The LIGO Scientific Collaboration has reported identification of a second gravitational wave event in the data from Advanced LIGO detectors. On December 26, 2015, at 3:38:53 UTC, scientists observed gravitational waves — ripples in the fabric of spacetime — for the second time. The waves were detected by both of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors. 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. This illustration shows the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other. The black holes — which represent those detected by LIGO on Dec. 26, 2015 — were 14 and 8 times the mass of the sun, until they merged, forming a single black hole 21 times the mass of the sun. In reality, the area near the black holes would appear highly warped, and the gravitational waves would be difficult to see directly. Courtesy of LIGO/T. Pyle. "It is very significant that these black holes were much less massive than those observed in the first detection," says Gabriela González, LIGO Scientific Collaboration (LSC) 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 black hole 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 ms before the Hanford detector — the position of the source in the sky can be roughly determined. This timeline shows the dates for two confirmed gravitational-wave detections by LIGO and one candidate detection, which was too weak to unambiguously confirm. All three events occurred during the first four-month run of Advanced LIGO — the upgraded, more-sensitive version of the facilities. The three events are GW150914 (Sept. 14, 2015), LVT151012 (Oct. 12, 2015), and GW151226 (Dec. 26, 2015). Courtesy of LIGO. The discovery was published in Physical Review Letters. The LIGO Observatories are funded by the National Science Foundation (NSF), and were conceived, built, and are operated by Caltech and MIT. Photonics Media reported on the first detection of gravitational waves, announced on February 11, 2016, which 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. 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, a third interferometer located in Cascina, Italy, is expected to join in the latter half of the upcoming observing run. LIGO research is carried out by the LSC, a group of more than 1,000 scientists from universities around the United States and in 14 other countries. Virgo research is carried out by the Virgo Collaboration, consisting of more than 250 physicists and engineers belonging to 19 different European research groups. This artist's animation shows the merger of two black holes and the gravitational waves that ripple outward during the event. Courtesy of LIGO/T. Pyle.