When it comes to black holes, the universe is a bit like a two-year-old. It loves making them and then it loves smashing them together.
Although the discovery is worded a bit more scientifically, that's what the latest results from the Laser Interferometer Gravitational Observatory (LIGO) are saying.
Twice last year, the groundbreaking project made headlines by reporting the detection of gravitational waves – ripples in spacetime that were predicted by Albert Einstein a century ago. (A third, weaker detection was suspected but deemed too marginal to publish.) In each case, the ripples were produced by the violent collision of two black holes – massive objects with gravitational fields so strong not even light can escape them.
Now, scientists working with LIGO and a sister project based in Europe say they have once again picked up the signal of a pair of black holes that spiralled in toward one another and merged into one.
"It's a very confident detection," said Harald Pfeiffer, a researcher at the Canadian Institute for Theoretical Astrophysics in Toronto and a member of the international collaboration that operates LIGO.
Based on the characteristics of the vibrations that momentarily disturbed LIGO's sensitive laser-based detectors, the two black holes were about 20 and 30 times the mass of our sun. Their coming together formed a new black hole about 49 times the sun's mass.
That result fits neatly into a gap between the two earlier detections and suggests that "black holes exist in a variety of sizes," Dr. Pfeiffer said.
The find, described Thursday in the journal Physical Review Letters, was made on Jan. 4, about a month after the experiment was switched on for its second run of observation. It further reinforces the impression that black holes were once formed in great abundance in our universe and that they are colliding often enough for LIGO researchers to start reaching more general conclusions based on their properties.
"We're really moving from novelty to a new observational science," said David Shoemaker, a physicist at the Massachusetts Institute of Technology and spokesman for the collaboration.
What is most impressive about the latest detection is that it was made from a distance of about three billion light years, more than double the distances calculated during last year's detections. The greater distance means means that the new signal provides an even stronger test of Einstein's predictions for how gravitational waves should behave as they propagate. So far, it appears that Einstein was right.
In this case, researchers were also able to tease out details that suggest at least one of the black holes was spinning in a direction opposite to the direction that the black holes were orbiting each other as they converged.
That makes it likely that the black holes originated separately and then later captured each other, possibly when they converged at the heart of a dense cluster of stars.
"This is an important clue in understanding how black holes form," said Laura Cadonati, a LIGO researcher based at Georgia Tech University in Atlanta.
The new find was notable for Canadian team members, said Dr. Pfeiffer. For the first time, researchers at the University of Toronto led the effort to characterize the properties of the colliding black holes – a task that rotates between participating teams in the collaboration in two-week shifts during the experiment's round-the-clock monitoring of the heavens.