UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item Gravitational Radiation in the Limit of High Frequency(1967) Isaacson, Richard Allen; Misner, Charles W.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)This dissertation deals with a technique for obtaining approximate radiative solutions to the Einstein equations of general relativity in situations where the gravitational fields of interest are quite strong. In the first chapter, we review the history of the problem and discuss previous work along related lines. In the second chapter, we assume the radiation to be of high frequency and expand the field equations in powers of the small wavelength this supplies. This assumption provides an approximation scheme valid for all orders of 1/r, for arbitrary velocities up to that of light, and for arbitrary intensities of the gravitational field. To lowest order we obtain a gauge invariant linear wave equation for gravitational radiation, which is a covariant generalization of that for massless spin-two fields in flat space, This wave equation is then solved by the W.K.B. approximation to show that gravitational waves travel on null geodesics with amplitude and frequency modified by gravitational fields in exactly the same way as are those of light waves, and with their polarization parallel transported along the geodesics, again as is the case for light. The metric containing high frequency gravitational waves is shown to be type N to lowest order, and some limits to the methods used are discussed. In the third chapter we go beyond the linear terms in the high frequency expansion, and consider the lowest order non-linear terms. They are shown to provide a natural, gauge invariant, averaged effective stress tensor for the energy localized in the high frequency radiation. By assuming the W.K.B. form for the field, this tensor is found to have the same structure as that for an electromagnetic null field. A Poynting vector is used to investigate the flow of energy and momentum in the gravitational wave field, and it is seen that high frequency waves propagate along null hypersurfaces and are not backscattered off by the curvature of space. Expressions for the total energy and momentum carried by the field to flat null infinity are given in terms of coordinate independent integrals valid within regions of strong field strength. The formalism is applied to the case of spherical gravitational waves where a news function is obtained, and where the source is found to lose exactly the energy and momentum contained in the radiation field.Item Black Hole Dynamics and Gravitational Radiation in Galactic Nuclei(2009) Lauburg, Vanessa; Miller, Michael C.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, we present new channels for the production of gravitational radiation sources: mergers of black holes in the nuclear star clusters found in many small galaxies, and mergers and tidal separations of black hole binaries in galaxies that host supermassive black holes. Mergers between stellar-mass black holes will be key sources of gravitational radiation for ground-based detectors. However, the rates of these events are highly uncertain, because we can not observe these binaries electromagnetically. In this work, we show that the nuclear star clusters found in the centers of small galaxies are conducive environments for black hole mergers. These clusters have large escape velocities, high stellar densities, and large numbers of black holes that will have multiple close encounters, which often lead to mergers. We present simulations of the three-body dynamics of black holes in this environment and estimate that, if many nuclear star clusters do not have supermassive black holes, tens of events per year will be detectable with Advanced LIGO. Larger galaxies that host supermassive black holes can produce extreme-mass ratio inspiral (EMRI) events, which are important sources for the future space-based detector, LISA. Here, we show that tidal separation of black hole binaries by supermassive black holes will produce a distinct class of EMRIs with near-zero eccentricities, and we estimate that rates from tidal separation could be comparable to or larger than those from the traditionally-discussed two-body capture formation scenario. Before tidal separation can occur, a binary encounters multiple stars as it sinks through the nucleus toward the supermassive black hole. In this region, velocities are high, and interactions with stars can destroy binaries through ionization. We investigate wide ranges in initial mass function and internal energy of the binaries, and find that tidal separations, mergers, and ionizations are all likely outcomes for binaries near the galactic center. Tidally separated binaries will contribute to the LISA detection rate, and mergers will produce tens of events per year for Advanced LIGO. We show, therefore, that galactic nuclei are promising hosts of gravitational wave sources for both LISA and LIGO.