1,997 research outputs found
Analytic Results for the Gravitational Radiation from a Class of Cosmic String Loops
Cosmic string loops are defined by a pair of periodic functions and
, which trace out unit-length closed curves in three-dimensional
space. We consider a particular class of loops, for which lies along
a line and lies in the plane orthogonal to that line. For this class
of cosmic string loops one may give a simple analytic expression for the power
radiated in gravitational waves. We evaluate exactly in
closed form for several special cases: (1) a circle traversed
times; (2) a regular polygon with sides and interior vertex angle
; (3) an isosceles triangle with semi-angle .
We prove that case (1) with is the absolute minimum of within
our special class of loops, and identify all the stationary points of
in this class.Comment: 15 pages, RevTex 3.0, 7 figures available via anonymous ftp from
directory pub/pcasper at alpha1.csd.uwm.edu, WISC-MILW-94-TH-1
Byzantine Stochastic Gradient Descent
This paper studies the problem of distributed stochastic optimization in an
adversarial setting where, out of the machines which allegedly compute
stochastic gradients every iteration, an -fraction are Byzantine, and
can behave arbitrarily and adversarially. Our main result is a variant of
stochastic gradient descent (SGD) which finds -approximate
minimizers of convex functions in iterations. In contrast, traditional
mini-batch SGD needs iterations,
but cannot tolerate Byzantine failures. Further, we provide a lower bound
showing that, up to logarithmic factors, our algorithm is
information-theoretically optimal both in terms of sampling complexity and time
complexity
Superconductivity Solves the Monopole Problem for Alice Strings
Alice strings are cosmic strings that turn matter into antimatter. Although
they arise naturally in many GUT's, it has long been believed that because of
the monopole problem they can have no cosmological effects. We show this
conclusion to be false; by using the Langacker-Pi mechanism, monopoles can in
fact be annihilated while Alice strings are left intact. This opens up the
possibility that they can after all contribute to cosmology, and we mention
some particularly important examples.Comment: 16 pages, 7 figures (not included
Herd-level bovine tuberculosis risk factors:assessing the role of low-level badger population disturbance
Bovine TB (bTB) is endemic in Irish cattle and has eluded eradication despite considerable expenditure, amid debate over the relative roles of badgers and cattle in disease transmission. Using a comprehensive dataset from Northern Ireland (>10,000 km2; 29,513 cattle herds), we investigated interactions between host populations in one of the first large-scale risk factor analyses for new herd breakdowns to combine data on both species. Cattle risk factors (movements, international imports, bTB history, neighbours with bTB) were more strongly associated with herd risk than area-level measures of badger social group density, habitat suitability or persecution (sett disturbance). Highest risks were in areas of high badger social group density and high rates of persecution, potentially representing both responsive persecution of badgers in high cattle risk areas and effects of persecution on cattle bTB risk through badger social group disruption. Average badger persecution was associated with reduced cattle bTB risk (compared with high persecution areas), so persecution may contribute towards sustaining bTB hotspots; findings with important implications for existing and planned disease control programmes
Detection of Anisotropies in the Gravitational-Wave Stochastic Background
By correlating the signals from a pair of gravitational-wave detectors, one
can undertake sensitive searches for a stochastic background of gravitational
radiation. If the stochastic background is anisotropic, then this correlated
signal varies harmonically with the earth's rotation. We calculate how the
harmonics of this varying signal are related to the multipole moments which
characterize the anisotropy, and give a formula for the signal-to-noise ratio
of a given harmonic. The specific case of the two LIGO (Laser Interferometric
Gravitational Observatory) detectors, which will begin operation around the
year 2000, is analyzed in detail. We consider two possible examples of
anisotropy. If the gravitational-wave stochastic background contains a dipole
intensity anisotropy whose origin (like that of the Cosmic Background
Radiation) is motion of our local system, then that anisotropy will be
observable by the advanced LIGO detector (with 90% confidence in one year of
observation) if \Omega_{gw} > 5.3 \times 10^{-8} h_{100}^{-2}. We also study
the signal produced by stochastic sources distributed in the same way as the
luminous matter in the galactic disk, and in the same way as the galactic halo.
The anisotropy due to sources distributed as the galactic disk or as the
galactic halo will be observable by the advanced LIGO detector (with 90%
confidence in one year of observation) if \Omega_{gw} > 1.8 \times 10^{-10}
h_{100}^{-2} or \Omega_{gw} > 6.7 \times 10^{-8} h_{100}^{-2}, respectively.Comment: 25 pages, Latex with RevTeX and epsfig, now includes S/N ratio
calculations, expected response from anisotropy due to local motion & sources
in galax
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Securities trading by banks and credit supply: Micro-evidence from the crisis
We analyze securities trading by banks during the crisis and the associated spillovers to the supply of credit. We use a proprietary data set that has the investments of banks at the security level for 2005–2012 in conjunction with the credit register from Germany. We find that—during the crisis—banks with higher trading expertise (trading banks) increase their investments in securities, especially in those that had a larger price drop, with the strongest impact in low-rated and long-term securities. Moreover, trading banks reduce their credit supply, and the credit crunch is binding at the firm level. All of the effects are more pronounced for trading banks with higher capital levels. Finally, banks use central bank liquidity and government subsidies like public recapitalization and implicit guarantees mainly to support trading of securities. Overall, our results suggest an externality arising from fire sales in securities markets on credit supply via the trading behavior of banks
Quantifying mesoscale-driven nitrate supply: a case study
The supply of nitrate to surface waters plays a crucial role in maintaining marine life. Physical processes at the mesoscale (~10-100?km) and smaller have been advocated to provide a major fraction of the global supply. Whilst observational studies have focussed on well-defined features, such as isolated eddies, the vertical circulation and nutrient supply in a typical 100-200?km square of ocean will involve a turbulent spectrum of interacting, evolving and decaying features. A crucial step in closing the ocean nitrogen budget is to be able to rank the importance of mesoscale fluxes against other sources of nitrate for surface waters for a representative area of open ocean. While this has been done using models, the vital observational equivalent is still lacking.To illustrate the difficulties that prevent us from putting a global estimate on the significance of the mesoscale observationally, we use data from a cruise in the Iceland Basin where vertical velocity and nitrate observations were made simultaneously at the same high spatial resolution. Local mesoscale nitrate flux is found to be an order of magnitude greater than that due to small-scale vertical mixing and exceeds coincident nitrate uptake rates and estimates of nitrate supply due to winter convection. However, a non-zero net vertical velocity for the region introduces a significant bias in regional estimates of the mesoscale vertical nitrate transport. The need for synopticity means that a more accurate estimate can not be simply found by using a larger survey area. It is argued that time-series, rather than spatial surveys, may be the best means to quantify the contribution of mesoscale processes to the nitrate budget of the surface ocean
Editorial : Bovine Tuberculosis—International perspectives on epidemiology and management
CITATION: Andrew W. Byrne, Adrian R. Allen, Daniel J. O’Brien and Michele A. Miller. (2019) Bovine tuberculosis—international perspectives on epidemiology and management. Frontiers in Veterinary Science, 6. doi: 10.3389/fvets.2019.00202The original publication is available at https://www.frontiersin.org/journals/veterinary-scienceIntroduction: Bovine tuberculosis (bTB) remains a prominent zoonotic pathogen on the world stage, with significant impacts on animal and human health, and economic well-being. Eradication is hampered by a complex epidemiology, which in many countries involves wildlife hosts. Indeed, despite advances in understanding gleaned from national programs of bTB eradication, much of our understanding of transmission mechanisms, diagnostics, control, and multi-host infection systems remains opaque.
In this collection of Frontiers in Veterinary Science, as editors, we felt these limitations could best be addressed by adopting an international perspective. Localism understandably focuses on the fine details of problems at hand, but can perhaps overlook issues that only become apparent when compared to the experiences of others.
Below we summarize the papers published in this truly international collection, and highlight some themes. We trust readers will find these articles as stimulating to read as they were to edit.https://www.frontiersin.org/articles/10.3389/fvets.2019.00202/fullPublisher’s versio
One-loop Quantum Gravity in Schwarzschild Spacetime
The quantum theory of linearized perturbations of the gravitational field of
a Schwarzschild black hole is presented. The fundamental operators are seen to
be the perturbed Weyl scalars and associated with the
Newman-Penrose description of the classical theory. Formulae are obtained for
the expectation values of the modulus squared of these operators in the
Boulware, Unruh and Hartle-Hawking quantum states. Differences between the
renormalized expectation values of both and
in the three quantum states are evaluated
numerically.Comment: 39 pages, 11 Postscript figures, using revte
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