Gravitational Wave Astrophysics with LIGO: The Oregon Experimental Relativity Group

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The initial discovery of gravitational waves in 2015 by LIGO represented a triumph of experimental science and a momentous breakthrough in fundamental physics. It confirmed a crucial prediction of General Relativity made by Einstein a century earlier. Its importance was reflected by the 2017 Nobel Prize in physics awarded to NSF PIs Weiss, Thorne, and Barish. The gravitational waves observed by NSF's Laser Interferometric Gravitational-Wave Observatory (LIGO) in 2015 resulted from the collision of two black holes, each roughly thirty times more massive than our sun. Such an amazing event is only observable by gravitational waves -- the ripples in spacetime which propagate outward across the cosmos for millions of light-years and may eventually be noticed by the LIGO observatories. It underscores the potential of LIGO to make unique contributions to astronomy, astrophysics, and cosmology. Since 2015, LIGO has recorded approximately 100 such collisions, mostly of pairs of black holes, but also of black holes with neutron stars, and pairs of neutron stars. Indeed, with this unique collection in hand, LIGO has been able to make new statements about the origins of massive stars in the universe, and paired with more conventional astronomical observations, provided new insights into the origin of heavy elements which are crucial to our life on Earth. Meanwhile, through hard work and innovation, the LIGO observatories have become increasingly sensitive to ever smaller spacetime ripples. This has enabled the potential for new discoveries -- the observation of gravitational waves from different types of astrophysical objects, objects which are not pairs of black holes and/or neutron stars. This award will set up LIGO gravitational wave searches from new sources, namely collapsing black holes and extremely magnetized neutron stars. The former are associated with the explosive phenomena known as (long) gamma-ray bursts (GRBs), while the latter are associated with magnetar x-ray flares and fast radio bursts. As was the case for the 2015 discovery, it might be expected that initial discoveries from new sources will be followed by new astrophysical insight and understanding. This award will largely be used to support graduate students at University of Oregon (UO), who will develop new methods to carry out these novel searches. In keeping with the recent history at UO, it is expected that these students will either continue in the field or will enter the private sector, where they will carry their expertise in data science and analysis to other targets. The UO group will continue to present results to the public and carry out workshops for Oregon high school teachers, so they can, in turn, bring the latest in research to their students.This award will combine two emerging areas of astrophysics to probe sources of gravitational waves from progenitors which are not compact binary mergers. The first area is multi-messenger astronomy (MMA). The second area is the search for gravitational-wave signals without waveform templates – the so-called burst analyses. The searches will fundamentally rely on MMA methods. X-ray, gamma-ray, or radio transient signals from astrophysically energetic events provide a time and sky location which also mark an episode of putative gravitational-wave emission. These provide the so-called triggers for the gravitational-wave data analysis, which labels the time near the trigger as the “on-source” (or signal) data segment, and the off-source data as the background. Excess signal power relative to background is an indicator of a potential gravitational-wave detection in association with the triggering gamma-ray burst (GRB), magnetar x-ray flare, or fast radio bursts (FRB), as the case may be. This method does not rely on any predetermined mathematical form for the gravitational waves. Alongside the searches, the group will develop methods for determining astrophysically interesting parameters from a confident detection, or for constraining such parameters from non-observations, as the case may be. Such parameters might include, for example, those related to the neutron star equation of state for the magnetar searches, or accretion disk properties for the long GRBs. The Oregon group has a strong history of leadership for previous MMA-GW burst searches and the corresponding methods developments. The ever improving sensitivity of the LIGO observatories now makes it plausible for such searches to provide new discoveries.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.