talk at RAS specialist discussion meeting “Neutron star and black hole binary mergers: the first results of the LIGO-Virgo era”

Gravitational waves (GWs) from distant sources, such as binary black hole mergers, can be affected by intervening masses acting as gravitational lenses. In the case of strong lensing this can produce significant magnifications, as well as multiple images of the same GW signal with relative time delays and different magnification factors. Hence, the possibility was brought up that some of the observed GWs from binary black hole mergers could be multiple images of a single event, or that the whole set of events corresponds to a population of lensed lower-mass, higher-redshift sources. In this presentation, I will summarise recent work on a comprehensive statistical answer to this question.
I will also cover ongoing work on a sub-threshold search for lower-magnification lensed counterparts of observed signals, and the basics of wave-optics effects that may distort the GW waveforms beyond the usual geometric approximation. Upper limits on lensing effects can constrain binary black hole population models, and a successful detection of a multiply-imaged GW signal would yield exciting possibilities for cosmology.