, Latex; original 8 page version extended to include
estimates of relevant orders of magnitude. To be published in Physical
Review,
Symplectic structure for elastic and chiral conducting cosmic string models
This article is based on the covariant canonical formalism and corresponding
symplectic structure on phase space developed by Witten, Zuckerman and others
in the context of field theory. After recalling the basic principles of this
procedure, we construct the conserved bilinear symplectic current for generic
elastic string models. These models describe current carrying cosmic strings
evolving in an arbitrary curved background spacetime. Particular attention is
paid to the special case of the chiral string for which the worldsheet current
is null. Different formulations of the chiral string action are discussed in
detail, and as a result the integrability property of the chiral string is
clarified.Comment: 18 page
A high excitation HII region in the faint dwarf elliptical galaxy A0951+68
We present the results of BVRI imaging and optical spectroscopy of the dwarf
galaxy A0951+68. The images reveal that, although this galaxy is classified as
a dwarf elliptical, it has some properties that are similar to dwarf irregular
galaxies. It contains two bright knots of emission, one of which is red and
unresolved and the other blue and resolved. The blue knot also shows a high
excitation emission line spectrum. The observed line ratios indicate that this
is an HII region, although with some line ratios that are border-line with
those in AGN. The emission line luminosity is consistent with ionisation by a
single, very luminous O star, or several smaller O stars, but the extended blue
light in the knot shows that this has occurred as part of a substantial recent
star formation event. We find that the metal abundance, while low compared to
typical large galaxies, actually seems to be high for such a low luminosity
dwarf. The position of A0951 in the literature is incorrect and we provide the
correct value.Comment: 8 pages, Latex, 4 encapsulated postscript figures included, 1
separate JPEG figure; to be published in Monthly Notice
Cosmic Vortons and Particle Physics Constraints
We investigate the cosmological consequences of particle physics theories
that admit stable loops of superconducting cosmic string - {\it vortons}.
General symmetry breaking schemes are considered, in which strings are formed
at one energy scale and subsequently become superconducting in a secondary
phase transition at what may be a considerably lower energy scale. We estimate
the abundances of the ensuing vortons, and thereby derive constraints on the
relevant particle physics models from cosmological observations. These
constraints significantly restrict the category of admissible Grand Unified
theories, but are quite compatible with recently proposed effects whereby
superconducting strings may have been formed close to the electroweak phase
transition.Comment: 33 pages, 2 figures, RevTe
A single-board preprocessor and pulse generator
The Aeronomy Lab. of NOAA has designed and built a single board, programmable radar controller for use with VHF ST (stratosphere troposphere) radars. The controller consists of a coherent integrator preprocessor and a radar pulse generator, both of which are described, as well as interfaces to an antenna beam switch and a receiver bandwidth switch. The controller occupies a single slot in a Data General Nova of Eclipse computer. The integrator and pulse generator take advantage of high density, dual port FIFO chips such as the 512 x 9 Mostek MK 4501. These FIFOs have separate input and output ports and independent read and write cycles with cycle times of less than 200 ns, making them very fast and easy to interface. A simple block diagram of the coherent integrator is shown. The integrator is designed to handle inputs from one receiver (2 channels) with 1 sec sample spacing. The pulse generator is based on controllers designed by R. F. Woodman for the Arecibo and SOUSY radars us a recirculating memory scheme
Rigidity and stability of cold dark solid universe model
Observational evidence suggests that the large scale dynamics of the universe
is presently dominated by dark energy, meaning a non-luminous cosmological
constituent with a negative value of the pressure to density ratio ,
which would be unstable if purely fluid, but could be stable if effectively
solid with sufficient rigidity. It was suggested by Bucher and Spergel that
such a solid constituent might be constituted by an effectively cold (meaning
approximately static) distribution of cosmic strings with , or
membranes with the observationally more favoured value , but it was not
established whether the rigidity in such models actually would be sufficient
for stabilisation. The present article provides an explicit evaluation of the
rigidity to density ratio, which is shown to be given in both string and
membrane cases by , and it is confirmed that this is indeed
sufficient for stabilisation.Comment: 6 pages latex, revised version extended to include 4 figure
Recent developments in Vorton Theory
This article provides a concise overview of recent theoretical results
concerning the theory of vortons, which are defined to be (centrifugally
supported) equilibrium configurations of (current carrying) cosmic string
loops. Following a presentation of the results of work on the dynamical
evolution of small circular string loops, whose minimum energy states are the
simplest examples of vortons, recent order of magnitude estimates of the
cosmological density of vortons produced in various kinds of theoretical
scenario are briefly summarised.Comment: 6 pages Latex. Contribution to 1996 Cosmology Meeting, Peyresq,
Franc
Social capital and coping with economic shocks
"South African households live in an environment characterized by risks, and many face a significant probability of experiencing economic losses that threaten their daily subsistence. Using household panel data that include directly solicited information on economic shocks and employing household fixed-effects estimation, we explore how well households cope with shocks by examining the effects of shocks on child nutritional status. Unlike in the idealized village community, some households appear unable to insure against risk, particularly when others in their communities simultaneously suffer large losses. Households in communities with more social capital, however, seem better able to weather shocks." from Authors' Abstract
Linearized self-forces for branes
We compute the regularized force density and renormalized action due to
fields of external origin coupled to a brane of arbitrary dimension in a
spacetime of any dimension. Specifically, we consider forces generated by
gravitational, dilatonic and generalized antisymmetric form-fields. The force
density is regularized using a recently developed gradient operator. For the
case of a Nambu--Goto brane, we show that the regularization leads to a
renormalization of the tension, which is seen to be the same in both
approaches. We discuss the specific couplings which lead to cancellation of the
self-force in this case.Comment: 15 page
Entrainment coefficient and effective mass for conduction neutrons in neutron star crust: II Macroscopic treatment
Phenomena such as pulsar frequency glitches are believed to be attributable
to differential rotation of a current of ``free'' superfluid neutrons at
densities above the ``drip'' threshold in the ionic crust of a neutron star.
Such relative flow is shown to be locally describable by adaption of a
canonical two fluid treatment that emphasizes the role of the momentum
covectors constructed by differentiation of action with respect to the
currents, with allowance for stratification whereby the ionic number current
may be conserved even when the ionic charge number Z is altered by beta
processes. It is demonstrated that the gauge freedom to make different choices
of the chemical basis determining which neutrons are counted as ``free'' does
not affect their ``superfluid'' momentum covector, which must locally have the
form of a gradient (though it does affect the ``normal'' momentum covector
characterising the protons and those neutrons that are considered to be
``confined'' in the nuclei). It is shown how the effect of ``entrainment''
(whereby the momentum directions deviate from those of the currents) is
controlled by the (gauge independent) mobility coefficient K, estimated in
recent microscopical quantum mechanical investigations, which suggest that the
corresponding (gauge dependent) ``effective mass'' m* of the free neutrons can
become very large in some layers. The relation between this treatment of the
crust layers and related work (using different definitions of ``effective
mass'') intended for the deeper core layers is discussed.Comment: 21 pages Latex. Part II of article whose Part I (Simple microscopic
models) is given by nucl-th/0402057. New version extended to include figure
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