Utilizing gravitational-wave (GW) lensing opens a new way to understand the small-scale structure of the Universe. We show that, in spite of its coarse angular resolution and short duration of observation, LIGO can detect the GW lensing induced by small structures, in particular by compact dark matter (DM) or the primordial black hole of $10–{10}^5{M}_{\odot }$, which remains an interesting DM candidate. The lensing is detected through GW frequency chirping, creating the natural and rapid change of lensing patterns: frequency-dependent amplification and modulation of GW waveforms. As a highest-frequency GW detector, LIGO is a unique GW lab to probe such light compact DM. With the design sensitivity of Advanced LIGO, one-year observation by three detectors can optimistically constrain the compact DM density fraction $f_{\mathrm{DM}}$ to the level of a few percent.

• Received 13 December 2017
• Revised 31 July 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.041103