The new window onto the universe just opened a little bit wider. For the second time in history, an international team of scientists and engineers, including Northwestern University astrophysicists and a laser scientist, has detected gravitational waves - ripples in the fabric of spacetime - and a pair of colliding black holes.

LIGO's first detection of gravitational waves and merging black holes occurred Sept. 14, 2015 - an event that made Scientists detect second pair of colliding black holess worldwide, confirming a major prediction of Albert Einstein's 1915 general theory of relativity. The field of gravitational-wave astronomy was born with a little chirp "heard" on Earth that forever changed the way we see the universe.

The second detection occurred at 03:38:53 UTC (Coordinated Universal Time) Dec. 26, 2015, and is known as the "Boxing Day event" (after the holiday celebrated in the U.K.). Both of the twin, U.S.-based Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) detectors recorded the gravitational waves.

Gravitational waves carry information about the origins of black holes and about the nature of gravity that cannot otherwise be obtained. 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 21 times the mass of the sun. (In comparison, the black holes detected Sept. 14, 2015, were 36 and 29 times the sun's mass, merging into a black hole of 62 solar masses.)

This time, the gravitational waves released by the violent black hole merger resulted in a longer signal, or chirp, providing more data. The new chirp lasted one second; the Sept. 14 chirp lasted just one-fifth of a second. The higher-frequency gravitational waves from the lower-mass black holes better spread across the LIGO detectors' sweet spot of sensitivity.

Gravitational waves are not sound waves, but researchers have converted the gravitational wave's oscillation and frequency to a sound wave with the same frequency, producing a "chirp" people can hear.

The discovery, accepted for publication in the journal Physical Review Letters, was made by the international 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.

Northwestern alumnus David Reitze, now at Caltech and the executive director of the LIGO Laboratory, was one of three scientific leaders to announce the discovery today (June 15) at a media briefing at the summer meeting of the American Astronomical Society (AAS) in San Diego. Kalogera was present at the media briefing.

Scientists now have a small population of black holes from which to learn more about the universe. As Advanced LIGO becomes more and more sensitive, the number of detected black holes will only grow, producing a broad mass spectrum of black holes in nature.

"Scientifically, these black holes are important because it shows binary black holes exist as a population, with a range of masses, forming from a range of different stars," said Vicky Kalogera, director of Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and the Erastus O. Haven Professor of physics and astronomy in the Weinberg College of Arts and Sciences.

Kalogera, a member of the LIGO Scientific Collaboration (LSC), is attending the AAS meeting and was present at the media briefing.

"We expect black holes with a range of masses, which we now are seeing, showing us that black holes form ubiquitously in the universe," said Kalogera, an expert in black-hole formation in binary systems and in LIGO data analysis.

"This second detection also proves the first was not a fluke - the gravitational waves truly came from cosmic sources. Multiple events are exactly what we needed to be convinced beyond any doubt."

Kalogera leads Northwestern's LSC group, which includes Shane L. Larson, research associate professor of physics and astronomy at Northwestern and an astronomer at the Adler Planetarium in Chicago, and Selim Shahriar, professor of electrical engineering and computer science at Northwestern's McCormick School of Engineering.

Kalogera's and Larson's contributions to the new discovery include making predictions for anticipated detections, interpreting the astrophysics, analyzing the data and characterizing the LIGO detectors.

Shahriar, a laser scientist, leads the experimental portion of Northwestern's LSC group and is working to find means to improve the detectors' sensitivity by a factor of 20. Three postdoctoral fellows, five graduate students and approximately 20 undergraduate students also are involved in the Nothwestern research.

"It is very significant that these black holes were much less massive than those in the first detection," said Gabriela Gonzalez, spokesperson of the LIGO Scientific Collaboration and professor of physics and astronomy at Louisiana State University. She joined Reitze in announcing the discovery at AAS.

"Because of their lighter mass, 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," she said.

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 twin LIGO detectors are funded by the National Science Foundation (NSF) and were conceived, built and are operated by Caltech and the Massachusetts Institute of Technology. The NSF has funded all Northwestern LIGO-related research since 2001.