University news

University of Glasgow astrophysicists have played key roles in the development of the largest-ever catalogue of gravitational waves.

The catalogue, released today, is based on the detection of dozens of ripples in spacetime captured by a global network of detectors.

The aftershocks of huge astronomical events were picked up by a world-wide team of scientists, with leadership from the UK. 

The team has detected a further 35 gravitational wave signals, detailed in today’s paper, bringing the total number of observed events since detection began to 90.

The catalogue updates the list of all gravitational-wave events observed to date with events observed between November 2019 and March 2020, using international detectors:

• The two Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in Louisiana and Washington state in the US.
• Advanced Virgo detector in Italy. 

Scientists from 12 different universities across the UK are involved in the LIGO Scientific Collaboration, with many taking leading roles from detector calibration to data analysis. The UK’s contribution to the collaborations is funded by the Science and Technology Facilities Council (STFC).

Researchers from the University of Glasgow’s Institute for Gravitational Research and School of Physics & Astronomy have been involved in the search for gravitational waves for more than half a century.

They have been partners in the LIGO and Virgo collaborations since their inception, and played a leading role in the data analysis that underpinned the discoveries contained in the new catalogue.

Since the first gravitational wave detection in 2015, the frequency of detections has risen at a thundering rate. In a matter of years, gravitational wave scientists have gone from observing these vibrations in the fabric of the universe for the first time, to now observing many events every month, and even multiple events on the same day.

As the rate of gravitational wave detections increases, scientists have also improved their analytical techniques to ensure the high accuracy of results. The growing catalogue of observations will enable astrophysicists to study the properties of black holes and neutron stars with unprecedented precision.

Daniel Williams, research associate at the School of Physics & Astronomy, said: “Over the five years that we've been making detections with the LIGO and Virgo detectors we've seen a steady increase in the number of events that we've detected.

“In our third observing run, things really took off, going from detecting a few events per month to having several days where we made multiple detections! That proved to be a challenge for our analysis teams, who must prepare and then monitor the data analysis of each signal, which take weeks or even months to complete.

“At Glasgow we led the development, testing, and operation of software that is capable of automating almost the entire analysis of each gravitational wave event. We used this to reduce the total team to a handful of experts who were able to deal with unusual and unexpected results from the analysis, and coordinated the effort from Glasgow.

“It's very exciting to have gone from our  first detection to having almost 100 in just five years; detecting such large numbers allows us to learn more about the population of black holes and neutron stars in the universe, which in turn gives us new insights into its distant history, and how the modern universe came to be.”

Of the 35 events reported in the latest set of results, 32 of those were most likely to be black hole mergers – two black holes spiralling around each other and finally joining together, an event which emits a burst of  gravitational waves.

Several of the black holes formed from these mergers exceed 100 times the mass of our Sun, and are classed as intermediate-mass black holes. This type of black hole has long been theorised by astrophysicists, and has now been proven to exist thanks to gravitational wave observations.

Two of the 35 events spotted were likely to be neutron stars and black holes merging – a much rarer event, and one that was only discovered in the most recent observing run of LIGO and Virgo.

Of these rare neutron star and black hole mergers, one event seems to show a massive black hole (about 33 times the mass of our Sun) with a very low-mass neutron star (about 1.17 times the mass of our Sun). This one of the lowest-mass neutron stars ever detected, either using gravitational waves or electromagnetic observations.

The masses of black holes and neutron stars are key clues to how massive stars live their lives and die in supernova explosions.

Christopher Berry, a lecturer at the School of Physics and Astronomy, explains: “Only now are we starting to appreciate the wonderful diversity of black holes and neutron stars. Our latest results prove that they come in many sizes and combinations—we have solved some long-standing mysteries, but uncovered some new puzzles too. Using these observations, we are closer to unlocking the mysteries of how stars, the building blocks of our Universe, evolve.”

The final gravitational wave event of the observing run came from either a black hole and a neutron star or a black hole and a black hole. The mass of the lighter object crosses the expected divide between the two, a region where previously no black holes or neutron stars were expected to be formed, and remains a mystery.

Using the catalogue of detections, scientists are able to calculate the population of merging black holes and neutron stars across the Universe, key to understanding how these systems form, and also map the expansion rate of the Universe, key to understanding the evolution of the cosmos.

To achieve the monumental progress in gravitational wave detection, the pioneering instruments have been getting more sensitive thanks to a programme of constant upgrades and maintenance. In the third observing run, the gravitational wave detectors reached their best ever performance, as the lasers were tuned to even higher power.

Professor Sheila Rowan, director of the Institute for Gravitational Research, said: “Since the historic first detection of gravitational waves in 2015, we have made remarkable advances in establishing the field of gravitational wave astronomy.

“I’m proud that Glasgow researchers have played a vital role in both the detector upgrades and the development of the data analysis which made these discoveries possible. We expect that the next phase of observing runs will bring us even more detections as our hardware and software continues to improve, and I’m looking forward to seeing what they’ll have to teach us about the universe.”

The LIGO and Virgo observatories are currently undergoing improvement works before the upcoming fourth observing run, expected to begin next summer.

The KAGRA observatory in Japan will also join the next full observing run. Located deep under a mountain, KAGRA completed a successful first observing run in 2020, but has yet to join LIGO and Virgo in making joint observations.

With more detectors, potential events can be located more accurately. As more detections are confidently added to the gravitational wave catalogue, researchers are learning more and more about these astronomical phenomena.

Before the next observing run, scientists will be busy further analysing the existing information, learning more about neutron stars and black holes, and searching for new types of signals hidden in the data.

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First published: 8 November 2021