When two black holes merged 1.8 billion light-years 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, early Wednesday afternoon.
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 LIGO, 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 three-detector network for sensing gravitational waves is operational, astronomers hope to zoom in on the source of the waves.
Gabriela Gonzalez, a professor of physics and astronomy at Louisiana State University and former spokeswoman for the LIGO team, said this is a milestone detection for the collaboration.
The Virgo detector – a nearly 2-mile-long, V-shaped 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 in the collaboration 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 large 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 as well.
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.
Knowing exactly where a gravitational wave came from, Gonzalez said, will help LIGO scientists correlate their detection to optical counterparts – heralding a new era of astronomy in which celestial events are “heard” as well as “seen.”
“This proves we are now ready to do that kind of astronomy,” Gonzalez said.
In fact, since mid-August, rumors have circulated that the detectors sensed gravitational waves from colliding neutron stars on Aug. 17. If so, it is an event that might also have been seen on the electromagnetic spectrum by telescopes like Hubble and the Chandra X-ray Observatory. In mid-August, these telescopes were all abruptly aimed toward the origin of a gamma-ray burst, an explosion predicted to come from the impact of two neutron stars.
(In a news release for Tuesday’s announcement of the gravitational wave, the LIGO and Virgo researchers hoped to tamp down some of the speculation: “LIGO and VIRGO’s partner electromagnetic facilities around the world didn’t identify a counterpart for GW170814, [the gravitational wave from colliding black holes] which was similar to the three prior LIGO observations of black hole mergers. Black holes produce gravitational waves but not light.”)
Gonzalez said that the LIGO team is still seeking to confirm several potential detections from the latest observing run, which started in January and ended Aug. 25. She would not say whether the Aug. 17 signal was among them. But Gonzalez explained that gravitational wave events with optical counterparts can take longer to confirm because of the consistency tests involved.