Physics Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2800
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Item Low-Latency Searches for Gravitational Waves and their Electromagnetic Counterparts with Advanced LIGO and Virgo(2019) Cho, Min-A; Shawhan, Peter S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For the first time in history, advanced detectors are available to observe the stretching and squeezing of space---gravitational waves---from violent astrophysical events. This opens up the prospect of joint detections with instruments of traditional astronomy, creating the new field of multi-messenger astrophysics. Joint detections allow us to form a coherent picture of the unfolding event as told by the various channels of information: mass and energy dynamics from gravitational waves, charged particle environments (along with magnetic field and specific element environments) from electromagnetic radiation, and thermonuclear reactions/relativistic particle outflows from neutrinos. In this work, I motivate low-latency electromagnetic and neutrino follow-up of sources known to emit gravitational radiation in the sensitivity band of ground-based interferometric detectors, Advanced LIGO and Advanced Virgo. To this end, I describe the low-latency infrastructure I developed with colleagues to select and enable successful follow-up of the first few gravitational-wave candidate events in history, including the first binary black hole merger, named GW150914, and binary neutron star coalescence, named GW170817, from the first and second observing runs. As a review, I outline the theory behind gravitational waves and explain how the advanced detectors, low-latency searches, and data quality vetting procedures work. To highlight the newness of the field, I also share results from an offline search for a more speculative source of gravitational waves, intersecting cosmic strings, from the second observing run. Finally, I address how LIGO/Virgo is prepared to adapt to challenges that will arise during the upcoming third observing run, an era that will be marked by near-weekly binary black hole candidate events and near-monthly binary neutron star candidate events. To handle this load, we made several improvements to our low-latency infrastructure, including a new, streamlined candidate event selection process, expansions I helped develop for temporal coincidence searches with electromagnetic/neutrino triggers, and data quality products on source classification and probability of astrophysical origin to provide to our observing partners for potential compact binary coalescences. These measures will further our prospects for multi-messenger astrophysics and increase our science returns.Item Improving analytical templates and searching for gravitational waves from coalescing black hole binaries(2010) Ochsner, Evan Lee; Buonanno, Alessandra; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo are taking data at design sensitivity. They will be upgraded to Advanced LIGO and Virgo within the next 5 years and the detection of gravitational waves will be very likely. Binaries of two compact objects which inspiral and coalesce are one of the most promising sources for LIGO and Virgo. Most searches have focused solely on the inspiral portion of the waveform, and are consequently limited to low total mass. Recent breakthroughs in numerical relativity allow one to construct complete inspiral-merger-ringdown waveforms and search for the whole signal. This thesis will review some of the basic characteristics of gravitational waves from compact binaries and methods of searching for them. Analytical template waveforms for such systems will be presented including a comparison of different families of analytical waveforms, a study on the inclusion of spin effects in such waveforms, and a study of inspiral-merger-ringdown waveforms with amplitude corrections and the importance of these effects for parameter estimation. The thesis will culminate with a presentation of the first gravitational wave search to use inspiral-merger-ringdown templates, which was performed on data from the fifth science run of LIGO.Item APPLYING NUMERICAL RELATIVITY TO GRAVITATIONAL WAVE ASTRONOMY(2008-03-12) McWilliams, Sean Thomas; Shawhan, Peter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)General relativity predicts the existence of gravitational waves produced by the motion of massive objects. The inspiral, merger, and ringdown of black hole binaries is expected to be one of the brightest sources in the gravitational wave sky. Interferometric detectors, such as the current ground-based Laser Interferometer Gravitational Wave Observatory (LIGO) and the future space-based Laser Interferometer Space Antenna (LISA), measure the influx of gravitational radiation from the whole sky. Each physical process that emits gravitational radiation will have a unique waveform, and prior knowledge of these waveforms is needed to distinguish a signal from the noise inherent in the interferometer. In the strong field regime of the merger, only numerical relativity, which solves the full set of Einstein's equations numerically, has been able to provide accurate waveforms. We present a comprehensive study of the nonspinning portion of parameter space for which we have generated accurate simulations of the late inspiral through merger and ringdown, and a comparison of those results with predictions from the adiabatic Taylor-expanded post-Newtonian (PN) and effective-one-body (EOB) PN approximations. We then focus on data analysis questions using the equal-mass nonspinning as well as the 2:1, 4:1, and 6:1 mass ratio nonspinning black hole binary (BHB) waveforms. We construct a full waveform by combining our results from numerical relativity with a highly accurate Taylor PN approximation, and use it to calculate signal-to-noise ratios (SNRs) for several detectors. We measure the mass ratio scaling of the waveform amplitude through the inspiral and merger, and compare our observations with predictions from PN. Lastly, we turn our focus to parameter estimation with LISA, and investigate the increased accuracy with which parameters can be measured by including both the merger and inspiral waveforms, compared to estimates without numerical waveforms which can only incorporate the inspiral. We use the equal mass, nonspinning waveform as a test case and assess the parameter uncertainty by means of the Fisher matrix formalism.Item Energy, stars, and black holes in Einstein-aether theory(2007-07-12) Eling, Christopher; Jacobson, Theodore A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years there have been hints of Lorentz violation in various approaches to quantum gravity. Lorentz violating physics has also been proposed as an explanation for unexpected observational anomalies such as atmospheric cosmic rays apparently observed above the GZK cutoff, the flatness of galactic rotation curves and the accelerating expansion of the universe. In this dissertation we will consider an alternative theory of gravity that exhibits Lorentz violation. This ``Einstein-aether" theory is a four parameter class of theories where a dynamical unit timelike vector field (the ``aether") is coupled to gravity. We will focus particularly on energy, stars, and black holes in the theory. First, using pseudotensor methods we find expressions for the Einstein-aether energy. These are then applied to find the energy in both linear and non-linear regimes. Enforcing the energy positivity of linearized wave modes yields an important constraint on the four parameters. An expression for the energy of an asymptotically flat spacetime is also obtained, but a complete positive energy theorem remains elusive. Next, we study in detail non-linear spherically symmetric solutions in the theory. The time independent asymptotically flat solutions fall into two classes depending on whether the aether is aligned with the timelike Killing vector. ``Static" solutions aligned with the Killing vector describe the interior and vacuum regions of fluid stars. We characterize properties such as maximum masses and surface redshifts for candidate neutron star equations of state. Only tentative observational constraints on the theory are currently possible due to uncertainties in neutron star physics. Black hole solutions, which must be non-static, are shown to exist in a class of Einstein-aether theories using numerical integration. The geometry outside the horizon is very similar to the Schwarzschild solution of General Relativity, but there are qualitative differences inside. Finally, we investigate classical two-dimensional Einstein-aether theory as a toy model that could be used to study the Hawking effect and quantization in a Lorentz violating setting. We conclude by examining directions for future research.