At 9:38:53 CST on Dec. 25, 2015, scientists observed gravitational waves—ripples in the fabric of spacetime—for the second time.
The gravitational waves were detected by both of the twin Laser Interferometer Gravitational-Wave Observatory detectors, located in Livingston, La., and Hanford, Wash. University of Chicago scientists led by Daniel Holz, assistant professor in physics and astronomy, are members of the LIGO collaboration.
The LIGO detectors operated for approximately four months late last year, yielding about 50 days of data. An analysis of the first 16 days of data yielded the event that the LIGO Collaboration announced in February 2016.
“Now we’ve analyzed the rest of the data, and we have another event that’s particularly interesting,” Holz said. “It’s not quite as loud as the first one, but it’s quite beautiful in its own way. The event is composed of smaller black holes, and at least one is spinning. This marks the official turning point from ‘detector’ to ‘observatory.’
The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration and the Virgo Collaboration using data from the two LIGO detectors.
Gravitational 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—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, LIGO Scientific 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 Feb. 11, 2016, 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.
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,” said 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 coming observing run.
LIGO research is carried out by the LIGO Scientific Collaboration, a group of more than 1,000 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. UChicago’s LIGO group is composed of Holz, postdoctoral fellow Ben Farr, and graduate students Hsin-Yu Chen and Zoheyr Doctor.
The LIGO observatories are funded by the National Science Foundation, and were conceived, built and are operated by the California Institute of Technology and the Massachusetts Institute of Technology. Virgo research is carried out by the Virgo Collaboration, consisting of more than 250 physicists and engineers belonging to 19 different European research groups.
—Prof. Jacqueline Stewart
