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| Illustration of two exceptionally loud binary black hole coalescences, GW240925 and GW250207, detected via gravitational waves, showing their masses, wave patterns, and estimated distances from Earth |
GLASGOW - Gravitational waves from colliding black holes are so precise they can now be used to fine-tune the very instruments that detect them. Scientists have developed a breakthrough "astrophysical calibration" method that effectively auto-tunes misaligned gravitational-wave detectors. By using real cosmic signals as a reference, researchers can correct subtle instrument errors in real time. This innovation strengthens global observatories' ability to pinpoint cosmic events with unprecedented accuracy. It marks a major step toward more reliable and precise gravitational-wave astronomy.
By combining signals from other detectors with precise predictions from the laws of gravity
Gravitational wave detectors can now 'autotune' signals to harmonise the heavens. Gravitational wave researchers working on the world's most sensitive scientific instruments have found a way to tune their detectors using a process akin to the pitch-correction used in music production. Scientists at the international LIGO, Virgo and KAGRA (LVK) gravitational wave observatory collaboration have employed the technique, which they call astrophysical calibration, to use gravitational-wave signals to measure the response of their incredibly sensitive instruments. It enables them to ensure that they can clearly 'hear' the sounds of colossal cosmic events like the collision of black holes, even when one gravitational wave detector is slightly out of tune. This is crucial to accurately interpret the signals and find their source location. By combining signals from other detectors with precise predictions from the laws of gravity, researchers can identify and account for subtle distortions in the data.
A preprint on the arXiv ahead of publication in the journal Physical Review Letters
The process is similar to how music‑production software such as Auto-Tune can correct a singer's errant pitch to meet the intended note in a melody. In a new paper published as a preprint on the arXiv ahead of publication in the journal Physical Review Letters, 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. The results could help future observing runs of the international network of LVK detectors in the USA, Italy and Japan ensure that they produce the most reliable results, even when the circumstances of the detection are less than ideal. 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.
GW240925
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." The gravitational-wave signals that the team used to develop their astrophysical calibration technique are among the loudest ever detected by the collaboration. The first signal, picked up on 25th September 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 7th February 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 two black holes between 35 and 30 times the mass of our sun around 600 million light-years from Earth.
The technical hitches with LIGO Hanford
The LVK collaboration can be confident of their results because of the work they did to overcome initial uncertainties introduced by problems with the US National Science Foundation Laser Interferometer Gravitational-wave Observatory's (NSF LIGO) detector in Hanford, Washington. For GW240925, there was a temporary error with the calibration—this was monitored and later corrected, enabling scientists to check the performance of astrophysical calibration for a case with a known miscalibration. For GW250207, the detector was just coming online, so not all monitoring systems were up-and-running. The paper's editorial chair, Dr Ling Sun of the Australian National University, said: "The loudness of these signals was remarkable, with very high signal-to-noise ratios compared to many of our other detections. These are exactly the types of signals you want to be recorded by all of our detectors. However, given the technical hitches with LIGO Hanford, we might have had to throw out the detector's results altogether, losing a large chunk of the signal strength and our ability to precisely locate these events in the sky.
Having accurately calibrated data is essential to accurately characterise the signal and its source
By first verifying astrophysical calibration with the analysis of the September 2024 detection, we were much more prepared to deal with the more significant problems with the February 2025 data." The astrophysical calibration technique works because the telltale chirp of a black hole merger signal is well predicted by the theory of general relativity, Einstein's theory of gravity. By comparing the predicted and observed signals, the researchers were able to make accurate inferences about how the LIGO Hanford detector was distorting the data picked up at the same time by the LIGO's Livingston detector in Louisiana and the Virgo detector in Italy. For GW240925, this method matched known calibration errors measured on-site. For GW250207, however, it was essential to use astrophysical calibration because reliable on‑site calibration measurements were unavailable. Having accurately calibrated data is essential to accurately characterise the signal and its source.
Dr Daniel Williams from the University of Glasgow's Institute for Gravitational Research
By including the potential to auto-tune the detector data, using the signal as reference, as part of the analysis, the team can avoid introducing errors into their results. Using the corrected calibration for the LIGO Hanford detector, the team measured the black hole masses, distances, and spins more accurately, and significantly improved the precision of the sky location. Sky location depends critically on the number of detectors observing, and improves significantly going from two to three detectors. 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 backup methods to compensate and leverage data from the other detectors to give us the best-quality results."
To expand our understanding of the Universe
Cardiff University's Professor Stephen Fairhurst, 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. Being able to use astrophysical calibration so successfully during our fourth observing run is a demonstration of the maturation of the detector's capabilities and our ability to get the most out of every detection. Improving the quality of our results on sky localisation will also help us test key concepts like the expansion rate of the Universe, a value which is still being debated by scientists. 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."
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The University of Glasgow's research is supported by funding from UKRI's Science and Technology Facilities Council (STFC)
The team's paper, titled 'GW240925 and GW250207: Astrophysical Calibration of Gravitational-wave Detectors', is accepted in Physical Review Letters. The publication comes 10 years after the publication of the first observation of gravitational waves in Physical Review Letters, a discovery that was recognised with the Nobel Prize for Physics. The University of Glasgow's research is supported by funding from UKRI's Science and Technology Facilities Council (STFC), as are other gravitational research groups across the UK including the Universities of Birmingham, Cambridge, Cardiff, Kings College London, Nottingham, Portsmouth, Sheffield, Strathclyde, University College London, Queen Mary University, and the University of the West of Scotland.
Dr Ling Sun. Editorial Chair, The Australian National University
"Over the past decade, major upgrades to our detectors have significantly improved their sensitivity, allowing us to observe signals from colliding black holes with unprecedented clarity. General relativity provides precise predictions for how these gravitational-wave signals should evolve. By comparing those predictions with the beautiful signals recorded by our instruments, we can identify deviations that sometimes point to calibration imperfections. In this way, astrophysical black holes serve as independent cross-checks on our detector response. We are effectively using nature’s most extreme laboratories to validate the performance of our observatories!"
Dr Christopher Berry. Paper Manager, University of Glasgow
"Hundreds of millions of years ago, two black holes merged, creating ripples in spacetime that travelled across the cosmos to Earth. We detected these signals, and used them to calibrate our instruments—measuring positions to the precision of a fraction of the size of an atomic nucleus. This is astounding, a true testament to human ingenuity. Gravitational-wave astronomy is maturing as a science. Thanks to the hard-work of scientists across the world, we are now doing precision astrophysics. Using gravitational-wave signals, we can build an understanding of the properties of black holes and the nature of gravity itself."
Dr Sylvia Biscoveanu. Analyst and paper writing team member, Princeton University
"The fact that we were able to make this measurement now is remarkable, as most previous works investigating the prospects for performing astrophysical calibration predicted it wouldn't be possible with the current generation of detectors. The demonstration and validation of astrophysical calibration included in this work are critical, proving that we can reliably use this method for future observations at times when we might not have reliable knowledge of the detector calibration via other means. This will allow us to search for more gravitational-wave signals and better constrain their properties by including more data in the future. These two events are among the best-localized binary black hole mergers we’ve ever detected. This highlights the power and utility of astrophysical calibration. Such precise constraints on the sky location of these sources would not have been possible if we had to discard the miscalibrated data instead of measuring the calibration astrophysically."
Dr Leo Tsukada. Analyst and paper writing team member, University of Nevada Las Vegas
"While data calibration plays a critical role in the gravitational-wave detections, the fact that we managed to detect them even from miscalibrated data demonstrates the robustness of detection pipelines, without which none of downstream analysis including astrophysical calibration could have happened. Now together with upgraded detector sensitivities, we can find many more signals hiding from us in the data!"
August Muller (she/her) Paper team member, University of Glasgow
"We can do the best science with gravitational waves when they are heard by multiple detectors, so we want all the data we can get. Usually, we think about our detectors teaching us about black holes, but in this case the black holes actually taught us something about our detectors as well! It's like letting the black holes tell us how to hear them clearly. It's really exciting that we're at a point where we can use gravitational-wave signals themselves to make sense of our data, so we can get as much information as possible from every detection."
Dr Alan Weinstein. Paper writing team member, Caltech
"The idea of using real astrophysical signals to calibrate the LIGO-Virgo-KAGRA (LVK) detectors, or at least to check that our calibration procedure is quantitatively accurate, has been around for a long time. Doing so assumes that the astrophysical signals are well modeled by our waveform templates, based on General Relativity (GR); so first, we needed to confirm that with a suite of tests. These tests have been performed and reported in a set of LVK papers on tests of GR, demonstrating that real astrophysical signals agree well with the GR-based waveform templates; great! But then, even before we were ready to check our calibration with real events, GW240925 happened - a nice loud BBH event that came just as we mistakenly uploaded wrong calibration information to our low-latency calibration pipeline. The information was not so wrong as to prevent us from identifying the event at low-latency. We discovered the mistake and fixed it within 2 hours; but it took some time to re-analyze the event with correct calibration information. That mistake gave us the opportunity and impetus to, at last, confirm the quantitative accuracy of our calibration (after fixing the mistake), using real astrophysical events, and provide more tests of GR with this event and with GW250207. For me, it's wonderful to turn a regrettable mistake into a great scientific opportunity!"
Elizabeth Todd (she/her). Science Summary co-author, University of Glasgow
"It's so cool that we're able to bring so many different ideas and people together to contribute to this result! From the experimentalists who built detectors sensitive enough to detect the mergers of black holes over a hundred million light years away, to the scientists that have modelled waveforms derived from general relativity so accurately that we are able to detect tiny deviations and use those to tell us when our detectors are miscalibrated. Astrophysical calibration allows us to showcase so much of the expertise and skills of the people within the LVK."
Dr. Parthapratim Mahapatra. Analyst team member, Cardiff University
"What is especially exciting about this work is that gravitational-wave signals can be used directly to calibrate our detectors — much like a tuning fork that tells a musician whether their instrument is perfectly in tune. The waveform models describing black hole mergers are one of the great triumphs of modern theoretical physics, and tests of general relativity confirm their accuracy with extraordinary confidence. This makes the signals themselves nature's own tuning fork: when our detectors hear them slightly off-key, we know it is our instruments, not the universe, that need adjusting — allowing us to extract more from every detection while sharpening our tests of Einstein's theory itself."
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