Description
Like light, gravitational waves (GWs) are gravitationally lensed by massive objects in the Universe. Their long wavelengths, phase coherence, and lack of absorption make GW lensing highly complementary to its electromagnetic counterpart. Beyond magnification and the formation of multiple images, lensed GWs can exhibit genuine wave-optics effects such as diffraction and interference. These imprint frequency-dependent modulations on the signal that encode information about the lens mass and its distribution.
I will describe how wave-optics lensing can be used to discover high-redshift, magnified binary mergers and probe small-scale structure, including dark matter. After reviewing the basic theory, I will focus on two applications: 1) the interpretation of GW231123 as a diffracted and magnified compact binary coalescence, and 2) the potential of LISA to probe dark-matter halos from 10 to 10,000 solar masses. These examples will highlight the need to go beyond the isolated-lens picture, as external gravitational fields or the collective effect of many structures produce qualitatively new diffraction signatures.