Most massive gravitational wave signal yet poses new mysteries

Issued: Wed, 02 Sep 2020 13:00:00 BST

The most massive gravitational-wave source yet has been detected – a binary black hole merger, which produced a blast equal to the energy of eight Suns, sending shockwaves through the universe.
The detection provides answers to some fundamental questions about how black holes are formed – and poses some intriguing new ones.
Gravitational waves are produced when an extreme cosmic event occurs somewhere in the universe and, like dropping a rock in a pond, these events ripple across the cosmos, bending and stretching the fabric of space-time itself.

Since gravitational waves were first detected in 2015, from the merger of two black holes more than a billion light years distant, astronomers have witnessed a slew of signals from different cosmic collisions.  Together these events have opened an entirely new window on the universe that offers a unique and powerful probe of the most extreme cosmic phenomena.
This time, researchers believe the gravitational wave detectors have picked up the signal of the most massive black hole merger yet to be observed. The collision involved two inspiralling black holes, the first about 85 times as massive as the Sun, and the second measuring about 66 times the Sun’s mass.
When the two giant, spinning black holes smashed into each other, it created a behemoth black hole – with a mass of about 142 Suns, and a short burst of gravitational-wave energy equivalent to the mass of around eight Suns.  The remnant black hole is the first clear detection of a so-called “intermediate mass black hole”, with a mass between 100 and 1,000 times that of the Sun.
It also appears the signal came from a source about 17 billion light years from Earth, making it one of the most distant gravitational-wave sources detected so far.
They detected the signal, which they have labelled GW190521, on May 21, 2019, with the National Science Foundation’s LIGO (Laser Interferometry Gravitational-wave Observatory (LIGO), based in the United States, and the Virgo detector in Italy.
The international team of scientists, who make up the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration, have reported their findings in two papers published today.
Scientists from the University of Glasgow are key members of the LIGO and Virgo collaborations. They have helped to design and build the mirrors which are at the hearts of the detectors, made vital contributions to the upgrades to the LIGO detectors to make them even more sensitive to gravitational wave signals, and assisted with the complicated data analysis process which picks the signals out from the background noise of the universe.
Daniel Williams, from the University of Glasgow’s School of Physics and Astronomy, said: “Gravitational wave astronomy continues to help us answer questions about how our universe works, as well as present us with exciting new problems to solve. This detection gives us a fascinating first look at the physics of intermediate-mass black holes, and opens up the opportunity for future detections to solve the mystery of just how they are formed.
“One possibility is that the black holes involved in this merger were themselves the product of previous mergers - what we call second-generation black holes. It’s also possible that black holes of this size might have been formed by stripping gas from other nearby stars to add to their own mass before they collided with each other. We’re very much looking forward to finding more pieces of this puzzle in future detections.”
The UK Government, through STFC, helped to fund the upgrades carried out between 2010 and 2015 that turned initial LIGO into Advanced LIGO, and enabled the first ground-breaking detections to be made.  The UK is also investing in the upcoming phase of further improvements (2020-2025) that will upgrade Advanced LIGO to Advanced LIGO+ and which will greatly improve the sensitivity of the detectors to allow even more detections to take place. This next phase of improvements will be funded through UKRI’s Fund for International Collaboration, which aims to enhance the UK’s excellence in research and innovation through forging new bilateral and multilateral research and innovation programmes with global partners.
Professor Sheila Rowan, director of the University of Glasgow’s Institute for Gravitational Research, said: “One of the lessons we’ve learned since the first LIGO observing run is the importance of being able to pause occasionally to upgrade the instruments and improve their sensitivity, because the return on that investment of time in the form of new science is tremendous.
“It translates into more detections, an improved rate of detections, and also detections of individual events made at higher sensitivities. That enables detections like this one, where the very low frequency of the signal might well have been impossible to pick out of the background noise without our improvements. 
“It’s an exciting preview of the kinds of science we can look forward to as we continue to develop the new field of gravitational wave astronomy.”
The detections were only made possible by combining UK innovations in technology, sustained international funding, and enormous dedication and hard work by more than a thousand scientists from around the world. The LIGO Scientific Collaboration comprises over 1,300 scientists from 18 countries, and includes researchers from 11 UK universities (Glasgow, Portsmouth, Birmingham, Cardiff, Strathclyde, West of Scotland, Sheffield, Edinburgh, Cambridge, King College London and Southampton).