2 September 2020
This artist's concept illustrates a hierarchical scheme for merging black holes. LIGO and Virgo recently observed a black hole merger with a final mass of 142 times that of the sun, making it the largest of its kind observed in gravitational waves to date. The event is thought to have occurred when two black holes of about 66 and 85 solar masses spiraled into each other and coalesced. Theoretical models indicate that nature is not likely to form black holes of this heft; in particular models identify a range of masses between 65 and 120 solar masses, called the "pair instability mass gap," in which it is thought that black holes cannot be formed by a collapsing star. So how did the two merging black holes observed by LIGO and Virgo originate? Scientists think that these black holes may have themselves formed from the earlier mergers of two smaller black holes, as indicated in the illustration.
(Credit: LIGO/Caltech/MIT/R. Hurt (IPAC))
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.
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.
The signal, named GW190521, was observed on May 21, 2019 by the Virgo detector in Italy and the twin detectors at LIGO (Laser Interferometry Gravitational-Wave Observatory) in the United States – which is partially-funded by STFC, which is part of UK Research and Innovation (UKRI).
Science Minister Amanda Solloway said: "Ever since Albert Einstein first predicted the existence of gravitational waves over 100 years ago, the global science community has committed its efforts to identifying them, culminating in the first sighting in 2015.
“Through the ground-breaking work of British and international scientists, this latest discovery is another momentous step forward in advancing our knowledge of the universe, providing more exciting clues about the existence of black holes, how they are formed and how common they are.”
UK teams have played important roles in the development and construction of both LIGO and the data analysis which allows the collaboration to pick out the gravitational wave signals. The UK’s contribution to the collaborations is funded by STFC.
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.
Professor Alberto Vecchio, Director of the Institute of Gravitational Wave Astronomy, University of Birmingham, said: “When stars are too massive they blow up completely when they collapse, leaving nothing behind. A black hole of 85 solar masses should not exist. This is a beautiful discovery and a fascinating puzzle. Now we need to figure out how nature could have possibly assembled such an object.”
Based on how we understand the internal workings of stars, scientists believed that a black hole of this mass could not be formed by a collapsing star. So now, with firm evidence that such massive black holes do exist, astronomers need to rethink what is known about how black holes form.
Professor Stephen Fairhurst, Director of the Gravity Exploration Institute, Cardiff University, said: “Once again, the latest gravitational wave observation challenges our understanding of the universe. We have observed the merger of the most massive pair of black holes to date, including one which is too massive to have formed directly from the collapse of a massive star. Future observations will tell us how common these massive black holes are, and provide further clues to their origins. However, they’re also sure to provide new and unexpected observations to challenge our theories in new ways.”
Dr Laura Nuttall, who is a member of the LIGO Scientific Collaboration, a UKRI Future Leaders Fellow and a senior lecturer at the University of Portsmouth, said: “GW190521 is an extremely exciting event. This is the first time we have detected gravitational waves from the collision of such massive black holes – 85 and 65 times the size of the Sun.
“What is particularly intriguing is the size of these black holes. Up to now, all the black holes that LIGO/Virgo have seen can be explained by the collapse of a massive star to form a black hole. But you can't produce a black hole that is 85 times the size of the sun this way, due to a phenomenon called 'pair instability'. This phenomenon means that collapsing stars, no matter their size, will not produce a black hole between 65 and 120 times the size of the Sun. So how did one of our black holes form?”
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.
LIGO is funded by the US-based National Science Foundation (NSF) and operated by Caltech and MIT, which conceived of LIGO and lead the project. 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). More information is available here.
The Virgo Collaboration is currently composed of approximately 520 members from 99 institutes in 11 different countries. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. More information is available here.
The first detection of gravitational waves in 2015 was a milestone in physics and astronomy; it confirmed a major prediction of Albert Einstein’s 1915 theory of general relativity, and marked the beginning of the new field of gravitational-wave astronomy.
In 2017, scientists announced that they had directly detected gravitational waves in addition to light from the spectacular collision of two neutron stars, marking the first time that a cosmic event has been viewed in both gravitational waves and light. That event was widely reported as helping usher in an era of multi-messenger astronomy. Earlier this year, the LIGO and Virgo Collaborations detected GW190814, a signal in the 'Mass Gap'.
Last updated: 04 September 2020