Collaborative Research: Testing General Relativity with Gravitational-Wave Observations

A collaboration between research groups at the University of Mississippi and Pennsylvania State University will use data from upcoming observations of LIGO and other detectors around the world to subject Einstein’s theory of space, time and gravity to new precision tests. Einstein’s General Theory of Relativity has been a highly successful theory of physics whose prediction of the bending of light was famously confirmed by Sir Arthur Eddington more than a hundred years ago. That same theory predicted the existence of gravitational waves–a new kind of radiation that is produced in extreme astronomical phenomena such as colliding black holes. On September 14, 2015, NSF's Laser Interferometer Gravitational-wave Observatory (LIGO) detected for the first-time gravitational waves from two colliding black holes. Since then, almost 100 such events have been observed. Gravitational waves from colliding black holes are the best grounds for testing if Einstein’s theory is the correct description of spacetime and gravity. Although Einstein’s theory has been remarkably successful in explaining precision terrestrial experiments and astronomical observations, there are hints that the theory is incomplete. Researchers at the University of Mississippi and Penn State have found a way to combine data from multiple events to increase the efficacy of the methods used. They will make use of sophisticated statistical inference tools to ascertain that the results are sound and are of high confidence, but at the same time make sure that noise artifacts in the data and other unrelated physical effects are not misinterpreted as a failure of the theory. The groups will train students in standard scientific practices and advanced analysis techniques and provide them with a platform to work with experts in gravitational wave astronomy globally. The team will also organize Physics and Astrophysics at the 'eXtreme' workshops to stimulate novel ideas via panel discussions and brainstorming sessions. The improved sensitivities of LIGO, Virgo, and KAGRA to detect gravitational waves over the next three years will allow the detection of hundreds of colliding neutron stars and black holes. These collisions will release vast amounts of energy into gravitational waves and the detected signals carry the signature of relativistic gravity in action in unprecedented detail and will have the potential to falsify general relativity (GR). Due to the enormous success of GR in explaining observational and experimental results, the prior probability that the theory is correct is very high. Falsifying GR will require robust statistical inference that accounts for missing physics in the waveform models used in detection and measurement and mitigation of artifacts due to non-stationary noise. For example, most of the current tests assume that binary black holes are in quasi-circular orbits in vacuum, but this is not necessarily true. The principal goal of the study is to strengthen the standard tests of GR using singular value decomposition and to implement a new test utilizing the multipole structure of the emitted waves. These refined tests could reveal GR violations present in high-fidelity signals. It is vital, however, to have a comprehensive list of systematic effects that could be misinterpreted as a GR violation. The second principal goal is to assemble such a list, explore their effect on the various tests of GR, and prepare the collaboration to account for false alarms. This two-pronged approach reinforces the impact of the tests of GR undertaken by the collaborations, while preparing the path to discovering new physics should it show up in the data. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.