Could any of the gravitational wave signals observed in the LVC observing run O3a be lensed by massive astrophysical objects, such as galaxies, galaxy clusters, black holes, or stars?
Like electromagnetic waves, gravitational waves can be gravitationally lensed. While the lensing of electromagnetic waves has had a rich observational history, we are still taking the first steps in the hunt for lensed gravitational waves. The theory behind light lensing and gravitational-wave lensing are similar, but the methodologies to detect these waves are entirely different — and so are the scientific goals.
If we observed lensed gravitational waves in practice, they would open the door to several exciting scientific pursuits. When the lens system is sufficiently unique, one could locate merging black holes, invisible to conventional telescopes, by combining gravitational-wave and electromagnetic lensing surveys. When an electromagnetic counterpart accompanies the lensed waves, precision cosmology studies may become feasible owing to the sub-millisecond lensing time-delay gravitational-wave measurements. By contrasting the time delays between lensed gravitational waves with their transient electromagnetic counterparts, one could measure the speed of gravity relative to light. As lensed gravitational waves allow us to observe the same event multiple times at different detector orientations, they can also probe the full polarization of the waves, testing general relativity and alternative theories. Microlensing, on the other hand, might help in studying populations of objects such as primordial and intermediate-mass black holes. Interestingly, the detection of lensed waves has been forecasted in the coming years by recent studies (e.g., ArXiv:2106.06303, ArXiv:2105.14390).
In a recent LVC study, we have looked for lensing signatures in the gravitational wave signals detected in the first half of the third LIGO-Virgo observing run.