Gravitational lensing of waves: a new window into astrophysics, dark matter and gravity

Abstract: Two of Einstein’s key predictions — the bending of light by gravity and the existence of gravitational waves — are now meeting observationally. Just as light is deflected and magnified by massive objects, gravitational waves from merging black holes can also be gravitationally lensed. But because gravitational waves have long wavelengths, coherent phases, and accurately modeled emission, they can reveal lensing phenomena that are nearly impossible to access with electromagnetic signals.

I will describe how gravitational-wave lensing opens a new window on the Universe. In addition to magnification and multiple images, lensed gravitational waves can exhibit diffraction and interference: frequency-dependent distortions that encode the mass and structure of the lens. These effects offer a way to identify high-redshift magnified mergers and to probe compact objects and small-scale dark-matter structure. I will also discuss how propagation effects beyond general relativity, such as birefringence and dispersion, can test gravity on cosmological scales. Finally, I will present recent results on GW231123, a compelling candidate for a magnified and diffracted black-hole merger.