A light-cone gauge for black-hole perturbation theory

The geometrical meaning of the Eddington-Finkelstein coordinates of Schwarzschild spacetime is well understood: (i) the advanced-time coordinate v is constant on incoming light cones that converge toward r=0, (ii) the angles theta and phi are constant on the null generators of each light cone, (iii) the radial coordinate r is an affine-parameter distance along each generator, and (iv) r is an areal radius, in the sense that 4 pi r^2 is the area of each two-surface (v,r) = constant. The light-cone gauge of black-hole perturbation theory, which is formulated in this paper, places conditions on a perturbation of the Schwarzschild metric that ensure that properties (i)--(iii) of the coordinates are preserved in the perturbed spacetime. Property (iv) is lost in general, but it is retained in exceptional situations that are identified in this paper. Unlike other popular choices of gauge, the light-cone gauge produces a perturbed metric that is expressed in a meaningful coordinate system; this is a considerable asset that greatly facilitates the task of extracting physical consequences. We illustrate the use of the light-cone gauge by calculating the metric of a black hole immersed in a uniform magnetic field. We construct a three-parameter family of solutions to the perturbative Einstein-Maxwell equations and argue that it is applicable to a broader range of physical situations than the exact, two-parameter Schwarzschild-Melvin family.Comment: 12 page

A fast-neutron spectrometer of advanced design

Fast neutron spectrometer combines helium filled proportional counters with solid-state detectors to achieve the properties of high efficiency, good resolution, rapid response, and effective gamma ray rejection

Development of an electronically-scanned pressure module for operation at cryogenic temperatures

Pressure and temperature characteristics were measured for a number of multichannel electronically scanned pressure sensors. The tests were made on commercially available units designed to operate in a controlled temperature environment. Measurements of zero shift, sensitivity, and nonlinearity for each transducer were taken over a temperature range from 100 K to 340 K using a computer controlled data acquisition system. The units tested failed to meet accuracy specifications over the complete temperature range, which was expected. However, the sensors showed acceptable and predictable behavior over the temperature range from approximately -40 C (233 K) to 70 C (343 K). It was determined that a combination of local heating and accurate temperature monitoring can result in a device that can be compensated for temperature as well as its other physical properties. The design of a prototype for operation in a cryogenic environment is proposed, and a method for compensation is developed

Anti-deSitter gravitational collapse

We describe a formalism for studying spherically symmetric collapse of the massless scalar field in any spacetime dimension, and for any value of the cosmological constant $\Lambda$. The formalism is used for numerical simulations of gravitational collapse in four spacetime dimensions with negative $\Lambda$. We observe critical behaviour at the onset of black hole formation, and find that the critical exponent is independent of $\Lambda$.Comment: 4 pages, 2 figures, revtex4, version to appear in CQ

Light-cone coordinates based at a geodesic world line

Continuing work initiated in an earlier publication [Phys. Rev. D 69, 084007 (2004)], we construct a system of light-cone coordinates based at a geodesic world line of an arbitrary curved spacetime. The construction involves (i) an advanced-time or a retarded-time coordinate that labels past or future light cones centered on the world line, (ii) a radial coordinate that is an affine parameter on the null generators of these light cones, and (iii) angular coordinates that are constant on each generator. The spacetime metric is calculated in the light-cone coordinates, and it is expressed as an expansion in powers of the radial coordinate in terms of the irreducible components of the Riemann tensor evaluated on the world line. The formalism is illustrated in two simple applications, the first involving a comoving world line of a spatially-flat cosmology, the other featuring an observer placed on the axis of symmetry of Melvin's magnetic universe.Comment: 11 pages, 1 figur

Cooperation, conflict and warfare in wild banded mongooses

Intergroup conflict can be a strong force in the lives of social species. Conflict can be dramatic, leading to serious injury or death, loss of territory or dominance status, and can impact behaviour, reproductive success and fitness. The impact of intergroup conflict on within-group behaviour is a growing area of research, and evidence for increased affiliation between group members after exposure to intergroup conflict has been found in several species. However, these studies focus on short timescales, the minutes and hours post-conflict, and it is unclear what effect intergroup conflict has on within-group behaviour in the longer term. In this thesis I use the banded mongoose (Mungos mungo) as a model system to investigate the effects of intergroup conflict on within-group behaviour in the longer term. I discovered that group level within-group affiliation was only affected in the hour after exposure to conflict, but individual social relationships were affected into the longer term, up to two days after exposure. Unlike other studied populations, banded mongooses reduced within-group affiliation and aggression, and these changes differed between males and females, and between younger and older mongooses. I found only tentative evidence that intergroup conflict affected group movement or home range use, however, the risk of intergroup conflict affected leadership, with evidence that females lead more successfully in areas of high risk at the edge of the territory, which may indirectly affect movements in the longer term. This thesis gives evidence that intergroup conflict affects behaviour in the longer term, beginning to bridge the gap between evolutionary theory and empirical observations, and highlights that groups do not respond in a heterogeneous way, as different sex and age classes react differently, potentially due to differential costs and benefits