The LIGO–Virgo–KAGRA (LVK) detector network has a new trick up its sleeve to improve the instruments’ sensitivity to gravitational waves: it’s called Astrophysical Calibration and it plays a role similar to auto-tune in music production.
When a gravitational wave passes through the Earth, the LIGO, Virgo and KAGRA detectors are ready to detect it, but their sensitivity depends on many factors and it is possible that one of them may not be operating at full capacity at that moment. In moments like these, it is essential to be able to process the data collected by that detector to improve its quality, and the network of detectors now has a new efficient tool to do so: Astrophysical Calibration.
Gravitational waves distort space, stretching and compressing it as they pass through. This effect on the detector arms is around 10-19 m, far smaller than the diameter of a proton! To be sensitive to such tiny changes, the detectors must be carefully calibrated in real time, using feedback control circuits and a precise procedure that models how the detector changes as the waves pass through it, whilst also taking into account the effects generated by the control circuits themselves. If the calibration is not optimal, the ‘reading’ of the signal and therefore the interpretation of the cosmic phenomenon that generated it are compromised.
However, if the gravitational signal detected is sufficiently strong – that is, when it clearly outweighs the background noise – comparing the signal to predictions from general relativity (together with the signals observed in other detectors) can be used to recalibrate the data from a ‘mis-tuned’ detector retrospectively. Theoretical models are, in fact, like musical scores that suggest the shape of the signal (i.e. which notes the signal plays); together with data from well-‘tuned’ detectors, they allow us to clean the data from the poorly calibrated detector of spurious effects, thereby recording it correctly. 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.
“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,” said Christopher Berry of the University of Glasgow’s Institute for Gravitational Research. “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 its own distinctive chirp. Those chirps encode a wealth of information we can analyse to learn about their sources—their masses, spins, distance, and location. Specifically in the case of the merger of two black holes, the astrophysical calibration technique works because the characteristic ‘chirp’ of the signal is described with extreme precision by Einstein’s theory of general relativity.”
In an article accepted in Physical Review Letters, researchers from the LIGO–Virgo–KAGRA (LVK) Collaboration demonstrate how this technique has been applied to two particularly loud and scientifically interesting signals from the LVK Collaboration’s fourth observing run, GW240925 and GW250207. As is typical for gravitational wave events, the signal names indicate the dates of each detection: September 2024 and February 2025 respectively. At the time both these signals arrived, the LIGO Hanford detector (in Washington, USA) was not in optimal condition, making the interpretation of its data particularly difficult.
“GW240925 happened just as we had mistakenly uploaded the wrong calibration information to our low-latency pipeline,” said Alan Weinstein, Professor of Physics at Caltech. “We discovered the mistake and fixed it within two hours, but the event gave us the opportunity to at last confirm the quantitative accuracy of our calibration using real astrophysical signals. For me, it’s wonderful to turn a regrettable mistake into a great scientific opportunity.”
By comparing the predicted signals with the observed ones, the researchers were able to draw precise conclusions about how the LIGO Hanford detector distorted its own data, using the signals recorded simultaneously by the LIGO Livingston detector in Louisiana and the Virgo detector in Italy. For GW240925, this method confirmed the known calibration errors measured on-site. For GW250207, however, it was essential to use astrophysical calibration as no reliable on-site calibration measurements were available.
“The fact that we were able to make this measurement now is remarkable — most previous works predicted it wouldn’t be possible with the current generation of detectors,” said Sylvia Biscoveanu of Princeton University. “These two events are among the best-localised binary black hole mergers we’ve ever detected, and such precise constraints on the sky location wouldn’t have been possible if we’d had to discard the miscalibrated data.”
Using the corrected calibration for the LIGO Hanford detector, LVK researchers have discovered that GW240925 was generated by black holes with masses 9 and 7 times that of the Sun at a distance of approximately 350 megaparsecs from Earth, whilst GW250207 was generated by two black holes with masses 35 and 30 times that of the Sun at a distance of approximately 200 megaparsecs from Earth. Without taking calibration uncertainties into proper account, these estimates could have been biased towards an incorrect value.
“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,” said August Muller, a member of the paper team at the University of Glasgow. “It’s like letting the black holes tell us how to hear them clearly.”