Title: Lensing or luck? False alarm probabilities for gravitational lensing of gravitational waves
Link: https://arxiv.org/abs/2201.04619 (accepted to MNRAS)
Gravitational lensing is a fascinating phenomenon in which light from a distant object is bent and magnified by the gravity of an intervening massive object, allowing us to study objects that would otherwise be too faint or too distant to observe. This effect was first predicted by Albert Einstein’s theory of general relativity in 1915, and was famously confirmed during the solar eclipse of 1919 by the British astrophysicist Arthur Eddington (see Figure above).
Fast forward to today, and we’re now able to study gravitational lensing in a new way: using gravitational waves. These ripples in the fabric of spacetime are generated by some of the most energetic events in the universe, such as the merger of two black holes or neutron stars. As gravitational waves travel through space, they can be bent and distorted by massive objects just like light, creating a gravitational lensing effect that can magnify or even multiply the signal we receive on Earth.
However, as we report in our recently accepted publication in the Physical Review D, distinguishing genuine lensing events from chance coincidences can be a difficult task. The problem is that the parameters of a genuine lensed event – such as its chirp mass, sky location, and coalescence phase – can overlap with those of an unrelated event, making it difficult to tell them apart.
To investigate this issue, we constructed a mock catalog of lensed and unlensed gravitational wave events and calculated the false alarm probability (FAP) based on coincidental overlaps of the parameters. We found that the FAP based on chirp mass, sky location, and coalescence phase are approximately 9%, 1%, and 10% per pair, respectively. Combining all three, the overall FAP per pair is around one part in 10 thousand. This means that for sufficiently high numbers of events in the gravitational wave catalogs, false alarms will always dominate over the true lensing events.
To combat this problem, we proposed alternative identification criteria that go beyond simple waveform and sky location overlap. We also explored the possibility of using statistical modelling of strong lenses to help distinguish genuine lensing events from false alarms.
Our study, led by Mesut Çalışkan, a PhD student from Johns Hopkins, highlights the challenges of detecting and characterizing gravitational lensing of gravitational waves. But it also provides a roadmap for developing more sophisticated methods of identifying and studying these rare events. By improving our ability to detect and study gravitational lensing, we can unlock new insights into the nature of the universe and the physics of gravity itself.