In the data from the Advanced LIGO detectors, the LIGO Scientific Collaboration and the Virgo Collaboration have identified a further gravitational wave event.
On December 26th, 2015 at 03:38:53 GMT, both the LIGO instruments noticed a binary black hole coalescence, called GW151226.
The new outcomes from the LIGO-Virgo team, including scientists from the Gravitational Wave group of the University of Birmingham’s School of Physics and Astronomy, are out just a few months following the first direct observation of gravitational waves—ripples in the space-time fabric—and the first-ever detection of a binary black hole merger reported on February 11th, 2016.
The finding, reported in Physical Review Letters, is the most recent observation by the Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, situated in Livingston, Louisiana, and Hanford, Washington, United States.
The gravitational waves of the Boxing Day signal were generated by a pair of black holes, of about 14 and 8 solar masses, that traveled for more than a billion years before they reached Earth. LIGO detected the final second of this black hole pair before their collision, at half of the speed of light, to form a new black hole.
During the collision, the equivalent of a solar mass of energy was emitted into ripples of space-time, and a new black hole was produced, which was 21 times heavier when compared to the Sun.
In the first four months of its operation, LIGO has explicitly detected two binary black hole mergers. The next most important candidate event at the time of the entire observation run was recorded on October 12th, 2015. Although it is consistent with a binary black hole’s signal, it is more silent when compared to the two confirmed observations and hence cannot be as surely claimed as an observation.
Gravitational waves include exclusive information related to some of the most fierce phenomena of the Universe. Yet, they interact with particles very weakly and necessitate extremely sensitive instruments to observe.
Already, the LIGO discoveries have unraveled a group of binary black holes, which was earlier unidentified, and have substantiated that the behavior of gravity in the dynamic and strong system is consistent with Einstein’s theory. It is expected that observations in the years to come will offer significant understanding about gamma-ray bursts, neutron stars, supernovae, and the evolution of stars.
As per the plan, at present, the Advanced LIGO instruments are experiencing further commissioning activities. It is anticipated that they will recommence science observations later this year.
In the years to come, the Advanced LIGO detectors will be optimized to full power, thus improving their sensitivity to gravitational waves and enabling more distant events to be evaluated. Further detectors will be added, first in Italy and then in other sites across the globe, and the researchers consider that early observations are just the “tip of the iceberg” of gravitational astronomy.
Birmingham researchers continue to significantly contribute to the search for gravitational waves and the interpretation of the outcomes using existing instruments, as well as to design and develop next generations of gravitational wave detectors.
As members of the international team that observed the gravitational waves, they were proferred with the Special Breakthrough Prize in Fundamental Physics and the Gruber Cosmology Prize in May 2019.
What the Gravitational Wave Group Says About the Findings
According to Dr John Veitch, Ernest Rutherford Fellow at Birmingham and co-chair of the LIGO-Virgo group that led the analysis, “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.”
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.
Andreas Freise, Professor, School of Physics and Astronomy, University of Birmingham
Freise continued, “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 stated, “I have been amazed to watch our gravitational-wave detectors, the most sensitive devices ever built, transform into astronomical discovery machines.”
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 Christopher Berry, School of Physics and Astronomy, University of Birmingham
According to Dr Walter Del Pozzo, from the University of Birmingham’s School of Physics and Astronomy, “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 added, “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!”
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.
Ilya Mandel, Professor, School of Physics and Astronomy, University of Birmingham
In the words of Professor Alberto Vecchio, from the University of Birmingham’s School of Physics and Astronomy, “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.”
Professor Alberto Vecchio and Professor Andreas Freise
Video credit: University of Birmingham.