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Browsing by Subject "LIGO"

Now showing 1 - 3 of 3
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    Cross-correlation searches for persistent gravitational waves with Advanced LIGO and noise studies for current and future ground-based gravitational-wave detectors.
    (2018-05) Meyers, Patrick
    Over the last three years, the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) has detected signals from colliding black holes and a signal from colliding neutron stars. These detections ushered in a new era of gravitational-wave (GW) astrophysics and multimessenger astronomy that allows us to probe new regions of the universe. One of the next frontiers for gravitational-wave astronomy is the detection and characterization of the stochastic GW background (SGWB). A measurement of the SGWB from unresolved compact binary systems could come as Advanced LIGO reaches design sensitivity, and future detectors will be important for digging beyond that astrophysical background towards trying to measure signals from relic gravitational waves produced in the early universe. In this dissertation, I present cross-correlation-based searches for a SGWB and other persistent sources of GWs. I introduce and use a new method for setting limits on the strain amplitude of a potential source of GWs in the directions of Scorpius X-1, the galactic center, and Supernova 1987a in the frequency band from 20-1726 Hz. I also set limits on persistent, broadband point sources of GWs across the whole sky. Finally, I show how we can implement data analysis techniques to improve the Advanced LIGO detector sensitivity to persistent sources of GWs. Improving sensitivity of current detectors and planning for future detectors is vital to the effort to measure and understand the SGWB. This will requires a better understanding of the noise sources that limit sensitivity, especially at lower frequencies. To this end, I outline a method for estimating and modeling correlated magnetic noise between spatially separated GW detectors. I also present results from a 3D seismometer array deployed at the Homestake Mine, aimed at characterizing seismic and Newtonian noise for future GW detectors. I estimate the fundamental Rayleigh-wave eigenfunction, and then use it in a seismic radiometer algorithm to separate different components of the seismic field that contribute differently to the Newtonian noise. Finally, I present estimates of the Newtonian noise as a function of depth in the frequency band from 0.5-5 Hz based on results from the seismic radiometer.
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    Gravitational Waves As Tools For Astrophysics And Cosmology
    (2021-05) Banagiri, Narayana Sri Sharan
    The discovery of gravitational waves by LIGO and Virgo have unveiled a sector of the Universe previously hidden from us. Gravitational waves allow us to detect and observe the dynamics of phenomena usually hidden from electromagnetism probes like binary black hole mergers, providing new tools to study astrophysics and cosmology in the process. As the number of detections increase, the statistics of the mergers are starting to inform us about their progenitor distributions. This dissertation consists of three parts. First, analyses to detect potential gravitational waves from a post-merger remnant of the binary neutron star merger GW170817 are described, with particular emphasis on the STAMP pipeline-based search targeting gravitational waves from a long-lived remnant. Bayesian parameter estimation techniques for the poorly modeled post-merger signals are then described. A novel likelihood formalism is developed to account for the inaccuracies in models, focusing in particular on the phase evolution of the waveform. In the second part, techniques are developed, using hierarchical Bayesian modeling to measure N-point correlations of the distributions of black hole mergers, with a focus on two-point correlations. These methods allow us to use black hole mergers as a tool to measure the angular distribution and the large-scale structure of the matter in the Universe. The two-point correlation method is validated with simulations for the angular structure of the mergers in the Universe. Finally, Bayesian methods are devised to probe anisotropies in the angular distribution of the stochastic gravitational-wave backgrounds and foregrounds in the LISA band. A novel decomposition using \cg coefficients in the spherical harmonic basis is developed that allows us to infer the anisotropy of arbitrary distributions of gravitational-wave power. This method is employed and tested using different kinds of simulations, including that of the galactic gravitational-wave foreground from galactic white dwarf binaries.
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    Unmodeled searches for long-lasting gravitational-wave signals with LIGO and studies of underground seismic noise for future gravitational-wave detectors
    (2016-07) Prestegard, Tanner
    The Laser Interferometer Gravitational-wave Observatory (LIGO) has recently reported the first two direct detections of gravitational waves, confirming yet another prediction of general relativity and providing an arena for testing gravity in the strong-field, high-velocity regime. These detections herald the beginning of the era of gravitational-wave astronomy, in which frequent gravitational-wave observations will provide a means for probing currently inaccessible regions of the universe and periods in its history. In this dissertation, I focus on detecting and characterizing a stochastic gravitational-wave background and long-lasting gravitational-wave transients, which will be important components of this era for the information they can yield about the beginnings of the universe and the objects contained within it. I implement a method for estimating the parameters of a model of a stochastic gravitational-wave background and apply it to a model based on core-collapse supernovae. Using the expected sensitivities of Advanced LIGO and the Einstein Telescope, I estimate the detectability of such a background and compare the results to simulations of core-collapse events. I also develop an unmodeled all-sky search for long-lasting gravitational-wave transients and apply it to LIGO S5 and S6 data, setting the first upper limits on signals lasting between 10-500 s in the 40-1000 Hz band. Finally, I describe the implementation of a 3D array of seismometers in and around the Homestake Mine, with the goal of mitigating seismic and Newtonian noise for future generations of gravitational-wave detectors. A seismic radiometer algorithm is developed and applied to simulated and real data; I demonstrate its ability to separate various components of the seismic field and map their directional dependence. In order to improve models of surface seismic waves in the radiometer algorithm, I use measurements of Rayleigh waves from the Homestake array and perform parameter estimation to fit a biexponential model of the Rayleigh wave eigenfunctions.