This award supports a program of research and education in computational physics at Bowdoin College, a highly selective undergraduate institution located in Brunswick, Maine. The supported research, in the areas of gravitational wave source simulations (numerical relativity) and computational astrophysics, provides opportunities for undergraduate students to participate. The research goals, involving numerical simulations of neutron stars and black holes in binary orbit, are primarily motivated by the prospect of detecting gravitational radiation from such objects with the new generation of gravitational wave laser interferometers, including the Laser Interferometer Gravitational wave Observatory (LIGO.) To identify and interpret any observed signals, and to extract their astrophysical content, requires theoretical models of the sources. The extreme environments of neutron stars and black holes require them to be modeled within the framework of Einstein's theory of general relativity. The research activities supported by this award are aimed at developing the necessary numerical algorithms, at numerically constructing such theoretical models, at predicting gravitational wave signals for comparison with future observation, and at answering related astrophysical questions.
Undergraduate students will actively participate in a computational physics research group, providing them with a ?hands-on? research experience and generating a research-enriched learning environment. The students will work on a variety of well-defined, manageable and meaningful projects, including, for example, the generalization and extension of previous results and the exploration of simple model problems. They will also have the opportunity to collaborate with scientists at other institutions and to present their work at conferences and in publications, making this research experience a very valuable preparation for graduate training and other future careers in the sciences and beyond.
Einstein's General Relativity describes gravitation. It governs the attraction that we feel to the Earth, the orbits of the Moon and the planets, the structure of our galaxy, and even the evolution and fate of the Universe itself. It has also predicted the existence of objects as exotic as black holes and neutron stars, which we now know to be abundant even in our own galaxy. Black holes are objects whose gravitational fields are so strong that even light cannot escape. Neutron stars are made up of densely packed neutrons; their mass is typically a little more than that of the Sun, yet these stars have a radius of only about 10 miles or so. Because of their large mass contained in a small volume, they also have extremely strong gravitational fields. Another prediction of general relativity is the existence of gravitational waves. Gravitational waves are tiny ripples in the fabric of space and time that travel at the speed of light. They are extremely weak, and are emitted in appreciable amounts only by objects that move very rapidly and that have very strong gravitational fields. Among the most promising sources of gravitational waves, therefore, are binary systems in which two black holes or neutron stars orbit each other. Physicists are optimistic that we will soon be able to detect gravitational waves directly, possibly as early as 2014 or 2015, when the NSF-funded Advanced Laser Interferometer Gravitational-Wave Observatory (or ``Advanced LIGO" for short) will become operational. Such a detection will mark a true milestone in our understanding of gravitation - it will constitute the strongest test of general relativity to date and will help us explore the universe by providing new means of observing black holes and other exotic astrophysical objects. Detecting and interpreting gravitational wave signals requires accurate theoretical models of likely gravitational wave patters. Such patterns can be predicted by finding mathematical solutions to the equations of General Relativity, called Einstein's equations. Since Einstein's equations are very complicated, they can be solved exactly only for very special cases. Modeling more general scenarios, like two black holes or neutron stars orbiting each other, requires numerical solutions - i.e. solutions that are found by writing and running suitable programs on a computer. My on-going research effort addresses a wide range of issues related to finding numerical solutions to Einstein's equations. Some projects answer questions that are directly related to astrophysical problems and aim at predicting the gravitational wave signal from certain objects. For example, much effort during the funding period of this proposal went into a better understanding of black hole-neutron star binaries. In a collaboration with researchers at the University of Illinois, we studied the effect of black hole spin on the merger of a black hole and a neutron star. We found that the more rapidly the black hole spins (in the direction of the orbit), the less likely it is that the entire neutron star is swallowed by the black hole intact. This affects not only the emitted gravitational wave signal, but also the potential of such a merger to emit so-called Gamma-Ray Bursts - enigmatic signals that astronomers have observed for decades, but whose origin is still only poorly understood. I have also solved more abstract conceptional problems that are related to how Einstein's equations are best solved. In this past funding period, in particular, I have explored new approaches to constructing models of black holes. In addition to my research effort, I have also engaged in numerous educational and outreach activities. Most notably my co-author Stuart Shapiro and I have completed a comprehensive textbook on numerical relativity, which was published by Cambridge University Press in the summer of 2010. We also wrote an invited feature article for the magazine Physics Today (October 2011). Finally, my research activities inform my teaching here at Bowdoin College, and help me attract undergraduate students into the physics program. Over the past years I have always had undergraduate students work with me on research projects; many of these students become co-authors on journal publications, present talks at professional conferences, and go on to prestigious graduate schools.
