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Colliding black holes detected for fourth time


    A NASA/JPL-Caltech infrared image from NASA’s Spitzer Space Telescope shows the center of the Milky Way galaxy.

In another step forward for the rapidly expanding universe of invisible astronomy, scientists said today that on Aug. 14 they had recorded the space-time reverberations known as gravitational waves from the collision of a pair of black holes 1.8 billion light years away from here.

It was the fourth time, officially, in the last two years that astronomers have detected such ripples from the cataclysmic mergers of black holes — objects so dense that space and time are wrapped around them like a glove so that not even light can escape.

In the August event, one black hole with about 31 times the mass of the Sun and another, with 25 solar masses, combined to make a hole of 53 solar masses. The remaining three solar masses were converted into gravitational waves that radiated more energy than all the stars in the known universe. The observation is in line with earlier gravitational wave detections, confirming an evolving view of the cosmic night.

The detection, announced at a G7 meeting of science ministers in Turin, Italy, and in a paper in the journal Physical Review Letters, marked the successful debut of a new gravitational wave detector known as Virgo, built by a European collaboration and located in Cascina, close to Pisa, Italy.

The first detections of gravitational waves had been made by a pair of L-shaped antennas, called LIGO, in Hanford, Washington, and Livingston, Louisiana, which monitor the squeezing and stretching of space between a pair of delicately positioned mirrors as a gravitational wave goes by. That announcement in February 2016 confirmed the existence of gravitational waves first predicted by Albert Einstein a century ago, and verified the nature of black holes, causing a sensation. LIGO’s leaders are now front-runners for the Nobel Prize in Physics, to be announced next week.

On Aug. 1, the Virgo antenna, built by the European Gravitational Observatory, came on line to join the existing LIGO antennas.

The addition paid off almost immediately, scientists for the observatories said on Wednesday, when a pair of black holes in collision rattled the antennas on Aug. 14. Although the Virgo antenna is still only about one-fourth as sensitive as the LIGO antennas, it greatly increases the network’s ability to triangulate the sources of gravitational waves so that optical telescopes can search for any accompanying fireworks in the visible sky.

To date none have been detected given that black holes are composed essentially of empty twisted space. But gravitational wave astronomers have hopes of finding other types of collisions, involving dense balls of matter called neutron stars, that will spark up the night aplenty.

The current observing run ended on Aug. 25. After a year of work improving the sensitivities of their instruments, a new run will begin in the fall of 2018. Hopes are, you might say, sky high.

In a news release from the University of Glasgow sent out before the G7 meeting, Sheila Rowan, director of the university’s Institute for Gravitational Research, said: “We now are demonstrating the capabilities of a network of gravitational wave detectors, which deepens the pool of data we’ll be able to draw from in future and will help to further expand our understanding of the universe.”

MIT’s David Shoemaker, spokesman for the LIGO Scientific Collaboration, said: “This is just the beginning of observations with the network enabled by Virgo and LIGO working together. With the next observing run planned for Fall 2018, we can expect such detections weekly or even more often.”

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