In a new paper published online, scientists from the LIGO and Virgo research collaborations have presented data from a total of 10 stellar-mass binary black hole mergers and one merger of neutron stars. Seven of these events had been reported before, while four of the black hole detections are newly announced. 

The paper represents a significant step forward in the fledgling field of gravitational wave astronomy, which searches for tiny ripples in spacetime left in the aftermath of massive astronomical events. 

University of Glasgow physicists have made major contributions to the search for gravitational waves, including the development and fabrication of the delicate mirror suspensions which make the detections possible and the data analysis which sorts the gravitational wave signals from the background noise of the universe. 

Professor Sheila Rowan is director of the University of Glasgow’s Institute for Gravitational Research. Professor Rowan said: “This remarkable crop of detections show just how valuable gravitational wave astronomy is in developing our understanding of the universe. 

“In less than three years gravitational wave detections have given us direct evidence of the existence of black holes and binary neutron star collisions. Today we present a wealth of new data from LIGO and Virgo to stand alongside the ground-breaking discoveries already made during their initial observing runs.

“It took science a century to confirm Einstein’s prediction of the existence of gravitational waves, but the pace of our discoveries since then has been exhilarating, and we’re anticipating many more exciting detections to come.” 

Professor Martin Hendry is head of the University of Glasgow’s School of Physics and Astronomy. Professor Hendry said: “The four new detections discussed in this paper are exciting, particularly the largest and most distant black hole collision we’ve seen to date, but what’s equally significant is that we now have data from 11 detections collected together as a catalogue. 

“That represents a big step forward in our understanding of the universe, and is a ringing endorsement of the effectiveness 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 University of Glasgow’s gravitational wave research is funded by the Science and Technology Facilities Council (STFC) and in partnership with the Universities of Cardiff and Birmingham, among other UK universities. 

From September 12, 2015, to January 19, 2016, during the first LIGO observing run since undergoing upgrades in a program called Advanced LIGO, gravitational waves from three binary black hole mergers were detected.

The second observing run, which lasted from November 30, 2016, to August 25, 2017, yielded a binary neutron star merger and seven additional binary black hole mergers, including the four new gravitational wave events being reported now.

The new events are known as GW170729, GW170809, GW170818 and GW170823 based on the dates on which they were detected. 

The new event GW170729, 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 5 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 polarization (analogous to light polarization).

Three days later, the event GW170817 was detected. This was the first time that gravitational waves were ever observed from the merger of a binary neutron star system. What’s more, 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 (located in the United States and Italy, respectively), 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. 

A total of 11 confident gravitational-wave detections were obtained by three independent analyses of the O1 and O2 data.

The scientific paper describing these new findings, which has been published on the arXiv repository of electronic preprints, presents detailed information in the form of a catalog of all the gravitational wave detections and candidate events of the two observing runs.

Thanks to more advanced data processing and better calibration of the instruments, the accuracy of the astrophysical parameters of the previously announced events increased considerably.  



University of Glasgow