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Gravitational waves detected 100 years after Einstein’s prediction

Published: 28 June 2016
two black holes merging
Graphic representation of two black holes merging. Credit: SXS Collaboration/CITA/SciNet

An international team of scientists has announced a second instance of the detection of gravitational waves—ripples in the fabric of space-time. The observation was made on Boxing Day, 26 December 2015 – following the first ever detection of its kind in September 2015, which was at the centre of much publicity earlier this year.

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 and neutron stars, as well as black holes.

Both discoveries were made by a global team of scientists as part of the Advanced LIGO (aLIGO) project – involving many researchers from the UK, including mathematicians from the University of Southampton.

This latest observation is of two black holes, 14 and eight times the mass of the sun, orbiting each other over 25 times, before merging into a more massive spinning single black hole, 21 times the mass of the sun. 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 event 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. The latest discovery transforms the LIGO detector into a true astronomical observatory.

Mathematician at the University of Southampton, Dr Ian Jones works on aLIGO with his PhD student Greg Ashton. Dr Jones has spent 13 years on the international gravitational wave detection project, providing colleagues with models for what the gravitational wave signals from small dense stars, known as neutron stars, might look like and advising how best to search for these signals.

He comments: “This second detection is important for two reasons. Firstly, it gives us confidence that we weren't just lucky with the first detection, and that the era of gravitational wave astronomy truly has arrived. Secondly, we can now, for the first time, compare and contrast two different signals, in this case seeing how black holes of lower mass are visible to us for longer before they finally collide to form one larger black hole.”

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.

Scientists believe that in the future we’ll see many of these binary black hole systems, with aLIGO. 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 LIGO Scientific Collaboration comprises over 1,000 scientists from 17 countries, and includes researchers from ten UK universities (Glasgow, Birmingham, Cardiff, Strathclyde, West of Scotland, Sheffield, Edinburgh, Cambridge, Kings College London and Southampton).

theory, the distance between the mirrors will change by a tiny amount when a gravitational wave passes by the detector. A change in the lengths of the arms of close to 10-19 metres (just one-ten-thousandth the diameter of a proton) can be detected.

Independent and widely separated observatories are necessary to verify the direction of the event causing the gravitational waves, and also to determine that the signals come from space and are not from some other local phenomenon. To ensure absolute accuracy, the LIGO consortium spent several months carefully checking and re-checking the data.

The discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed—and the discovery of gravitational waves during its first observation run.

The US National Science Foundation leads in financial support for Advanced LIGO. Funding organisations in Germany (Max Planck Society), the UK (Science and Technology Facilities Council, STFC) and Australia (Australian Research Council) also have made significant commitments to the project.

Gravity Group, University of Southampton


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