New detections of gravitational waves from four black holes have been announced by an international project involving the University of Strathclyde.
A total of 11 gravitational wave events have now been detected - 10 from stellar-mass binary black hole mergers and one from a merger of neutron stars, which are the dense, spherical remains of stellar explosions.
The new discoveries have been jointly announced by the National Science Foundation's LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-based VIRGO gravitational-wave detector.
They follow previous discoveries, announced in 2016 and 2017, of gravitational wave activity, which confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity, and marked the beginning of the new field of gravitational-wave astronomy.
Professor Nicholas Lockerbie, of Strathclyde’s Department of Physics, is a member of the LIGO project. He said: “The two LIGO detectors in the USA, and the VIRGO detector in Europe, have been decades in their conception, construction, bringing into operation, and refinement, requiring the work of more than 1000 scientists and engineers, internationally. Indeed, in collaboration with the Institute for Gravitational Research at the University of Glasgow, my colleagues and I in the Department of Physics at the University of Strathclyde have contributed to this effort.
“However, it is only a little over three years ago now that the very first direct detection of gravitation waves was ever made - by LIGO— the gravitational wave signal from two colliding black holes. It is also only just over one year ago that the very first gravitational wave signal from two colliding neutron stars was seen.
“Subsequently, this event was recorded by telescopes and various types of detector right across the electromagnetic spectrum, and it marked the beginning of true ‘multi-messenger’ astronomy. It even provided a possible explanation for the origin of much of the gold and heavy elements in the Universe!
“The work which is about to be published, and which catalogues the results from the first two observing runs, demonstrates the unparalleled capability of this ground-breaking gravitational-wave network to make new science. The systematic exploitation of the scientific output from these gravitational wave Observatories is a now a reality.”
The new discoveries were announced at the Gravitational Wave Physics and Astronomy Workshop in College Park, Maryland.
One of the new events, codenamed GW170729 and detected in the second observing run on July 29, 2017, is the most massive and distant gravitational-wave source ever observed. In this coalescence, which happened roughly five billion years ago, an equivalent energy of almost five solar masses was converted into gravitational radiation.
The Virgo interferometer joined the two LIGO detectors on August 1, 2017, while LIGO was in its second observing run. Although the LIGO-Virgo three-detector network was operational for only three-and-a-half weeks, five events were observed in this period. Two events detected jointly by LIGO and Virgo, GW170814 and GW170817, have already been reported. GW170814 was the first binary black hole merger measured by the three-detector network, and allowed for first tests of gravitational-wave polarisation, which is analogous to light polarisation.
Three days later, event GW170817 was detected. This was the first time that gravitational waves were ever observed from the merger of a binary neutron star system. Furthermore, this collision was seen in gravitational waves and light, and marked an exciting new chapter in multi-messenger astronomy, in which cosmic objects are observed simultaneously in different forms of radiation.
One of the new events, GW170818, detected by the global network formed by the LIGO and Virgo observatories, was very precisely pinpointed in the sky. The position of the binary black holes, located 2.5 billion light years from Earth, was identified in the sky with a precision of 39 square degrees. That makes it the next best localized gravitational-wave source after the GW170817 neutron star merger.
“It is gratifying to see the new capabilities that become available through the addition of Advanced Virgo to the global network,” said Jo van den Brand of Nikhef - the Dutch National Institute for Subatomic Physics - and VU University Amsterdam, who is the spokesperson for the Virgo collaboration.
“Especially our greatly improved pointing precision will allow astronomers to rapidly find any other cosmic messengers emitted by the gravitational wave sources.”
“The next observing run, starting in Spring 2019, 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 in MIT’s Kavli Institute for Astrophysics and Space Research. “It’s an incredibly exciting time.”
Professor Lockerbie has received the President’s Medal from the Royal Society of Edinburgh and a Special Breakthrough Prize Medal for Fundamental Physics for his work on gravitational waves.