U-M physicists are members of the LIGO team who have, for the second time, definitively observed gravitational waves from the collision of black holes more than a billion years ago, in addition to having likely observed a weak signal from a collision more than three billion years ago. The two definitive detections are GW150914 and GW151226, and the likely detection is LVT151012, where the numbers indicate the year, month and day of detection. For each detection, the black circles indicate the sizes of the two black holes that merged and the size of the final black hole formed from the merger. No light can escape the volumes depicted by the circles, superposed on a map of the northeastern United States.

ANN ARBOR—For the second time, scientists have observed gravitational waves—ripples in the fabric of spacetime caused by the merger of black holes. The observation confirms physicists’ original gravitational wave discovery, which was announced in February.

“This second detection confirms that the original discovery of gravitational waves was no fluke,” says University of Michigan physics professor Keith Riles. He is the principal investigator of the Michigan Gravitational Wave Group and a member of the LIGO Scientific Collaboration’s Executive Committee. “The future of gravitational wave science looks remarkably bright,” he says.

The newly observed gravitational waves were detected by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA.

Gravitational waves carry information about their origins and about the nature of gravity that cannot otherwise be obtained, and physicists have concluded that the detected gravitational waves once again were produced during the final moments of the merger of two black holes—14 and 8 times the mass of the sun—to produce a single, more massive spinning black hole that is 21 times the mass of the sun.

“It is very significant that these black holes were much less massive than those observed in the first detection,” said Gabriela Gonzalez, collaboration spokesperson and professor of physics and astronomy at Louisiana State University. “Because of their lighter masses compared to the first detection, they spent more time—about one second—in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our universe.”

During the merger, which occurred approximately 1.4 billion years ago, a quantity of energy roughly equivalent to the mass of the sun was converted into gravitational waves. The detected signal comes from the last 27 orbits of the black holes before their merger. Based on the arrival time of the signals—with the Livingston detector measuring the waves 1.1 milliseconds before the Hanford detector—the position of the source in the sky can be roughly determined.

The first detection of gravitational waves, announced on February 11, 2016, was a milestone in physics; 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.

The second discovery ‘has truly put the 'O' for Observatory in LIGO,” said Caltech's Albert Lazzarini, deputy director of the LIGO Laboratory. "With detections of two strong events in the four months of our first observing run, we can begin to make predictions about how often we might be hearing gravitational waves in the future. LIGO is bringing us a new way to observe some of the darkest yet most energetic events in our universe."

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.

“With the advent of Advanced LIGO, we anticipated researchers would eventually succeed at detecting unexpected phenomena, but these two detections thus far have surpassed our expectations,” says NSF Director France A. Córdova. “NSF’s 40-year investment in this foundational research is already yielding new information about the nature of the dark universe.”

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.

Riles, at U-M, is especially eager to analyze the upcoming new data. The Michigan group focuses on searching the data for signals even tinier – but longer lasting – than the signals seen on September 14 and, most recently, December 16.

“We look for continuous-wave signals 1000-10,000 times weaker in amplitude, but which are always present in the data,” said Riles who co-leads the LIGO Continuous Waves Search Group. This team of about 40 physicists and astronomers from the United States, Europe, Australia and Asia looks for such waves, which could be emitted by fast-spinning neutron stars in our galaxy. The longer data run planned to start in the fall will make it easier to detect these minute, continuous signals.

“The signals we seek are so weak that we must collect many months of data at a time, and then must run thousands of computers 24 hours per day for several more months, to look for every possible signal from unknown neutron stars with unknown rotation frequencies.”

The LIGO Observatories are funded by the National Science Foundation, and were conceived, built, and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors. LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than 1000 scientists from universities around the United States and in 14 other countries. More than 90 universities and research institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing members of the collaboration. The LSC detector network includes the LIGO interferometers and the GEO600 detector.