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Blue digital image of a gravitational wave on black background
The aftershocks of huge astronomical events were picked up by a worldwide team of scientists, with leadership from the UK.

The largest catalogue of gravitational wave events ever assembled has been released today, with dozens of ripples in space time 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. 

Experts from the University of Birmingham’s Institute for Gravitational Wave Astronomy were part of the parameter estimation team, responsible for characterising the gravitational wave sources.

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

An international effort

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:

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).

What did we see?

Of the 35 events detected, 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.

University of Birmingham lecturer Dr Patricia Schmidt said: “After only three observing runs, we are just shy of 100 gravitational wave observations. This tremendous achievement in only five years is testament to the significant advancements in both technology and analysis techniques. The third observing run has brought us exciting new discoveries including the first confirmed intermediate mass black hole and the strongest evidence to date for the mergers of neutron stars with black holes. We can’t wait to see what surprises the next observing run will bring.”

This publication brings our total of gravitational wave detections from compact binary mergers to 90. But far from becoming routine, each one gives clues as to how these binaries form, what properties they have as a population, and even the inner workings of gravity. With each new detection we are slotting another piece into the puzzle of our universe.

Lucy Thomas, PhD student at the University of Birmingham

The final gravitational wave event 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.

Monumental progress

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.

To achieve this monumental progress, 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.

Dr Denis Martynov, a reader at the University of Birmingham, said: “After the first science run started in 2015, the detectors continue to improve and observe more signals every new science run. Before O3, the improvements came from the quantum technology of squeezed states of light, from resonating more optical power in the detectors, and from suppressing a number of technical noise sources.”

Eyes on the skies

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.

University of Birmingham researcher Dr Geraint Pratten said: “Ever more sophisticated waveform models, such as those developed here in Birmingham, play a vital role in precision measurements of the properties of astrophysical black holes and neutron stars. Each new detection brings us one step closer towards understanding the fundamental properties of these exciting discoveries.”

The future of the field

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.

Natalie Williams, a PhD student at the University of Birmingham, said: “After watching the very first gravitational wave detection as a first year undergraduate, having the opportunity to contribute to the O3b catalogue in the first year of my PhD feels like a true full circle moment. With more and more detections the picture is only getting clearer - and after reflecting on how far we’ve come, we can start getting ready for it all over again with O4!”