This award funds theoretical research in several different topics in relativistic astrophysics and general relativity, with a focus on sources of gravitational radiation that might be detected by the Laser Interferometer Gravitational Wave Observatory (LIGO) and by the proposed space based detector, the Laser Interferometer Space Antenna (LISA). The principal topics are: (A) the development and implementation of methods to compute the gravitational signals from stars or small black holes inspiralling into large spinning black holes; (B) the assessment of the importance of backreaction effects in cosmology, in order to show definitively that they cannot explain the observed acceleration of the expansion of the Universe; and (C) computing the experimental constraints on various alternative theories of gravity that have been proposed as explanations for the Universe's acceleration.
The research on development of computational methods for gravitational waveforms will eventually have a significant impact on gravitational wave astronomy, by facilitating the detection and analysis of gravitational wave signals, from which we can learn about properties of black holes. Education and training of graduate and undergraduate students will be integrated into the research program.
This project consisted of theoretical research in gravitational wave physics and cosmology. Gravitational waves are a predictionof Einstein's general theory of relativity, and have never yet been directly detected. Efforts are underway to detect gravitational wave signals from astrophysical sources using laser interferometer detector systems such as LIGO (the Laser Interferometer Gravitational Wave Observatory). This project improved the theoretical modeling and understanding of two important classes of gravitational wave sources, binary systems of two black holes, and binary systems of two neutron stars. We discovered a new qualitative effect in binary black holes, transient resonances, that make the signals richer and more complex than had been thought. Resonances are commonplace in dynamicalsystems with weak gravity such as the Solar System whenever three or more bodies are involved; our work showed that in the strong gravitational fields of black holes resonances arise with only two bodies due to nonlinearities in the general theory of relativity. Accurate theoretical models of these systems are important for detecting and extracting information from the gravitational wave signals. A second part of this project has been the development of theoretical models for the observed acceleration of the expansion of the Universe. This acceleration is thought to be due to a form of energy called "dark energy", or alternatively may be due to a modification of the laws of gravity on cosmological distance scales. We explored the observational and theoretical consequences of several classes of models of the cosmic acceleration. In particular, we explored the consequences of a class of theories where a dynamical dark energy component is coupled to cold dark matter, and computed the implications for future high precision cosmological observations.
