An international team of scientists from the LIGO, Virgo, and KAGRA collaborations, including researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) , has shown for the first time that gravitational waves, ripples in space and time produced by some of the most violent events in the Universe, such as the collision of two black holes, can be used to measure and correct the calibration of the detectors that observe them.
The breakthrough comes from the study of two exceptionally strong gravitational-wave signals, known as GW240925 and GW250207, produced by the collisions of pairs of black holes and detected by the twin detectors of the US National Science Foundation Laser Interferometer Gravitational-wave Observatory (NSF LIGO). These events were so strong that they allowed researchers not only to study the black holes that created them, but also to check how accurately the detectors were recording the signals.
"In a way, we are using black holes to help check the accuracy of our detectors. How cool is that!" said Dr Ling (Lilli) Sun from The Australian National University (ANU).
Three OzGrav researchers from three different Australian universities played key scientific roles in the study. Dr Ling (Lilli) Sun from ANU provided scientific leadership on the paper, while Mallika Sinha, a PhD student at Monash University, and Dr Yi Shuen Christine Lee, a Postdoctoral researcher at the University of Melbourne, made important contributions to the analysis and interpretation of the results.
The LIGO-Virgo-KAGRA collaboration has now confidently detected more than 200 gravitational-wave signals from merging black holes and neutron stars. Each signal carries information about its source and the extreme physics governing these collisions. Extracting that information requires the detectors to measure gravitational waves with extraordinary precision and to carefully account for any uncertainties in those measurements.
Gravitational waves stretch and squeeze spacetime as they pass through Earth. The detectors measure this by sending laser light down two perpendicular arms and looking for tiny differences in the time it takes the light to travel back and forth. A typical gravitational wave changes the arm length by about one ten-billionth of a billionth of a metre, smaller than the width of a proton.
"Thanks to major upgrades over the past decade, our detectors are now so sensitive that signals from colliding black holes come through loud and clear," said Dr Sun. "If Einstein's theory of general relativity is correct, those signals should follow a very specific pattern."
Turning those minute measurements into a physical gravitational-wave signal requires a detailed model of the detector's response. This includes accounting for the complex control systems used to keep the instruments stable. Normally, calibration uncertainties are measured and estimated using auxiliary lasers, sensors, and engineering data. However, during the detections of GW240925 and GW250207, the LIGO Hanford detector happened to have a larger calibration error than usual.
According to Dr Sun "by comparing the predicted signal with what we actually record, we can spot tiny mismatches that sometimes reveal the detector wasn't perfectly calibrated at the time."
Because both signals were exceptionally loud, the researchers were able to disentangle the true gravitational-wave signal from the detector's calibration error, a process known as astrophysical calibration. GW240925 served as a verification case, allowing the team to compare results from astrophysical calibration with data that was later corrected using standard methods.
GW250207, meanwhile, is the second-loudest gravitational-wave event ever observed and provides a unique window into extreme physics. For this event, astrophysical calibration was essential to ensure the data could be trusted at all.
Accurate calibration is critical because even small errors can bias estimates of key source properties, such as the masses of the black holes, whether they are spinning, and where the signal originated in the sky.
"It was simply bad luck that such a loud event was observed while LIGO Hanford was in an unsettled state," said Mallika Sinha.
"As our detectors become more sensitive and we observe more events, situations like this will only become more common. Without astrophysical calibration, we might not be able to reliably analyse these interesting events and miss out on some nifty science."
The researchers found that GW240925 was produced by black holes around nine and seven times the mass of the Sun, while GW250207 involved black holes roughly 35 and 30 times the Sun's mass.
"Using three detectors instead of two helps us pinpoint the location of gravitational-wave sources much more precisely, which also means we can better understand the physical properties of the sources themselves," said Dr Yi Shuen Christine Lee.
"This successful astrophysical calibration using GW240925 and GW250207 is an exciting step forward for gravitational-wave astronomy. It improves our chances for extracting important astrophysical information from gravitational-wave sources, even when traditional detector calibration methods are not accurate or feasible!"
Because of its strength and position in the sky, GW250207 is considered one of the most promising gravitational-wave signals for future measurements of the Hubble constant, although many such "dark siren" events, gravitational-wave signals from black hole mergers that produce no visible light, will be needed to resolve the long-standing tension between different cosmological measurements.
Together, GW240925 and GW250207 mark the first successful tests of astrophysical calibration, a technique that could allow scientists to trust gravitational-wave data even when detectors are in an unsettled state. As gravitational-wave astronomy moves from discovery to precision science, using the Universe itself to help calibrate our instruments may become an increasingly powerful tool.
This research is published in Physical Review Letters .
