This award will support an integrated program of teaching and research designed to educate scientists in the fundamentals of gravitational-wave (GW) sources and measurement. Three major components will comprise this program: (1) The design of a new course in GW science focusing largely on the astrophysics of GW sources. This course will be made accessible to the community via MIT's OpenCourseWare Initiative (http://web.mit.edu/ocw). (2) The production of a publicly accessible catalog presenting the "sounds" of various GW sources. Such sounds have been demonstrated to very effectively illustrate the principles of GW science, both to technical audiences and more broadly. Audio encodings of GW science will be coupled to source dynamical visualizations where possible. Much of this work will be done by undergraduate students at MIT. (3) A research program in GW source science and measurement. This program will focus in particular on studies of radiation emission from binary systems; a formalism for mapping the characteristics of massive black holes; and the astronomy that can be done with networks of ground-based GW detectors. The bulk of the work in these research projects will be performed by graduate students.
This work will help ensure that GW detectors reach their full potential as GW observatories, opening a new field of observational astronomy. The course that will be developed will help educate the young scientists who will work with these new instruments in the coming years; the catalog of sounds and visualizations will serve to inform the broader scientific community about the promise of GW science, as well as serve as an effective public outreach tool.
The most important outcomes of this CAREER grant are the scientists we trained and mentored. Four Ph.D. theses were completed (Ryan Lang, Pranesh Sundararajan, Leo Stein, and Sarah Vigeland), and two others started (Stephen O'Sullivan and Uchupol Ruangsri) under its support. This grant also supported two MIT bachelor's theses (Terrence Torres and William Throwe), six summer researchers (Pei-Lan Hsu, Terral Jordan, Colin Hill, Michael Shaw, Hugo Marrochio, and Peter Reinhardt), and one postdoctoral researcher (Samaya Nissanke). Our work is summarized at http://gmunu.mit.edu, which includes a catalog of gravitational waveforms, their audio encodings, and visualizations of gravitational-wave sources, plus an index of publications this grant produced. Our major scientific accomplishments fall into five categories: 1. Radiation reaction. A major component of our research has been the evolution of binary systems due to gravitational wave emission. Much of our work focuses on large mass ratio systems, which can treated more easily than the general case. We now have a toolkit which allows complete analysis of inspiral, at least when we can approximate a binary's evolution as "slow" in a well-defined sense. Our code models systems with arbitrary spin, eccentricity, and orbital inclination. We have adapted a spectral technique which vastly improved our code's speed and accuracy. 2. Waveform generation. Item 1 taught us to model the inspiral of binary systems. We now have new tools for constructing, in the time domain, waveforms produced by coalescences, including the final plunge and merger of the binary's smaller member with the larger black hole. Graduate student Pranesh Sundararajan discovered a technique for accurately describing the source of the time-domain equations. Prior to this, time-domain perturbation codes computed waveforms with errors of 10% or more. Pranesh's discovery reduced these errors to roughly 0.01%, allowing many physically interesting investigations. We continue to refine these techniques and their applications. 3. A framework for testing the nature of black hole candidates. Astronomers tells us about many black holes in our universe: stellar mass holes in disks of galaxies, and supermassive ones in galaxy cores. What astronomers actually tell us is that they have found dark concentrations of mass which, most likely, are black holes. Are these objects in fact black holes? To answer this, we want to test whether these objects are accurately described by the black hole solutions of general relativity. We have developed a framework to test this, based on the fact that a black hole's gravity has a special shape. The "No hair" theorems of general relativity, which tell us that a black hole is described only by its mass and spin, tightly constrain black holes shapes. Any deviation from these constraints tells us that these objects are not perfect black holes: Some other matter pollutes the spacetime, or general relativity does not work in the strong field. We have shown that if a black hole has the "wrong" shape, it strongly affects the frequencies of orbits. This in turn affects all observables arising from these orbits. Future work will examine how well one can test black holes using gravitational waves and using planned measurements of the shape of a black hole's shadow. 4. The impact of spin-induced precession. In general relativity, spin both generates and couples to spacetime curvature. Spinning bodies in a binary precess in their orbits due to these couplings. This makes the waveform substantially more ornate than in the absence of spin precession, which makes it harder to measure. However, these complicated motions offer an opportunity: Precessions encode information about the spins of the binary's members. This enables measurements to break degeneracies between strongly correlated parameters. Focusing on measurements with planned space-based gravitational-wave detectors, we have shown that spin-induced precession substantially improves the ability of detectors to determine a binary's sky position and provides accurate information about its spins. A complete waveform model enables gravitational-wave detectors to learn an immense amount from the signals we detect. 5. Standard "sirens." Measuring distances is one of the hardest tasks in astronomy. Many measurements use "standard candles," sources whose intrinsic brightness is very well understood. By comparing their measured apparent brightness with their known intrinsic brightness, we infer their distance. Gravitational waves from inspiraling binaries are standard candles: the waves' amplitude is fixed by general relativity and source parameters. Because it is best to think of gravitational waves as sound-like rather than light-like, we call them "standard sirens." We have examined how well gravitational wave measurements of coalescing binaries will be able to take advantage of their standard-siren nature. Focusing on events with an electromagnetic counterpart (e.g., an accompanying gamma-ray burst), we find that advanced LIGO measurements of binaries will constitute good quality standard sirens. If multiple standard sirens occur in the advanced LIGO era, their measurements will provide an independent check on universe's expansion, reducing systematics errors in the measured Hubble constant.
