Infinitesimal ripples in space called gravitational waves have revealed four more instances in which two massive black holes have spiraled into each other and merged with mind-bending violence. Spied between 30 November 2016 and 25 August 2017, the events bring the total number of black hole mergers to 10, report physicists with the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo gravitational wave detector. With help from Virgo’s detector in Pisa, Italy, LIGO’s twin detectors—in Livingston, Louisiana, and Hanford, Washington—are seeing such mergers about once every 15 days of observations, physicists report today at a workshop at the University of Maryland in College Park.
Gravitational waves are mind-bogglingly small distortions of space-time itself that can be set off when two massive objects whirl into each other. LIGO researchers electrified the world in February 2016 when they reported the first observation of such waves, which emanated from two black holes 29 and 36 times as massive as the sun, spiraling together. Twenty months later, LIGO and Virgo wowed the world again when they reported the merger of two much smaller neutron stars. For astronomers, that collision was even more of a gold mine because it produced a gamma ray burst and other electromagnetic signals that, for example, revealed the birth of copious heavy nuclei. (Because they involve no matter, black hole mergers produce only invisible gravitational waves.) Just over a year ago, the developers of LIGO shared the 2017 Nobel Prize in Physics. Virgo came on in 2017 and has also seen three of 11 total sources, helping pinpoint their locations on the sky.
The latest observations set some new records. In particular, a merger spotted on 29 July 2017 was a staggering 9 billion light-years from Earth, and it involved black holes 50 and 34 times as massive as the sun. Physicists still aren’t sure how such big stellar mass black holes form or how they pair. For example, scientists don’t know whether they start out as pairs of stars that collapse into their own black holes or instead start as individual black holes that somehow latch onto each other. The details from a bigger sample of such events could help sort out the correct models. LIGO and Virgo are currently down for maintenance and tuning, and they should resume their searching early next year.