Scientists from the LIGO Scientific Collaboration have observed gravitational waves for a second time, according to an announcement made on June 15 during the 228th meeting of the American Astronomical Society in San Diego. The detection of the waves—ripples in the fabric of spacetime—was made on Dec. 25 by both of the twin Laser Interferometer Gravitational Wave Observatory (LIGO) detectors, located in Livingston LA and Hanford WA, and thereafter confirmed and analyzed by scientists from the LIGO and Virgo Collaborations prior to the announcement.

As with the first observation on Sept. 14, 2015, the detected waves began with the collision of a pair of black holes. The second discovery “has truly put the ‘O’ for Observatory in LIGO,” says 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.”

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,” says Gabriela Gonzalez, a spokeswoman for the LIGO Scientific Collaboration 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.

“In the near future, Virgo, the European interferometer, will join a growing network of gravitational wave detectors, which work together with ground-based telescopes that follow-up on the signals,” says Fulvio Ricci, a spokesman for the Virgo Collaboration. “The three interferometers together will permit a far better localization in the sky of the signals.”

The first detection of gravitational waves, announced on February 11, 2016, was a milestone in physics and astronomy, confirming a major prediction of Albert Einstein’s 1915 general theory of relativity and marking 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,” says France Córdova, director of the National Science Foundation (NSF). “NSF’s 40-­year investment in this foundational research is already yielding new information about the nature of the dark universe.”

The LIGO Observatories are funded by the NSF 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.

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.