Gravity speaks to us again – 100 years after Einstein’s prediction

An international team of scientists has this week confirmed the detection of gravitational waves from a second instance of two black holes colliding, opening the door to a new age of astronomy.

The new observation, supported by a global team that includes researchers at Strathclyde, took place on 26 December 2015. Two black holes, 14 and 8 times the mass of the sun, orbited each other more than 25 times before merging into a more massive spinning single black hole 21 times the mass of the sun. Unlike the discovery from September 2015, in the Boxing Day binary, at least one of the black holes was spinning.

This latest observation indicates that there is a rich population of binary black holes in the Universe, whose properties are gradually starting to emerge.  Gravitational-wave astronomy is no longer a field of single detections, but of regular observations. This latest discovery transforms the LIGO detector into a true astronomical observatory.

‘Extraordinary events’

Dr Nicholas Lockerbie, of Strathclyde’s Department of Physics, said: “This black-hole-black-hole merger took place approximately 1.4 billion light-years away, and its remoteness in space and time underscores the fact that gravitational waves allow us to detect such events far out into the Cosmos, and far back in time.

“Here on Earth, the present is certainly an exciting time for science, as the dark Universe begins to reveal itself to us through these extraordinary astrophysical events—which have been detected by the immensely sensitive aLIGO detectors.”

What happened?

On December 26, 2015 at 03:38:53 UTC, scientists observed gravitational waves—ripples in the fabric of spacetime—for the second time. The gravitational waves were detected by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA.

This second observation means that when in future we see many of these binary black hole (BBH systems), with aLIGO, and with future detectors of increasing sensitivity, we will start to do cosmology with Gravitational Wave signals – aiming to use a totally new way to probe the mysteries of the expansion of our Universe. 

The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.

What are Gravitational Waves?

Gravitational waves carry unique information about the origins of our Universe and studying them is expected to provide important insights into the evolution of stars, supernovae, gamma-ray bursts, neutron stars and black holes. However, they interact very weakly with particles and require incredibly sensitive equipment to detect. The British and German teams, working with US, Australian, Italian and French colleagues as part of the LIGO Scientific Collaboration and the VIRGO Collaboration, are using a technique called laser interferometry.

Mapping black holes in our universe

“It is very significant that these black holes were much less massive than those in the first detection," said Gabriela Gonzalez, spokesperson of the international LIGO Scientific Collaboration (LSC) and Professor of Physics and Astronomy at Louisiana State University. "Because of their lighter mass, they spent more time – about one second – in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our universe.”

Award-winning science 

In recent months the LIGO project has received recognition for its achievements from around the world. On 3 May, the Special Breakthrough Prize in Fundamental Physics was awarded to the international team responsible for the detection of gravitational waves and this was closely followed by the announcement of the award of the Gruber Cosmology Prize. 

In both cases, the awards acknowledge the profound achievement of LIGO's interferometers.

What happens next?

Over coming years, the Advanced LIGO detectors will be ramped up to full power, increasing their sensitivity to gravitational waves, and in particular allowing more distant events to be measured. With the addition of further detectors, initially in Italy and later in other locations around the world, this is surely just the beginning of gravitational astronomy. UK scientists continue to contribute to the design and development of future generations of gravitational wave detectors. 

The LIGO Scientific Collaboration comprises over 1,000 scientists from 17 countries, and includes researchers from 10 UK universities (Glasgow, Birmingham, Cardiff, Strathclyde, West of Scotland, Sheffield, Edinburgh, Cambridge, King College London and Southampton). The Science and Technology Facilities Council supports this work for the UK.

June 2016