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Gravitational wave discovery confirms new population of black holes

Gravitational wave discovery confirms new population of black holes

Issued: Thu, 01 Jun 2017 16:00:00 BST

University of Glasgow astrophysicists are celebrating the third detection of gravitational waves – ripples in spacetime which are beginning to underpin an entirely new form of astronomy.

This new gravitational wave detection comes from the merger of two massive black holes. The waves cast out across the cosmos by the immense event reached Earth on January 4, 2017, having travelled around three billion light-years on their journey. The merger of the black holes created an even larger black hole, with a mass of about 49 times that of our sun.

Researchers from our School of Physics and Astronomy discuss the latest detection

One light-year measures the distance that light travels in one year, meaning the merger event which caused the spacetime ripples happened billions of years ago, when the earth was just a fraction of the age it is today.

The very faint gravitational wave signal was picked up by the Laser Interferometer Gravitational-wave Observatory (LIGO), twin detectors based in the United States at Hanford, Washington, and Livingston, Louisiana.

LIGO is operated by Caltech and MIT with funding from the National Science Foundation (NSF), and supported by vital input from more than 1,000 researchers around the world – including the Universities of Glasgow, Cardiff and Birmingham amongst others in the UK.

Scientists from the University of Glasgow’s Institute for Gravitational Research led on the conception, development, construction and installation of the sensitive mirror suspensions in the heart of the LIGO detectors.

LIGO made the first-ever direct observation of gravitational waves in September 2015 during its first observing run since undergoing major upgrades in a program called Advanced LIGO. A second detection was made in December 2015. The third detection, called GW170104 and made on January 4, 2017, is described in a new paper accepted for publication in the journal Physical Review Letters.

Dr John Veitch, of the University’s School of Physics and Astronomy, co-chaired the group that led analysis of the data from LIGO.

Dr Veitch said: “GW170104, like its predecessors, is a very exciting discovery. Firstly, it’s from an event almost twice as far away from the Earth than the previous two – at a distance of about three billion light years.  This also means we’re looking further back into the universe’s past than we’ve been able to with gravitational waves before now.

“That has helped us further strengthen the case that Einstein’s general theory of relativity is correct.  According to general relativity, apart from an overall ‘stretching’ due to the expansion of the Universe, the shape of the GW170104 gravitational wave signal shouldn’t have been distorted as it travelled to us across space.  Sure enough, we’ve shown that this distorting effect, known as dispersion, didn’t happen even over the immense distances involved in this detection.

“Secondly, as only the third confirmed signal ever detected from a binary black hole system, it’s a crucial step forward in our understanding of black holes; until the first detection of gravitational waves we didn’t even have direct evidence that black holes existed.

“This new detection will help us to understand better how black holes can be formed when massive stars collapse at the end of their lives.”

There are two main models to explain how pairs of black holes can be formed, and these models make different predictions about how the black holes are spinning as they orbit each other. In one model, the black holes come together later in life within crowded star clusters. The black holes pair up after they sink to the centre of a star cluster. In this scenario, the black holes can spin in any direction relative to their orbital motion.

The other model proposes that the black holes are born together: they form when each star in a pair of stars explodes, and then, because the original stars may have been spinning in alignment, the black holes’ spin remains aligned too.

Combining the GW170104 black hole data with the earlier discoveries, LIGO sees some evidence that the spin axes of black hole pairs may typically point in opposite directions, mildly favoring the dense stellar cluster theory.

Professor Sheila Rowan, director of the University’s Institute for Gravitational Research, said: “We’re pleased and proud to have been heavily involved for the third time in detecting gravitational waves through our roles in the LIGO collaboration.

Finding the first direct evidence of the existence of gravitational waves was a long, difficult job, but these first three confirmed detections have given us so much new data that we just couldn’t have collected any other way.

“Although we’re only in the infancy of gravitational wave astronomy as a revolutionary new way to explore the universe, it’s already clear that we have a lot more discoveries to look forward to.”

Professor James Hough, of the University of Glasgow’s School of Physics and Astronomy, added: “With this third discovery I think we can safely conclude that the first two detections were no flash in the cosmic pan.  We can expect to be regularly observing gravitational waves from events such as these for as long as LIGO is looking for them.

“The detectors are not yet working at their full design capacity, so we can anticipate even more frequent detections as LIGO becomes even more sensitive. It’s a very exciting time to be an astrophysicist.”

Scientists will 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 improvements for LIGO's next run, scheduled to begin in late 2018, aimed at improving the detectors' sensitivity even further.

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.