Listening in on a black hole collision

When two black holes merged 1.8 billion lightyears away, their violent union sent shock waves through space and time. On Aug. 14, three precisely tuned machines sensed the cosmic fallout, a ripple known as a gravitational wave. August’s event marked only the fourth time that astronomers have observed black hole collisions.

An international team of scientists announced the discovery from Turin, Italy, Wednesday.

The science of hunting gravitational waves is old on paper and young in practice. Albert Einstein, through his General Theory of Relativity, predicted in 1916 that the waves should exist. It would remain a prediction for 98 years, until the LIGO Scientific Collaboration detected the first gravitational wave in September 2015.

The two detectors that make up the Laser Interferometer Gravitational-Wave Observatory, located in Washington state and Louisiana, recently partnered with a third: the Virgo detector near Pisa, Italy. The detectors hear waves as a spike in frequency sometimes called a cosmic chirp. August’s chirp was the first signal detected by all three observatories; Virgo was online for just two weeks when it detected the gravitational wave.

Now that Earth’s threedetector network for sensing gravitational waves is operational, astronomers hope to zoom in on the source of the waves.

The Virgo detector — a nearly 2-mile-long, Vshaped instrument dug into the Italian countryside — is less sensitive than the two American detectors that first found gravitational waves two years ago. But its inclusion allows scientists to triangulate the origin of these space-time ripples with greater precision.

“We go from hundreds of square degrees, almost thousands, to only 30 square degrees,” Gonzalez said. (“Square degrees” are the units with which astronomers measure the celestial sphere of the night sky; the full moon takes up about 0.2 square degrees, the constellation Hydra covers 1,303.)

Astronomers said they were able to trace the gravitational wave, GW170814, to a region of sky of 60 square degrees. That’s a space 10 times smaller than if Virgo hadn’t been listening in.

The precision becomes even more important as gravitational wave detectors begin to detect signals from events involving objects other than black holes. Whereas black holes emit no radiation and are impossible to directly observe, other potential sources of gravitational waves — colliding neutron stars, supernovas, binary star mergers — can be seen through conventional telescopes.