For the second time in history, an international team of scientists and engineers have detected gravitational waves -- ripples in the fabric of space-time -- and a pair of colliding black holes.
Using the twin, US-based Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, the second detection occurred on December 26 last year and is named as the "Boxing Day event" (after the holiday celebrated in Britain).
LIGO's first detection of gravitational waves and merging black holes occurred on September 14, 2015 -- an event that made headlines worldwide, confirming a major prediction of Albert Einstein's 1915 general theory of relativity.
"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 University's centre for interdisciplinary exploration and research in astrophysics (CIERA).
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 eight 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 on September 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 September 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 at the summer meeting of the American Astronomical Society (AAS) in San Diego.
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.
"We expect black holes with a range of masses, which we now are seeing, showing us that black holes form ubiquitously in the universe," Kalogera added.
This second detection also proves the first was not a fluke -- the gravitational waves truly came from cosmic sources.
"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.
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 noted.
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.
Scientists from Rochester Institute of Technology and University of Maryland physicists also contributed to the identification of second gravitational wave event.
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