Yes, foreground massive objects can magnify gravitational waves emitted by a background source (the foreground objects can also induce a beating pattern on the

Yes, foreground massive objects can magnify gravitational waves emitted by a background source (the foreground objects can also induce a beating pattern on the background waves, or split the background waves into multiple wavefronts). The "foreground" emitter in a massive cluster of thousands of galaxies can be deep inside the cluster, and those are probably the gravitationally-lensed gravitational-wave sources we will identify first [more below at [1]]. A good starting point for a non-specialist is probably <https://astrobites.org/2021/05/25/gw-lensing/ https://astrobites.org/2021/05/25/gw-lensing/>. (You could compare a specialist paper <https://www.aanda.org/articles/aa/full_html/2020/11/aa38730-20/aa38730-20.html https://www.aanda.org/articles/aa/full_html/2020/11/aa38730-...> which describes the lensingGW software package https://gpagano.gitlab.io/lensinggw/ https://gpagano.gitlab.io/lensinggw/ ). - -- [1] When a candidate gravitational wave (GW) detection is found at LIGO et al., an electromagnetic (EM) detection may follow. Neutron stars collide explosively, generating a huge burst of very high-frequency (gamma ray) EM. Black holes don't explode but their accretion structures can interact very strongly producing high-frequency (X-ray) EM. These bright high-frequency EM flashes decay into a glow that can last for many days after a merger. Consequently they are looked for, as part of the overall project of multi-messenger astronomy (where EM and GW are two "messengers"; neutrinos and so forth make it "multi-"). Close to the source, peak EM tends to lag a bit behind the merger, mostly because of how the EM is produced at some distance outside merged black holes. Additionally, EM scatters/refracts/etc with dust and gas that one finds deep inside galaxies, which are more transparent to GW than to EM. Consequently in general GWs will be detected before EMs from the same source. Candidate GW detections provoke searches across the electromagnetic spectrum, since EM may arrive hours or even days later. GW from an inspiral and merger in general have much much longer wavelengths than the X-rays from the gas around the merging objects. Black holes merge roughly when their horizons touch, and horizon size is proportional to mass. GW wavelength is proportional to the orbital diameter, which must be larger than the sum of horizon radiuses, so supermassive black hole binary (SMBHB) mergers have much longer-wavelength GW than stellar-mass black hole binary mergers. The relatively short wavelengths of electromagnetic radiation will follow the laws of geometrical optics and in particular will experience Shapiro delay. GW wavelengths, being much longer, will not. This explains a further delay on EM from SMBHB mergers that are deep within a massive galaxy or cluster of galaxies. Foreground lenses (e.g. massive galaxies that the source does not live within) will further increase the delay suffered by EM produced by the merger that sourced the GW. Moreover, since foreground lenses can split the wavefronts of the GW and EM, there may be ~four detections, at different times, of the same merger in GW and EM. Alternatively, the foreground lens can magnify the source merger, but with the delays described above and geometric optics vs not, the focal point for GW and for EM will differ. All of this is active research, close to cutting edge, so sadly is mostly to be found via a literature search on Einstein lensing of gravitational waves. AFAICT most "pop sci" attempts to summarize some of the academic literature is simply awful. Hopefully I'm merely just not great above. :-/