1,306 research outputs found
Cosmic Strings Lens Phenomenology: Model of Poisson Energy Distribution
We present a novel approach for investigating lens phenomenology of cosmic
strings in order to elaborate detection strategies in galaxy deep field images.
To account for the complexity of the projected energy distribution of string
networks we assume their lens effects to be similar to those of a straight
string carrying a {\em random} lineic energy distribution. In such a model we
show that, unlike the case of uniform strings, critical phenomena naturally
appear. We explore the properties of the critical lines and caustics. In
particular, assuming that the energy coherence length along the string is much
smaller than the observation scale, we succeeded in computing the total length
of critical lines per unit string length and found it to be . The length of the associated caustic lines can also be computed to be
. The picture we obtain here for the
phenomenology of cosmic string detection is clearly at variance with common
lore.Comment: 10 pages, 5 figures. Minor correction
Governance and Power Dynamics in a Small-Scale Hilsa Shad (Tenualosa ilisha) Fishery : A Case Study from Bangladesh
This paper considers the hilsa shad (Tenualosa ilisha) fishery of southern Bangladesh as a case study regarding governance and power dynamics at play in a small-scale fishery, and the relevance of these for the sustainable management of coastal fisheries. Qualitative methods, involving in-depth individual interviews (n = 128) and focus group discussions (n = 8) with key stakeholders in the hilsa fishery, were used to capture multiple perspectives on governance from those in different positions in the relative power structures studied, while facilitating insightful discussions and reflections. The analysis here is based on a power cube framework along three power dimensions (levels, spaces, and forms) in Bangladesh's hilsa fishery. The study displays an imbalance in the present hilsa governance structure, with some stakeholders exercising more power than others, sidelining small-scale fishers, and encouraging increasing illegal fishing levels that ultimately harm both the fisheries and those dependent on them. To overcome this, we propose a co-management system that can play a vital role in equalizing power asymmetry among hilsa fishery stakeholders and ensure effective hilsa fishery governance. Our results suggest that recognizing analyzed power dynamics has substantial implications for the planning and implementation of such co-management and the long-term sustainability of the hilsa fishery.Peer reviewe
Estimating Self-Sustainability in Peer-to-Peer Swarming Systems
Peer-to-peer swarming is one of the \emph{de facto} solutions for distributed
content dissemination in today's Internet. By leveraging resources provided by
clients, swarming systems reduce the load on and costs to publishers. However,
there is a limit to how much cost savings can be gained from swarming; for
example, for unpopular content peers will always depend on the publisher in
order to complete their downloads. In this paper, we investigate this
dependence. For this purpose, we propose a new metric, namely \emph{swarm
self-sustainability}. A swarm is referred to as self-sustaining if all its
blocks are collectively held by peers; the self-sustainability of a swarm is
the fraction of time in which the swarm is self-sustaining. We pose the
following question: how does the self-sustainability of a swarm vary as a
function of content popularity, the service capacity of the users, and the size
of the file? We present a model to answer the posed question. We then propose
efficient solution methods to compute self-sustainability. The accuracy of our
estimates is validated against simulation. Finally, we also provide closed-form
expressions for the fraction of time that a given number of blocks is
collectively held by peers.Comment: 27 pages, 5 figure
Selective ion sieving through arrays of sub-nanometer nanopores in chemically tunable 2D carbon membranes
Two-dimensional (2D) membranes featuring arrays of sub-nanometer pores have applications in purification, solvent separation and water desalination. Compared to channels in bulk membranes, 2D nanopores have lower resistance to transmembrane transport, leading to faster passage of ions. However, the formation of nanopores in 2D membranes requires expensive post-treatment using plasma or ion bombardment. Here, we study bottom-up synthesized porous carbon nanomembranes (CNMs) of biphenyl thiol (BPT) precursors. Sub-nanometer pores arise intrinsically during the BPT-CNM synthesis with a density of 2 ± 1 pore per 100 nm2. We employ BPT-CNM based pore arrays as efficient ion sieving channels, and demonstrate selectivity of the membrane towards ion transport when exposed to a range of concentration gradients of KCl, CsCl and MgCl2. The selectivity of the membrane towards K+ over Cl− ions is found be 16.6 mV at a 10 : 1 concentration ratio, which amounts to ∼30% efficiency relative to the Nernst potential for complete ion rejection. The pore arrays in the BPT-CNM show similar transport and selectivity properties to graphene and carbon nanotubes, whilst the fabrication method via self-assembly offers a facile means to control the chemical and physical properties of the membrane, such as surface charge, chemical nature and pore density. CNMs synthesized from self-assembled monolayers open the way towards the rational design of 2D membranes for selective ion sieving
Photon-axion conversion in intergalactic magnetic fields and cosmological consequences
Photon-axion conversion induced by intergalactic magnetic fields causes an
apparent dimming of distant sources, notably of cosmic standard candles such as
supernovae of type Ia (SNe Ia). We review the impact of this mechanism on the
luminosity-redshift relation of SNe Ia, on the dispersion of quasar spectra,
and on the spectrum of the cosmic microwave background. The original idea of
explaining the apparent dimming of distant SNe Ia without cosmic acceleration
is strongly constrained by these arguments. However, the cosmic equation of
state extracted from the SN Ia luminosity-redshift relation remains sensitive
to this mechanism. For example, it can mimic phantom energy.Comment: (14 pages, 9 eps figures) Contribution to appear in a volume of
Lecture Notes in Physics (Springer-Verlag) on Axion
Dark soliton states of Bose-Einstein condensates in anisotropic traps
Dark soliton states of Bose-Einstein condensates in harmonic traps are
studied both analytically and computationally by the direct solution of the
Gross-Pitaevskii equation in three dimensions. The ground and self-consistent
excited states are found numerically by relaxation in imaginary time. The
energy of a stationary soliton in a harmonic trap is shown to be independent of
density and geometry for large numbers of atoms. Large amplitude field
modulation at a frequency resonant with the energy of a dark soliton is found
to give rise to a state with multiple vortices. The Bogoliubov excitation
spectrum of the soliton state contains complex frequencies, which disappear for
sufficiently small numbers of atoms or large transverse confinement. The
relationship between these complex modes and the snake instability is
investigated numerically by propagation in real time.Comment: 11 pages, 8 embedded figures (two in color
Homegarden commercialization: extent, household characteristics, and effect on food security and food sovereignty in Rural Indonesia
Homegardens have long been recognized for contributing to household food security, nutritional status, and ecological sustainability in especially poor, rural areas in low-income countries. However, as markets and policies drive the commercialization of food and farming systems, and of rural livelihoods in general, it becomes increasingly difficult for small-holder farmers to maintain homegarden plots. Rather than autonomous spaces to grow food for self-consumption, farmers are transforming the land around their dwellings into an income-generating space by planting commercial crops for sale in urban and processing markets. The objective of this study was to examine homegarden commercialization in the Upper Citarum Watershed of West Java, Indonesia, and its effects on food security and food sovereignty. We employed a mixed-method approach to survey 81 village households involved in agricultural production. For quantitative analysis, we calculated a “homegarden commercialization index,” and developed indicator frameworks to examine relationships between commercialization, household food security, and food-related decision-making. Accompanied by insights from qualitative interviews, our results show that homegardens are highly commercialized, which contributes to the spread of monocultural production in the region. We argue that homegardens should be included and supported in food, agricultural, health, environmental, and rural development policy, in Indonesia and generally
The Galactic Halo in Mixed Dark Matter Cosmologies
A possible solution to the small scale problems of the cold dark matter (CDM)
scenario is that the dark matter consists of two components, a cold and a warm
one. We perform a set of high resolution simulations of the Milky Way halo
varying the mass of the WDM particle () and the cosmic dark matter
mass fraction in the WDM component (). The scaling ansatz
introduced in combined analysis of LHC and astroparticle searches postulates
that the relative contribution of each dark matter component is the same
locally as on average in the Universe (e.g. ). Here we find however, that the normalised local WDM fraction ( / ) depends strongly on for 1 keV. Using the scaling ansatz can therefore introduce significant
errors into the interpretation of dark matter searches. To correct this issue a
simple formula that fits the local dark matter densities of each component is
provided.Comment: 19 pages, 10 figures, accepted for publication in JCA
Investigating the topology of interacting networks - Theory and application to coupled climate subnetworks
Network theory provides various tools for investigating the structural or
functional topology of many complex systems found in nature, technology and
society. Nevertheless, it has recently been realised that a considerable number
of systems of interest should be treated, more appropriately, as interacting
networks or networks of networks. Here we introduce a novel graph-theoretical
framework for studying the interaction structure between subnetworks embedded
within a complex network of networks. This framework allows us to quantify the
structural role of single vertices or whole subnetworks with respect to the
interaction of a pair of subnetworks on local, mesoscopic and global
topological scales.
Climate networks have recently been shown to be a powerful tool for the
analysis of climatological data. Applying the general framework for studying
interacting networks, we introduce coupled climate subnetworks to represent and
investigate the topology of statistical relationships between the fields of
distinct climatological variables. Using coupled climate subnetworks to
investigate the terrestrial atmosphere's three-dimensional geopotential height
field uncovers known as well as interesting novel features of the atmosphere's
vertical stratification and general circulation. Specifically, the new measure
"cross-betweenness" identifies regions which are particularly important for
mediating vertical wind field interactions. The promising results obtained by
following the coupled climate subnetwork approach present a first step towards
an improved understanding of the Earth system and its complex interacting
components from a network perspective
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