In the observations of gravitational waves with the LIGO and Virgo detectors, there is no evidence so far of gravitational-wave lensing. This is what the collaboration concludes in a new paper.
In ordinary astronomy, gravitational lenses are now a familiar phenomenon. Near massive objects such as galaxies or clusters of galaxies, spacetime is so distorted that images from the distant background can be distorted.
Due to the lensing effect, multiple images or light arcs can be observed, among other things. Astronomers even use time differences between pulses of light from the same object to measure the expansion of the universe.
In principle, gravitational waves from colliding black holes or neutron stars are also signals that can be distorted and split into multiple images by gravitational lenses, astrophysicists believe. The LIGO and Virgo detectors (LVC for short) are forecasted to detect strong distortions within this decade. Nikhef researchers, among others, are closely involved in the search for lensing effects.
Among other things, lensing could produce gravitational waves that are so similar that they could have come from the same source. Such gravitational-wave “images” would arrive a few minutes to months apart when lensed by galaxies, and up to years apart when lensed by clusters of galaxies. Gravitational lensing could also magnify gravitational-wave signals and cause “microlensing” signatures to appear in the waveforms.
But the most extensive study of gravitational-wave lensing to date does not yet show evidence of lensing. Gravitational waves that resemble each other have been detected a few times at different times. But that could just as well be coincidence, the researchers think.
Astrophysicists expect a lot from the measurements of distortions of gravitational waves. Multiple images could pinpoint sources much more precisely, test the speed of gravity, test the polarization of gravitational waves and also reveal a high-redshift population of binary black holes.
Nikhef postdoc researcher Otto Hannuksela, who among others was intensively involved in the study, says the results are interesting, even if lensing has not yet been observed. Hannuksela: “This is the first LVC statement on a topic that has been hotly debated within the broader scientific community in recent years. Moreover, being the first collaboration study of its kind, it marks the beginning of collaboration-wide searches for gravitational-wave lensing.”
