Gravitational-wave millilensing as a probe of dark matter substructures

Gravitational waves will bring forth an altogether new way to probe millilensing, which may allow us to probe the existence of dark matter substructures, and ultimately shed light on the nature of dark matter. Theoretically predicted dark matter objects, such as small subhalos and primordial black holes, can act as millilenses and introduce beating patterns in gravitational waves - if they exist. While millilensing has been investigated more traditionally in the electromagnetic band, gravitational waves present a new complementary approach to millilensing. Here we show that should millilensing signatures appear in the observed gravitational waves, current-generation gravitational-wave detectors are not only able to detect them, but also reveal the astrophysical nature of the lens. Furthermore, we discuss the advantages of using gravitational waves to probe millilensing.