Unmodeled searches for long-lasting gravitational-wave signals with LIGO and studies of underground seismic noise for future gravitational-wave detectors

Abstract

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.