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Black holes
An artist’s conception shows two black holes in the process of merging. (LIGO / Caltech / MIT Illustration)

Four more mergers of black holes, including the biggest one recorded to date, have been added to a catalog generated by gravitational-wave detectors.

The additions were announced today by the teams in charge of the Laser Interferometer Gravitational-Wave Observatory, or LIGO, and the European-based Virgo detector. The full list of stellar-mass binary black hole mergers now stands at 10, with a neutron-star merger thrown in for good measure.

“The release of four additional binary black hole mergers further informs us of the nature of the population of these binary systems in the universe, and better constrains the event rate for these types of events,” Caltech physicist Albert Lazzarini, deputy director of the LIGO Laboratory, said in a news release

The four previously unreported detections came to light during a re-analysis of data from LIGO’s first two observing runs. The third run, known as O3, is scheduled to begin next spring.

“The next observing run … should yield many more gravitational-wave candidates, and the science the community can accomplish will grow accordingly,” said David Shoemaker, spokesperson for the LIGO Scientific Collaboration and senior research scientist at MIT’s Kavli Institute for Astrophysics and Space Research. “It’s an incredibly exciting time.”

LIGO consists of two L-shaped, 2.5-mile-long detectors. One of them is on the Hanford Nuclear Reservation in eastern Washington state, and the other is near Livingston, La. Both of them use a system of laser beams, mirrors and photon detectors to pick up the relativistic ripples created in spacetime by far-off gravitational disturbances.

The detectors are sensitive enough to sense spatial distortions that amount to less than a thousandth of the width of a proton.

LIGO’s first detection was announced in early 2016, and since then, the addition of the Virgo detector’s readings has greatly improved the ability to identify gravitational-wave disturbances and their origins.

A little more than a year ago, LIGO’s scientists reported the smallest-known black hole merger. The events reported today include the biggest-known such merger.

That event was recorded on July 29, 2017, and involved the smash-up of black holes weighing about 51 and 34 times the mass of our sun. The merger, detected at a distance of 9 billion light-years, produced a much bigger black hole — about 80 times as massive as our sun. The equivalent of nearly five solar masses was converted into gravitational radiation, in accordance with Albert Einstein’s famous equation, E=mc2.

Susan Scott, a physicist at the Australian National University, said it was by far the most distant black-hole merger observed to date. “This event also had black holes spinning the fastest of all mergers observed so far,” she said in a news release.

The three other newly reported black hole mergers were detected in August 2017, at distances ranging from 2.5 billion to 6 billion light-years, and resulted in black holes ranging from 56 to 66 times more massive than our sun.

The LIGO-Virgo detections are discussed in two research papers. One is titled “GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs.” The other is “Binary Black Hole Population Properties Inferred from the First and Second Observing Runs of Advanced LIGO and Advanced Virgo.” Results are also due to be published in Physical Review X.

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