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 presented this week at the American Astronomical Society's 228th meeting in San Diego, confirms that LIGO's initial groundbreaking discovery of gravitational waves was no fluke. It also 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 González, a spokeswoman for LIGO's scientific collaboration and an experimental physicist at Louisiana State University.
The signal — which passed through the LIGO detector in Livingston, La., at 7:38:52 p.m. Pacific time on Dec. 25 and hit the twin detector in Hanford, Wash., 1.1 milliseconds later — was produced by 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, 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 researchers, demonstrating that their initial discovery in September 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," González said. "Now we know that there are no doubts."
The masses of these black holes are also reassuring to scientists. Based on the few examples they knew about, they had predicted that such bodies would be somewhere around 10 solar masses. The first discovery, revealed in February, picked up black holes that were much larger, carrying 29 and 36 solar masses.
With two mash-ups in the books, the scientists now think the rate of these particular collisions may range from 9 to 240 per cubic gigaparsec 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, González said.
"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 allows researchers to start taking a broader view on the diversity of black-hole merger 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."
These events also offer scientists a singular method for probing the limits of Einstein's long-lived theories about the universe, Kamionkowski added.
The scientists were able to determine that in the second collision, one of the black holes was spinning at more than 20% of its maximum possible rate — a measurement that likely will be honed with future detections and could help researchers winnow their models of black-hole formation, he added.
The LIGO team analyzed the new data with the help of the Virgo Collaboration, which is building an interferometer in Europe. Once that detector comes online, the two teams will continue working together, using multiple facilities to help triangulate the origin of these mergers.
While LIGO can detect that an event happened, it can't tell exactly where the source is, David Reitze, LIGO's executive director, said at the astronomy meeting.
The more detectors there are to catch the same event, the better scientists can narrow down its location. Luckily, more seem to be on the way: Virgo should conduct its first run in early 2017, Japan is building the KAGRA detector, and there are tentative plans to build one in India, too.
"We don't want to just know that we saw black holes, we don't want to just know the masses — we want to know where on the sky they came from," Reitze said.
In the meantime, González said, researchers are still poring over the data, looking to turn up any other hidden astrophysical gems, such as neutron-star binaries.
"We could be lucky," she said.
Unlike black holes, colliding neutron stars give off not just gravitational waves but also light, from X-rays and gamma rays to visible, infrared and even radio waves. If both LIGO and traditional telescopes were to study the same neutron-star collision, it could paint an unparalleled portrait of these extreme interstellar events.
LIGO arose from a decades-long 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 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 the two L-shaped detectors in Louisiana and Washington state, each with legs 2.5 miles long. If a gravitational wave passes through a detector, its legs are alternately stretched and squeezed. The detector can pick up this distortion with a system of lasers and mirrors.
The initial version of LIGO ran from 2002 to 2010 but sensed nothing, which was expected. The advanced setup found its first hit in September, just three days after going live.
By the time LIGO starts its next run this fall, the detector's sensitivity should be 15% to 25% better than its run last year, Reitze said.
LIGO is on a winning streak. Its founders — Thorne, Weiss and former Caltech experimentalist Ronald Drever — were awarded the 2016 Kavli Prize this month for their work, just days after winning the Shaw Prize in Astronomy in late May.
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3:38 p.m.: This article was updated with additional details, including comments from David Reitze during Wednesday's briefing.