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The Eberly College of Science at the Frontier of Gravitational Wave Research

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The Institute for Gravitation and the Cosmos (IGC) is a multidisciplinary institute of Penn State researchers dedicated to the study of the most fundamental structure and constituents of the Universe. Thanks to the sustained support from the Eberly College of Science for over two decades, IGC researchers have been able to make seminal contributions at the forefront of fundamental science, including the physics, astrophysics, and mathematics of gravitational waves.
The Eberly College of Science at the Frontier of Gravitational Wave Research

Abhay Ashtekar

Gravitational waves are predicted by Einstein’s theory of general relativity.  In this theory, the fabric of space-time is curved and gravitational waves are ripples in this fabric, produced by catastrophic events such as black hole collisions. Analysis of gravitational waves requires a mathematical framework that can capture the intricacies of general relativity, powerful computational tools that can handle some of the most complicated nonlinear differential equations, a solid understanding of astrophysical processes, and the ability to efficiently analyze some of the largest data sets encountered in fundamental science.  Advances in all these areas were needed to arrive at the conclusion that the event seen by the LIGO collaboration on September 14, 2015 came from a collision of two black holes that occurred over a billion years ago!

Over the years, IGC researchers have made key contributions to all these different areas:

On the computational side, Dr. Bernd Bruegmann’s group at IGC carried out the first numerical simulation of two black holes orbiting around each other.

On the analytic side, Dr. Jorge Pullin, together with a collaborator, developed a “close limit” approximation scheme that allows one to extract physics from the final stages of a black hole merger.  It continues to serve as a powerful tool some fifteen years later.

While they were graduate students at IGC, Drs. Badri Krishnan and Miguel Campiglia provided the comprehensive analysis of the dynamics of the surface of a black hole, formed by either a gravitational collapse or a merger.  

Another interesting discovery came from the fact that the LIGO detectors did not see any gravitational waves from 2002–2008 because they were then operating at a lower sensitivity.  It concerns the crab pulsar, which was formed in a supernova explosion, recorded by Chinese astronomers in 1054. Through a careful analysis of the astrophysics involved, Dr. Ben Owen used the absence of gravitational waves to show that there are no mountains higher than a centimeter on the crab pulsar!

Finally, as described in the accompanying article, Dr. Chad Hanna and his group played a key role in the identification of the source of the LIGO event of September 14.

In 2001, the National Science Foundation awarded Penn State a Physics Frontier Center with a mission to bring together scientists from different areas ranging from general relativity to astronomy and astrophysics to computer science, statistics, and data analysis. Under the leadership of Drs. Sam Finn and Pablo Laguna, the center organized a very large number of focus sessions, workshops, and outreach initiatives at Penn State. It is no exaggeration to say that the emergence of a cohesive field of gravitational-wave science in the United States owes much to these initiatives.

Hanna’s group has continued to play a leading role in the analysis of the rest of the data from the first scientific run of Advanced LIGO. The announcement of their findings is eagerly awaited. It is generally expected that, when the advanced detector reaches its design sensitivity, the LIGO collaboration will record dozens of events a year, thereby unveiling a new window on the Universe. This fall, Dr. B. Sathyaprakash, widely regarded as the foremost gravitational wave theorist  in the United Kingdom, will join the physics and IGC faculty. In addition to significantly enhancing Penn State’s role in the anticipated discoveries by LIGO in coming years, his presence here will place us in a leadership position in the international collaboration for the next-generation gravitational-wave detectors, which are now being planned.  Through its Swift mission center, Penn State is also actively involved in searches for electromagnetic follow-ups of sources of gravitational waves.

Finally, IGC has launched a new initiative called AMON, led by Drs. Douglas Cowen and Miguel Mostafá.  The goal is to weave together a holistic picture of the cosmos by imaginatively combining the information brought to us by all the cosmic messengers from the high-energy universe—gravitational waves, gamma rays, neutrinos, and cosmic rays. Since IGC is currently the only institute with this capability, eleven leading astrophysical observatories from all over the world have signed letters of collaboration with AMON.

The future of gravitational wave research at IGC is very bright, indeed!

—Abhay Ashtekar