New technique enables scientists to ‘hear’ black hole collisions

Scientists have discovered a way to “hear” cosmic events like the collision of black holes using a process similar to the pitch-correction used in music production.

Experts from the international Ligo, Virgo and Kagra (LVK) gravitational wave observatory collaboration used the technique to use gravitational-wave signals to measure the response of their incredibly sensitive instruments.

The technique, which they call astrophysical calibration, enables them to ensure that they can clearly “hear” the sounds of huge cosmic events even when one gravitational wave detector is slightly out of tune.

The researchers can account for subtle distortions in the data by combining signals from other detectors with precise predictions from the laws of gravity.

Experts said the process is similar to how music production software such as Auto-Tune can correct a singer’s pitch if it is off key to meet the right note.

The findings are described in a paper published as a preprint on arXiv ahead of publication in the journal Physical Review Letters.

Dr Christopher Berry, of the University of Glasgow’s Institute for Gravitational Research, is part of the LVK collaboration and an author of the paper.

He said: “Gravitational waves are ripples in spacetime that stretch and squeeze space. They are tiny by the time that they reach the Earth, millions of years after the events that first created them.

“They are not something which we can hear, but our detectors can output the signals as waveforms that we can increase in pitch to listen to, with each signal producing their own distinctive chirp.

“Those chirps encode a wealth of information we can analyse to learn about their sources: their masses, spins, distance, and location.”

Scientists said the gravitational-wave signals used by the team to develop their astrophysical calibration technique are among the loudest ever detected by the collaboration.

The first signal, picked up on September 25 2024 and named GW240925, was produced by the merger of two black holes between nine and seven times the mass of our sun more than a billion light-years away.

The second signal, on February 7 2025 and named GW250207, was the second-loudest signal in the nearly 200 detected by the collaboration in the decade since the first detection in 2015.

It was produced by the collision of two black holes between 35 and 30 times the mass of our sun about 600 million light-years from Earth.

In the paper LVK researchers demonstrate how they turned the challenge of analysing data collected from two gravitational wave signals detected when Ligo Hanford was up and running, but performing below its usual standard, into an opportunity to improve the collaboration’s ability to analyse data.

It is hoped results could help future observing runs of the international network of LVK detectors in the USA, Japan and Italy ensure that they produce the most reliable results, even when the circumstances of the detection are less than ideal.

Dr Daniel Williams from the University of Glasgow’s Institute for Gravitational Research, said: “These discoveries demonstrate that, over our decade of work since the first detection, we have developed a comprehensive understanding of our entire analysis pipeline, from the signals themselves to the detector behaviour.

“In the rare instance that something goes wrong with one detector, we now have robust back-up methods to compensate and leverage data from the other detectors to give us the best-quality results.”

Professor Stephen Fairhurst, of Cardiff University, who is the Ligo Scientific Collaboration’s spokesperson, said: “It’s remarkable that these massive cosmic events can not only be measured by our instruments, but actually used to check our measurements.”

He added: “We’re moving from the era of first discoveries to the era of precision gravitational wave astronomy. We can be confident that our next observing runs will continue to build our rapidly-growing catalogue of gravitational-wave discoveries, and expand our understanding of the universe.”

The LVK Collaboration is a network of scientists who co-ordinate and collaborate on the operation of the detectors and on the production of the scientific results.