Gravitational wave astrophysics is entering a new era of discovery and exploration, with first detections expected within the next few years with Advanced LIGO. One of the key missions of the LIGO project is to test Einstein's theory of General Relativity where it has never been tested before: where the gravitational forces are enormous compared to those found in the solar system. Such a task is achievable because gravitational waves encode information about the strong gravitational forces present when black holes and neutron stars collide to produce gravitational waves. This award supports a research program to develop and implement a generic and model-independent framework to test Einstein's theory of General Relativity and to search for anomalous deviations from it in gravitational wave data. The necessary tools to achieve this objective will be developed and implemented, so that they can be readily deployed in LIGO.
The research will enable exploitation of the full discovery potential of gravitational wave detection. This type of research is also timely and relevant, in view of upcoming, advanced detectors and the dawn of gravitational wave astrophysics. While the detection of a departure from Einstein's predictions could signal transformative new physics, the absence of such departures could provide strong confirmation for general relativity in an as yet untested regime.
This award focused on learning whether gravitational waves (ripples in spacetime predicted by Einstein) can be used to test General Relativity when black holes and neutron stars collide. We have learned that indeed this is the case, using a theory-independent tool we developed, which we call the parameterized post-Einsteinian or ``ppE'' scheme. This tool suggests the proposal of template filters that are small deformations away from the General Relativity prediction. Once a gravitational wave is detected with the US Laser Interferometer Gravitational Observatory, one can then determine whether the data is consistent with zero deformation or not. In this way, a particular modified gravity need not be studied, but rather, one tests the null hypothesis that General Relativity is correct. We have also discovered a set of almost universal relations among certain properties of neutron stars that should allow for stronger tests of General Relativity. These universal relations relate the star's moment of inertia (related to how much energy is stored in its rotation) to the star's quadrupole moment and Love number (related to how easily it can be deformed due to rotation or the presence of a second body). These relations depend only on the mass and spin of the star, but they are approximately independent of the star's internal structure. Thus, given a measurement of a neutron star's moment of inertia (for example through future binary pulsar observations) and of the star's Love number (for example through future gravitational wave observations), one should be able to place strong constraints on deviations from Einstein's theory.