9,594 research outputs found
Gravitational Waves Probe the Coalescence Rate of Massive Black Hole Binaries
We calculate the expected nHz--Hz gravitational wave (GW) spectrum from
coalescing Massive Black Hole (MBH) binaries resulting from mergers of their
host galaxies. We consider detection of this spectrum by precision pulsar
timing and a future Pulsar Timing Array. The spectrum depends on the merger
rate of massive galaxies, the demographics of MBHs at low and high redshift,
and the dynamics of MBH binaries. We apply recent theoretical and observational
work on all of these fronts. The spectrum has a characteristic strain
, just below the detection limit from
recent analysis of precision pulsar timing measurements. However, the amplitude
of the spectrum is still very uncertain owing to approximations in the
theoretical formulation of the model, to our lack of knowledge of the merger
rate and MBH population at high redshift, and to the dynamical problem of
removing enough angular momentum from the MBH binary to reach a GW-dominated
regime.Comment: 31 Pages, 8 Figures, small changes to match the published versio
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Results of an aqueous source term model for a radiological risk assessment of the Drigg LLW Site, U.K.
A radionuclide source term model has been developed which simulates the biogeochemical evolution of the Drigg low level waste (LLW) disposal site. The DRINK (DRIgg Near field Kinetic) model provides data regarding radionuclide concentrations in groundwater over a period of 100,000 years, which are used as input to assessment calculations for a groundwater pathway. The DRINK model also provides input to human intrusion and gaseous assessment calculations through simulation of the solid radionuclide inventory. These calculations are being used to support the Drigg post closure safety case. The DRINK model considers the coupled interaction of the effects of fluid flow, microbiology, corrosion, chemical reaction, sorption and radioactive decay. It represents the first direct use of a mechanistic reaction-transport model in risk assessment calculations
An Exact Test of Fit for the Gaussian Linear Model using Optimal Nonbipartite Matching
Fisher tested the fit of Gaussian linear models using replicated observations. We refine this method by (1) constructing near-replicates using an optimal nonbipartite matching and (2) defining a distance that focuses on predictors important to the modelâs predictions. Near-replicates may not exist unless the predictor set is low-dimensional; the test addresses dimensionality by betting that model failures involve a subset of predictors important in the old fit. Despite using the old fit to pair observations, the test has exactly its stated level under the null hypothesis. Simulations show the test has reasonable power even when many spurious predictors are present
Constructed Second Control Groups and Attenuation of Unmeasured Biases
The informal folklore of observational studies claims that if an irrelevant observed covariate is left uncontrolled, say unmatched, then it will influence treatment assignment in haphazard ways, thereby diminishing the biases from unmeasured covariates. We prove a result along these lines: it is true, in a certain sense, to a limited degree, under certain conditions. Alas, the conditions are neither inconsequential nor easy to check in empirical work; indeed, they are often dubious, more often implausible. We suggest the result is most useful in the computerized construction of a second control group, where the investigator can see more in available data without necessarily believing the required conditions. One of the two control groups controls for the possibly irrelevant observed covariate, the other control group either leaves it uncontrolled or forces separation; therefore, the investigator views one situation from two angles under different assumptions. A pair of sensitivity analyses for the two control groups is coordinated by a weighted Holm or recycling procedure built around the possibility of slight attenuation of bias in one control group. Issues are illustrated using an observational study of the possible effects of cigarette smoking as a cause of increased homocysteine levels, a risk factor for cardiovascular disease. Supplementary materials for this article are available online
Fitness-driven deactivation in network evolution
Individual nodes in evolving real-world networks typically experience growth
and decay --- that is, the popularity and influence of individuals peaks and
then fades. In this paper, we study this phenomenon via an intrinsic nodal
fitness function and an intuitive aging mechanism. Each node of the network is
endowed with a fitness which represents its activity. All the nodes have two
discrete stages: active and inactive. The evolution of the network combines the
addition of new active nodes randomly connected to existing active ones and the
deactivation of old active nodes with possibility inversely proportional to
their fitnesses. We obtain a structured exponential network when the fitness
distribution of the individuals is homogeneous and a structured scale-free
network with heterogeneous fitness distributions. Furthermore, we recover two
universal scaling laws of the clustering coefficient for both cases, and , where and refer to the node degree and the
number of active individuals, respectively. These results offer a new simple
description of the growth and aging of networks where intrinsic features of
individual nodes drive their popularity, and hence degree.Comment: IoP Styl
Elastic energy of polyhedral bilayer vesicles
In recent experiments [M. Dubois, B. Dem\'e, T. Gulik-Krzywicki, J.-C.
Dedieu, C. Vautrin, S. D\'esert, E. Perez, and T. Zemb, Nature (London) Vol.
411, 672 (2001)] the spontaneous formation of hollow bilayer vesicles with
polyhedral symmetry has been observed. On the basis of the experimental
phenomenology it was suggested [M. Dubois, V. Lizunov, A. Meister, T.
Gulik-Krzywicki, J. M. Verbavatz, E. Perez, J. Zimmerberg, and T. Zemb, Proc.
Natl. Acad. Sci. U.S.A. Vol. 101, 15082 (2004)] that the mechanism for the
formation of bilayer polyhedra is minimization of elastic bending energy.
Motivated by these experiments, we study the elastic bending energy of
polyhedral bilayer vesicles. In agreement with experiments, and provided that
excess amphiphiles exhibiting spontaneous curvature are present in sufficient
quantity, we find that polyhedral bilayer vesicles can indeed be energetically
favorable compared to spherical bilayer vesicles. Consistent with experimental
observations we also find that the bending energy associated with the vertices
of bilayer polyhedra can be locally reduced through the formation of pores.
However, the stabilization of polyhedral bilayer vesicles over spherical
bilayer vesicles relies crucially on molecular segregation of excess
amphiphiles along the ridges rather than the vertices of bilayer polyhedra.
Furthermore, our analysis implies that, contrary to what has been suggested on
the basis of experiments, the icosahedron does not minimize elastic bending
energy among arbitrary polyhedral shapes and sizes. Instead, we find that, for
large polyhedron sizes, the snub dodecahedron and the snub cube both have lower
total bending energies than the icosahedron
Accounting for the effect of heterogeneous plastic deformation on the formability of aluminium and steel sheets
Forming Limit Curves characterise âmeanâ failure strains of sheet metals. Safety levels from the curves define the deterministic upper limit of the processing and part design window, which can be small for high strength, low formability materials.
Effects of heterogeneity of plastic deformation, widely accepted to occur on the microscale, are neglected. Marciniak tests were carried out on aluminium alloys (AA6111-T4, NG5754-O), dual-phase steel (DP600) and mild steel (MS3). Digital image correlation was used to measure the effect of heterogeneity on failure. Heterogeneity, based on strain variance was modelled with the 2-component Gaussian Mixture Model and a framework was proposed to 1) identify the onset of necking and to 2) re-define formability as a probability to failure.
The result were âforming mapsâ in major-minor strain space of contours of constant probability (from probability, P=0 to P=1), which showed how failure risk increased with major strain. The contour bands indicated the unique degree of heterogeneity in each material. NG5754-O had the greatest width (0.07 strain) in plane strain and MS3 the lowest (0.03 strain). This novel characterisation will allow engineers to balance a desired forming window for a component design with the risk to failure of the material
Evolution of accretion disks around massive black holes: constraints from the demography of active galactic nuclei
Observations have shown that the Eddington ratios (the ratio of the
bolometric luminosity to the Eddington luminosity) in QSOs/active galactic
nuclei (AGNs) cover a wide range. In this paper we connect the demography of
AGNs obtained by the Sloan Digital Sky Survey with the accretion physics around
massive black holes and propose that the diversity in the Eddington ratios is a
natural result of the long-term evolution of accretion disks in AGNs. The
observed accretion rate distribution of AGNs (with host galaxy velocity
dispersion sigma~70-200 km/s) in the nearby universe (z<0.3) is consistent with
the predictions of simple theoretical models in which the accretion rates
evolve in a self-similar way. We also discuss the implications of the results
for the issues related to self-gravitating disks, coevolution of galaxies and
QSOs/AGNs, and the unification picture of AGNs.Comment: 18 pages, 2 figures; revised, main conclusions not changed; to appear
in ApJ, Oct., 200
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