'Einstein was right' - Third detection of gravitational waves sheds light on physics of merging black holes

The LIGO-Virgo Team – a worldwide team of research institutions, including the University of Birmingham – has detected a new gravitational-wave signal emanating from the collision of two black holes. The discovery adds further evidence for Einstein's famous theory of general relativity, and confirms the existence of a previously unknown population of black holes.

The Laser Interferometer Gravitational-wave Observatory (LIGO) has made a third confident detection of gravitational waves—ripples in space and time first theorised by Einstein a century ago. As was the case for LIGO's past detections, the gravitational waves were generated when two black holes merged to form a larger black hole. The new observation cements the case for a new class of black hole pairs, or binary black holes. The new-found black hole pair, once merged, has a mass of about 50 times that of our Sun, filling in a gap between the black holes previously observed by LIGO.

"We have further confirmation of the existence of stellar-mass black holes that are larger than 20 solar masses—these are objects we didn't know existed before LIGO detected them," says MIT's David Shoemaker, the newly elected spokesperson for the LIGO Scientific Collaboration (LSC), a body of more than 1,000 international scientists who perform LIGO research together with the European-based Virgo Collaboration.

"It is remarkable that humans can put together a story, and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us. The entire LIGO and Virgo scientific collaborations worked to put all these pieces together."

The gravitational waves travelled approximately 3 billion light-years before they reached Earth on January 4, 2017, just a month after LIGO started its second observing run. The new detection, named GW170104, now joins the family of those which took place during the first observing run: GW150914, the first-ever direct observation of gravitational waves in September of 2015, and GW151226, detected on Boxing Day 2015. The new results are published today in the journal Physical Review Letters.

"Black holes are beautifully simple, you just need two numbers to describe them completely: a mass - how much they bend space-time, and a spin - how much space-time swirls about them. But it takes lots of hard work to measure these from our data," says Dr Christopher Berry of the University of Birmingham, one of the editors of the new paper, which is authored by the entire LSC and Virgo Collaborations.

"We now have wonderful mass measurements, and are starting to uncover details about the spins of these black holes, which could reveal hints about how they formed."

The results demonstrate that LIGO has indeed opened a new window in astronomy, providing unique insight into the nature and prevalence of black holes.

“The black holes that generated GW170104, like the others observed by LIGO, seem to be spinning very differently than the stellar-mass black hole candidates in our galaxy we that we have known so far,” says Dr Will Farr of the University of Birmingham.

“It is very exciting to me that LIGO is providing a way, perhaps the only way possible, to observe this distinct population of black holes and, eventually, understand how they formed.”

The study of GW170104 puts Albert Einstein's theories to the test. The LIGO and Virgo team tested the idea that gravitational waves might travel at different speeds depending on its wavelength, a phenomenon called dispersion that, if observed, would disagree with Einstein's general theory of relativity. LIGO did not see evidence of dispersion.

“LIGO is a wonderful discovery machine that is unveiling some of the best kept secrets of the Universe,” says Professor Alberto Vecchio, director of the University of Birmingham’s Institute of Gravitational Wave Astronomy.

“At Birmingham we have worked for over fifteen years on LIGO and many aspects of gravitational-wave science, first building hardware for the instrument and then sophisticated analysis tools to tease out from the data the properties of gravitational-wave sources. We have now discovered a new and rich population of cosmic objects: every day about a hundred of these binary black holes merge somewhere in the Universe. We are beginning to understand their evolutionary paths, and those of their progenitors. We are putting what not long ago were considered Einstein’s outrageous predictions under the most exquisite experimental test. But this is just the beginning of a new era, and I am sure there’ll be many surprises in the future”.

Scientists will now continue to search the latest LIGO data for signs of space-time ripples from the far reaches of the cosmos. They are also working on technical upgrades for LIGO's next run, scheduled to begin in late 2018, in which the detectors' sensitivity will be improved.

“Once again we have caught the faint echoes from colliding black holes with the LIGO observatory”, says Professor Andreas Freise, deputy director of the Institute of Gravitational Wave Astronomy.

“The LIGO detectors are no ordinary telescopes but a completely new type of instrument developed over several decades just for this purpose. We are now beginning to map the sky for objects and events that no ordinary telescope on Earth or in Space can detect. I am proud to see that the instruments we built already deliver such exciting results.”

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Notes to Editors

Read the full paper.

LIGO is funded by the National Science Foundation (NSF), and operated by MIT and Caltech, which conceived and built the project. 

Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project.
More than 1,000 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration.
LIGO partners with the Virgo Collaboration, a consortium including 280 additional scientists throughout Europe supported by the Centre National de la Recherche Scientifique (CNRS), the Istituto Nazionale di Fisica Nucleare (INFN), and Nikhef, as well as Virgo’s host institution, the European Gravitational Observatory. Additional partners are listed at: http://ligo.org/partners.php.

The University of Birmingham is ranked amongst the world’s top 100 institutions, its work brings people from across the world to Birmingham, including researchers and teachers and more than 5,000 international students from over 150 countries.