Gravity 2004 - abstracts

Abstracts

Leo Brewin - "A Review of numerical methods in general relativity "

Geoffrey Bicknell - "Simulations of three dimensional outflows in a gravitational field"

Kin Wu - "Radiation Transfer in curved spacetimes "

Matthew Bailes - "A review of the techniques for high precision pulsar timing"

George Hobbs - "A Review of detecting gravitational waves using pulsar timing"

We have recently started a project to detect gravitational waves by observing millisecond pulsars. In this talk, I'll provide a review on work that has been carried out over the last 20 years in order to put constraints on the gravitational wave background and on individual sources. The talk will highlight techniques, results and possibilities for the near future.

Donald Payne - "Gravitational waves from an accreting neutron star"

There are several candidate sources of gravitational waves including supernovae, coalescing neutron star or black hole pairs, and rotating compact stars. We consider gravitational waves from an accreting neutron star undergoing polar magnetic burial. During accretion, the magnetic field of a neutron star is compressed into a narrow belt at the magnetic equator by material spreading

equatorward from the polar cap. The compressed field confines the accreted matter in a polar mountain which is generally misaligned with the rotation axis, producing gravitational waves. We have calculated the equilibrium hydromagnetic structure of the polar mountain, and its mass quadrupole moment as a function of the total mass accreted, by solving a Grad-Shafranov boundary value problem. We find the value of the orientation and polarisation-averaged gravitational wave strain to be higher than previous estimates that did not treat equatorward spreading and flux freezing self-consistently. We conclude that an accreting millisecond pulsar emits a persistent, harmonic, gravitational wave signal which is, in principle, detectable by long baseline interferometers after phase-coherent integration. Magnetic burial reduces the magnetic dipole moment monotonically, allowing an observationally testable scaling between gravitational wave strain and magnetic dipole moment.

David McClelland - "A Review of gravity wave detectors via laser interferometry"

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David Coward - "A cosmological background of gravitational waves from astrophysical source"

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In recent years it has become clear that a cosmological gravitational wave spectrum must include two stochastic background components, one, the Cosmological Gravitational wave Background (CGB), which would probe the Universe at about 10^(-22) s after the big bang, and a second one, the Astrophysical Gravitational wave Background (AGB) which probes the era from redshift z ~ 100-0.1. The very early CGB component is uncertain since its amplitude strongly depends on early Universe processes that include phase transitions, formation of cosmic strings, inflation and phase transition driven vacuum bubbles. In contrast, the AGB includes known sources of gravitational waves (neutron star coalescences) and others for which extensive modeling efforts have provided a range of plausible predictions. This presentation provides an overview of the interesting features that recent simulations of components of the AGB have provided.

Kirsty Rhook - "The LISA detection rate of supermassive black hole coalescence"

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The gravitational waves generated during supermassive black hole (SMBH) coalescence are prime candidates for detection by the gravitational wave satellite LISA. We calculate detection limits in both the ringdown and inspiral phases of SMBH coalescence. Our results indicate that only the higher frequency ringdown signal can detect the coalescence of black holes as massive as 10^8 - 10^9 Msolar. We use the extended Press-Schechter formalism combined with observationally motivated estimates for the SMBH-dark matter halo mass relation and SMBH occupation fraction to estimate the maximum rate of major SMBH mergers. Assuming efficient binary coalescence, and guided by the minimum black hole mass in local galactic bulges (approximately 10^5 Msolar) we predict approximately 15 detections per year in each of the inspiral and ringdown phases.

Alexander Knebe - "A Review of Newtonian N-body gravitational dynamics"

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In this talk I am going to review the most commonly used techniques for modeling the mutual gravity of N particles in simulations of cosmological structure formation. This includes not only a description of gravity solvers such as tree codes, adaptive mesh refinement codes and various hybrid methods, but also a detailed description of the appropriate time integrator for the equations of motions and potential problems related to the generation of the initial conditions. In the final part of the talk I am going to focus on a detailed comparison of one particular cosmological simulation run with three competing publicly available N-body codes highlighting the differences and similarities.

Andrew Melatos (CathrynTrott) - "Density profiles of CDM haloes in softened N-body gravity"

The statistical mechanics of N cold dark matter (CDM) particles interacting via a softened gravitational potential is reviewed in the microcanonical ensemble and mean-field limit. A phase diagram for the system is computed as a function of the total energy E and gravitational softening length epsilon. For softened systems, two stable phases exist: a collapsed phase, whose radial density profile rho(r) is a central Dirac cusp, and an extended phase, for which rho(r) has a central core and rho(r)~r^{-2.2} at large r. It is shown that many N-body simulations of CDM haloes in the literature inadvertently sample the collapsed phase only, even though this phase is unstable when there is zero softening. Consequently, there is no obvious reason to expect agreement between simulated and observed profiles unless the gravitational potential is appreciably softened in nature.

Rosemary Mardling - "A Review of planetary dynamics "

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Various applications of planetary dynamics are reviewed including planet formation, dynamics of small bodies in the Solar System, stability of planetary systems, non-point-mass dynamics (eg. spin-orbit coupling, tidal interactions), and other phenomena (eg. relativistic effects, Kozai cycles). Numerical and analytical methods are reviewed, followed by a brief primer on resonance in gravitating systems.

Geraint Lewis - "Tidal tails and the shape of the dark matter halo"

Rachel Webster - "Review of Gravitational lensing"

Zdenka Kuncic - "A Review of Black Holes in Galactic Nuclei, X-ray Binaries, and Ultraluminous X-ray Sources"

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Andrew Melatos - "A Review of gravity in Neutron stars and Pulsars "

John Webb - " Do constants change?"

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I describe the current status of a varying fine structure constant, summarising our results using 128 quasar absorption systems from the Keck telescope. I compare our results with a recent study based on a smaller sample of VLT data and discuss the inconsistencies. I also outline new work using light element observations and the Kawano BBN code, allowing the deuteron binding energy Q to vary. A significantly improved internal agreement between He, Li and D is obtained, and a better match to the WMAP baryonic density parameter, if Q is smaller at the BBN epoch.

Geoffrey Bicknell - "The fine structure constant, gravity and the 2nd law of thermodynamics"

Michael Kuchiev - "Reflection from the event horizon and absorption of long wavelength radiation by black holes"

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A new property of the event horizon of black holes is discussed. Usually it is assumed that a particle that approaches the horizon of a black hole crosses it smoothly, uneventfully, staying after that in the inside region. This type of behaviour justifies the name "black holes", which presumes that a black hole absorbs any matter that comes near by. However, the present work demonstrates that this conventional perception is based on purely classical arguments. Quantum treatment of the process reveals that there is an opportunity for a reflection of the incoming particle on the horizon, when it bounces on the horizon back into the outside world. The probability of reflection depends on the wavelength of the particle, being large in the infrared region. Thus, for the infrared radiation a black hole behaves not as an absorber, but as a perfect reflector, which is surprising. This effect forces the absorption cross section to vanish in the infrared region. It is shown that the effect of reflection and the well known Hawking radiation process originate from similar physical roots, both these phenomena are related to quantum processes that take place in a strong gravitational field on the horizon. The radiation phenomenon permits an observer to "see" the horizon, while the reflection process permits one to "touch" the horizon.