University of Glasgow astrophysicists are gearing up to resume the search for gravitational waves, the ripples in spacetime caused by some of the universe’s most spectacular events.

The Glasgow researchers played key roles in the development of the National Science Foundation's Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) project, based in the United States, which will be starting a new science run on Monday April 1 along with the Virgo gravitational detector, based in Italy. ‌

Glasgow physicists also made major contributions to the analysis of the data from both detectors during their first two periods of operation. They expect to have even more work to do during this third observing run because both projects have been significantly upgraded during their downtime – LIGO has a combined increase in sensitivity of about 40 percent over its last run and Virgo’s sensitivity has been almost doubled.

The detectors’ increased sensitivity means that they are capable of surveying an even larger volume of space for powerful, wave-making events, such as the collisions of black holes. In the next run, LIGO will be able to see those events out to an average of 550 million light-years away, or more than 190 million light-years farther out than before.

In 2015, after LIGO began observing for the first time in an upgraded program, it soon made history by making the first direct detection of gravitational waves.

Since then, the LIGO-Virgo detector network has uncovered nine additional black hole mergers and one explosive smashup of two neutron stars. That event, dubbed GW170817, generated not just gravitational waves but light, which was observed by dozens of telescopes in space and on the ground.

Professor Sheila Rowan is director of the University of Glasgow’s Institute for Gravitational Research, where Professor Rowan said: “This third run of observations marks an important step forward for the new field of gravitational wave astronomy.

“The upgraded LIGO-Virgo detectors will allow us to detect signals from further out in the universe, pushing back the boundaries of our understanding and delivering a wealth of new findings which are only possible by listening out for the sounds of those ripples in spacetime.”

Professor Martin Hendry, head of the University’s School of Physics and Astronomy, said: “We’ve already gathered a remarkable wealth of data from LIGO’s first two observing runs, which included the first direct evidence of the existence of black holes and the collision of neutron stars.

“We may well detect signals from similar events this time around, which will add further depth to our catalogue of gravitational wave data, but there’s also the very real possibility that we’ll find evidence of something completely new – perhaps a black hole colliding with a neutron star. It’s a really exciting prospect and we’re all looking forward to seeing what we can learn about our universe this time around.”

The University of Glasgow’s contribution to the LIGO and Virgo detectors is supported by the UK’s Science and Technology Funding Council (STFC). Alongside partner institutions including the Universities of Birmingham and Cardiff, Glasgow researchers play a key role in the sophisticated data analysis which underpins each detection. They also led the development of the silica mirror suspensions which are a critically important part of the detector technology.

Each LIGO installation consists of two long arms that form an L shaped interferometer. Laser beams are shot from the corner of the "L" and bounced off mirrors before traveling back down the arms and recombining. When gravitational waves pass by, they stretch and squeeze space itself, making nearly imperceptibly tiny changes to the distance the laser beams travel and thereby affecting how they recombine. In this next run, the laser power has been doubled to more precisely measure these distance changes, thereby increasing the detectors’ sensitivity to gravitational waves.

Other upgrades were made to LIGO's mirrors at both locations, with a total of five of eight mirrors being swapped out for better-performing versions.

This next run also includes upgrades designed to reduce levels of quantum noise. Quantum noise occurs due to random fluctuations of photons, which can lead to uncertainty in the measurements and can mask faint gravitational-wave signals. By employing a technique called "squeezing,” initially implemented at the Australian National University and routinely used since 2010 at the GEO600 detector, researchers can shift the uncertainty in the photons around, making their amplitudes less certain and their phases, or timing, more certain. The timing of photons is what is crucial for LIGO's ability to detect gravitational waves.

 

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First published: 26 March 2019