As the current ground-based gravitational-wave detectors are upgraded and more detectors come online, we expect to access new scientific avenues. One such avenue is observations of gravitational-wave lensing, with recent forecasts predicting detections in the coming years. Before the first detection, it is essential to investigate, outline, and build upon the various science cases of gravitational-wave lensing.
In recent work, we show that strong lensing could be used to localize merging black holes at high precision, even though they do not emit light.
Suppose a gravitational wave from a merging black hole is strongly lensed. In that case, the light from the galaxy that hosts the black holes is also lensed. Thus, if we point electromagnetic telescopes in this direction, the host galaxy would appear as a lensed galaxy.
Locating this lensed host galaxy allows us to localize merging black holes and study the lens system using electromagnetic and gravitational-wave channels.
In our study, we demonstrated the proof-of-principle for the method. Specifically, we demonstrated how to combine the image properties of lensed gravitational waves with electromagnetic galaxy lens searches to locate the host galaxy of merging black holes. Furthermore, we show that follow-up lens modeling allows us to locate where the merger occurred within the galaxy at a sub-arcsecond precision. Finally, we demonstrate a simple example application for the localization: Studying the expansion of the Universe at high redshift.
Besides the fundamental interest, this could plausibly enable several scientific pursuits such as cross-verification of lensing claims, measuring the cosmological expansion at high redshift, studying the binary black hole-host galaxy connection and binary formation channels, improved lens modeling, and improved tests of gravitational-wave polarization. It could also extend the current gravitational-wave lensing studies onto a multi-messenger playground.