They’ve done it again. Scientists using the Laser Interferometer Gravitational-Wave Observatory have detected a second collision between two black holes that sent telltale ripples through the cosmos.
The discovery, described in a paper accepted to Physical Review Letters and at the American Astronomical Society’s 228th meeting in San Diego, confirms that LIGO’s groundbreaking discovery of gravitational waves was no fluke – and draws the first brushstrokes that begin to fill in our understanding of these invisible denizens of the universe.
“It really feels like we are maturing as astronomers,” said Gabriela Gonzalez, spokeswoman for LIGO’s scientific collaboration and an experimental physicist at Louisiana State University.
The signal, which passed through the detector in Livingston, La., at 10:38:52 p.m. Eastern time on Dec. 25 and hit the detector 1.1 milliseconds later, came from the collision of two black holes some 1.4 billion years ago. The gravitationally bound bodies – weighing in at 14.2 and 7.5 solar masses, respectively – had been spinning inexorably toward one another. Their merger created a black hole with the mass of 20.8 suns and released the remaining 0.9 solar masses as energy in the form of gravitational waves.
The discovery may have eased the minds of some scientists, demonstrating that their initial discovery in September, which was revealed in February, was not simply a one-off.
“We were convinced it was real, but if it was the only one, we thought that other people might have some doubts,” Gonzalez said. “But now we know that there are no doubts.”
The masses of these black holes are also reassuring to scientists, who had predicted – based on the few stellar-mass black holes we know about – that such bodies would be somewhere around 10 solar masses. The first discovery picked up black holes that were much larger, carrying 29 and 36 solar masses, respectively.
The scientists now think the rate of these particular collisions occurs around 9 to 240 cubic gigaparsecs per year. (A gigaparsec is roughly 3.26 billion light-years. A cubic gigaparsec, then, is a very, very large volume of space.) That rate falls within their predictions, Gonzalez added.
“It’s consistent with the pre-LIGO predictions, but the pre-LIGO predictions were very, very uncertain. There was a broad range of possibilities,” said Marc Kamionkowski, a theoretical physicist at Johns Hopkins University who was not involved in the research. “The actual astrophysical rate that nature has chosen is near the high end of the range spanned by the pre-LIGO estimates.”
The discovery also allows researchers to start taking a broader view on the diversity of such black-hole events that may pervade the cosmos, said Kamionkowski, who predicted that thousands more would be discovered in the next decade.
“The first event was the discovery of black hole binaries, and the second event ushers in the era of population statistics,” he said. “There’s only so much that you can learn from one black hole merger, and it’s similar in astrophysics: There’s only so much you can learn about star formation or stellar evolution by looking at one star.”
The scientists also were able to determine that one of the black holes was spinning at 20 percent of its maximum possible rate – a rather blunt measurement that likely will be honed with later detections and could help researchers winnow their models of black-hole formation, he added.
The team worked with the Virgo Collaboration, which is building an interferometer in Europe, to analyze the data for this second black hole. Once that detector comes online, the two teams will continue working together, using multiple facilities to help triangulate the origin of these mergers.
In the meantime, Gonzalez said, researchers are still poring over the data, looking to turn up any more hidden astrophysical gems.
“We still are analyzing the data from a lot of other sources, and we could be lucky,” she said.
LIGO arose from a decadeslong effort to search for gravitational waves, which can be triggered by massive objects accelerating or decelerating through space. These ripples disturb space-time, squeezing and stretching it as they pass. Albert Einstein had predicted their existence as part of his general theory of relativity a century ago, but even he had his doubts.
Practically speaking, these ripples are so infinitesimally tiny, so difficult to detect, that you’d need an enormous machine to find them.
Kip Thorne of Caltech and Rainer Weiss of MIT started planning how to build a giant detector in the 1970s. Today, the observatory consists of two L-shaped detectors, one in Livingston, La., and one in Hanford, Wash., each with legs of equal length (2.5 miles). If a gravitational wave passes through the detector, the legs are alternately stretched and squeezed, and, using a system of lasers in mirrors inside, the detector can pick up this distortion.
The initial version of LIGO, which ran from 2002 to 2010, picked up nothing, although that was expected. The advanced setup found its first hit in September, just three days after going live.
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