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Gravitational Waves Detected from Second Pair of Colliding Black Holes

The LIGO Scientific Collaboration and the Virgo Collaboration identify a second gravitational wave event

by University of Wisconsin-Milwaukee
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LIGO research facilityUWM’s LIGO research faculty includes (from left) Alan Wiseman, Patrick Brady, Jolien Creighton, and Xavier Siemens, standing in the UWM supercomputer cluster.Photo Courtesy of: UWM Photo/Pete Amland


An international team of scientists—including physicists at the University of Wisconsin-Milwaukee—has observed gravitational waves for the second time.

The gravitational waves were detected on Dec. 26, 2015, at 03:38:53 UTC by both of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA.

The first detection of these waves, announced earlier this year, was a milestone in physics and astronomy; it confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity, and marked the beginning of the new field of gravitational-wave astronomy.

Related Article: Gravitational Waves Detected 100 Years After Einstein's Prediction

“This second detection tells us that the first detection wasn’t some sort of lucky break,” said Patrick Brady, director of the Leonard E. Parker Center for Gravitation, Cosmology and Astrophysics at UWM. “Gravitational-wave astronomy has truly started.”

Gravitational waves are ripples in the fabric of space-time that are caused by the movement of massive objects in space. The waves carry information about their origins and about the nature of gravity that cannot otherwise be obtained, and physicists have concluded that these gravitational waves were produced during the final moments of the merger of two black holes about 1.4 billion years ago. The collision of the two black holes, 14 and 8 times the mass of the sun, created a single, more massive spinning black hole that is 21 times the mass of the sun. 

The discovery, which will be described in an article accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration, a group of more than 1,000 scientists (including approximately 250 students) from universities around the U.S. and in 14 other countries, and the Virgo Collaboration, consisting of more than 250 scientists from 19 European research groups. 

In addition to Brady, UWM’s team includes faculty members Jolien Creighton, Xavier Siemens, and Alan Wiseman along with 26 scientists and students. UWM also contributed significant computer resources to the data analysis that identified both events.

It is a promising start to mapping the populations of black holes in our universe, said Brady. Because black holes are not visible even with the most powerful telescopes, scientists know relatively little about them.

“Black holes are formed when massive stars die,” he said. “But we know very little about how many black holes are out there, how massive they are, or how fast they spin. This discovery is a major step toward finding the answers to these questions.”

The first black hole merger detected was unexpectedly massive, said Creighton. Until then, there was no information that proved that mergers could exist at so large a mass.

“The signal of the first one stood out from the noise much more than this one,” he said. “This one is still a gold-plated event; it just required more sophistication to detect it in the data.”

The detected signal comes from the last 55 orbits of the black holes, before their merger. Based on the arrival time of the signals—with the observatory in Livingston, Louisiana measuring the waves 1.1 milliseconds before the observatory in Hanford, Washington—the position of the source in the sky can be roughly determined.

Both discoveries were made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed.

Related Article: Continuing the Search for Gravitational Waves

Advanced LIGO’s next data-taking run will begin this fall. By then, further improvements in detector sensitivity are expected to allow LIGO to reach as much as 1.5 to 2 times more of the volume of the universe. The Virgo detector is expected to join in the latter half of the upcoming observing run.

More about LIGO 

The LIGO Observatories are funded by the National Science Foundation (NSF), and were conceived, built, and are operated by Caltech and MIT.

The NSF also leads in financial support for Advanced LIGO. Funding organizations in Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council, STFC) and Australia (Australian Research Council) also have made significant commitments to the project.

Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German UK GEO collaboration. Significant computer resources have been contributed by the University of Wisconsin-Milwaukee, the AEI Hannover Atlas Cluster, the LIGO Laboratory, Syracuse University, the ARCCA cluster at Cardiff University, and the Open Science Grid.

Several universities designed, built, and tested key components and techniques for Advanced LIGO: The Australian National University, the University of Adelaide, the University of Western Australia, the University of Florida, Stanford University, Columbia University in the City of New York, and Louisiana State University. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom and Germany, and the University of the Balearic Islands in Spain.