The LIGO Scientific Collaboration and the Virgo Collaboration identify a further gravitational wave event in the data from the Advanced LIGO detectors.

On 26 December 2015 at 03:38:53 GMT, the twin LIGO instruments observed a binary black hole coalescence, named GW151226. 

The new findings from the LIGO-Virgo team, that includes researchers from the Gravitational Wave group of the University of Birmingham’s School of Physics and Astronomy, follows only a few months after the first direct detection of gravitational waves, ripples in the fabric of space-time, and the first observation of a binary black hole merger reported on 11 February 2016.

The discovery, published in Physical Review Letters, is the latest by the Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA.

The gravitational waves of the Boxing Day signal were produced by a pair of black holes, of around 14 and 8 solar masses, that travelled for over a billion years before reaching Earth. LIGO observed the final second of this pair of black holes before they collided, at half of the speed of light, to form a new black hole.

In the collision the equivalent of a solar mass of energy was released into ripples of space-time, and a new black hole was formed, 21 times heavier than the Sun.

In its first four months of operation LIGO has unambiguously observed two binary black hole mergers. The next most significant candidate event during the whole observing run was recorded on 12 October 2015. Though consistent with a signal from a binary black hole, it is quieter than the two confirmed observations and therefore cannot be as confidently claimed as a detection.

Gravitational waves carry unique information about the some of the most violent phenomena of our Universe. However, they interact very weakly with particles and require incredibly sensitive instruments to detect.

The LIGO discoveries have already revealed a population of binary black holes, which was previously unknown, and have confirmed that the behavior of gravity in the strong and dynamic regime is consistent with Einstein’s theory.  Future observations are expected to provide important insights into neutron stars, the evolution of stars, supernovae and gamma-ray bursts.

As planned, the Advanced LIGO instruments are currently undergoing further commissioning activities and are expected to resume science observations later this year.

Over the coming years, the Advanced LIGO detectors will be ramped up to full power, increasing their sensitivity to gravitational waves and allowing more distant events to be measured. With the addition of further detectors, initially in Italy and later in other locations around the world, the team believe that early detections are only the ‘tip of the iceberg’ of gravitational astronomy.

Birmingham scientists continue to play a significant role in the search for gravitational waves and the interpretation of the results using current instruments, and in the design and development of future generations of gravitational wave detectors.

As members of the international team responsible for the detection of gravitational waves, they were awarded the Special Breakthrough Prize in Fundamental Physics and the Gruber Cosmology Prize in May of this year.

Dr John Veitch, Ernest Rutherford Fellow at Birmingham and co-chair of the LIGO-Virgo group that led the analysis, said: “The Boxing Day signal was a great belated Christmas present that confirms the existence of a population of coalescing binary black holes. This signal was longer and quieter than the one we observed in September, making it more difficult to see by eye, but it sticks out clearly as the second most significant event in our search of the first Advanced LIGO dataset.”

Professor Andreas Freise, from the University of Birmingham’s School of Physics and Astronomy, said: “This second detection by LIGO shows us the path to the future. LIGO was not build for a single discovery, it is a new observatory with the aim to observe the universe in a completely new way, continuously taking data with many discoveries still to come.”

“The Advanced LIGO detectors are a masterpiece of experimental physics. They are the most sensitive gravitational wave detectors ever built, and this is what they were built to do: there was a ‘disturbance in the gravitational force’, and the LIGO detectors have felt it! We started with a well-known concept, a light interferometer, but it required new technologies that we have developed over several decades to create these extremely sensitive listening devices for gravity signals from the universe.”

Dr Conor Mow-Lowry, from the University of Birmingham’s School of Physics and Astronomy, added, I have been amazed to watch our gravitational-wave detectors, the most sensitive devices ever built, transform into astronomical discovery machines."

Dr Christopher Berry, from the University of Birmingham’s School of Physics and Astronomy, said: Black holes are actually remarkably simple: they are described by just their mass and their spin. Gravitational-wave observations are great for measuring masses, but figuring out spin is much trickier. The Boxing Day signal gives us the first clear evidence that one of the binary's black holes must be spinning."

Dr Walter Del Pozzo, from the University of Birmingham’s School of Physics and Astronomy, continued: There is no discordance between our mathematics and Nature. The sound of colliding black holes is in tune with Einstein's theory of gravity.”

Dr Will Farr, from the University of Birmingham’s School of Physics and Astronomy said, “One of the most exciting things for me is the confirmation that these events are much more common than we expected.  Every fifteen minutes or so a gravitational wave from a pair of merging black holes somewhere in the universe passes through you!”

Professor Ilya Mandel, from the University of Birmingham’s School of Physics and Astronomy said, Gravitational-wave observations allow us to probe the short, brilliant, turbulent, but somewhat secretive lives of massive stars.   Like a paleontologist who uses the skeletons of dinosaurs to discover what living dinosaurs looked like, we can begin to probe the evolutionary history of massive stars by observing their compact remnants, merging pairs of black holes.”

Professor Alberto Vecchio, from the University of Birmingham’s School of Physics and Astronomy said, “This was a late wonderful Christmas present. The curtain is definitely up on the stage of gravitational-wave astronomy. We are unveiling a population of binary black holes in the Universe, testing Einstein’s  predictions in new regimes, and I am looking forward to be surprised over and over again by what we are going to discover next.”

An animation explaining gravitational waves, audio files and videos with the Birmingham group are available for download. Please credit University of Birmingham.

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The University of Birmingham has been involved in the Advanced LIGO project since its inception and the Gravitational-Wave Group has developed and built components for the most sensitive instruments in the world – the high-performance sensors and control electronics for the LIGO suspension systems. Birmingham physicists have developed one of the main optical simulation tools, FINESSE, and contributed significantly to the design and commissioning of modern gravitational wave detectors. Experts in Birmingham are now investigating the behaviour of macroscopic quantum systems to explore new frontiers in precision measurement at the quantum limit.

The Birmingham group has also developed the techniques essential to tease out the signatures of gravitational waves from the data.  The group has pioneered the theoretical framework and analysis algorithms that are at the heart of the study of the physics of compact binary systems, their astrophysical evolution and tests of Einstein's theory with gravitational-wave observatories. The group is involved with the ground-based detectors LIGO, GEO600 and Virgo, the space-based mission eLISA and the European and International Pulsar Timing Array. 

LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than 1000 scientists from universities around the United States and in 14 other countries. More than 90 universities and research institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing members of the collaboration. The LSC detector network includes the LIGO interferometers and the GEO600 detector. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom, and the University of the Balearic Islands in Spain.

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 organizations in Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council, STFC) and Australia (Australian Research Council) also have made significant commitments to the project. Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German UK GEO collaboration. Significant computer resources have been contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory, Syracuse University, and the University of Wisconsin-Milwaukee.  Several universities designed, built, and tested key components for Advanced LIGO: The Australian National University, the University of Adelaide, the University of Florida, Stanford University, Columbia University in the City of New York, and Louisiana State University.