The cosmos sets its own lenses in the path of gravitational waves. When two black holes merge, the resulting ripple travels through the universe. If it encounters a massive galaxy cluster along the way, its gravity bends the path of the waves — much like glass focuses light. The signal can be amplified several times or even split into multiple copies arriving with delays. For detectors on Earth, this is a trap: the event appears closer and more massive than it actually is.
The author of the study took the catalog of mergers collected by the LIGO, Virgo, and KAGRA detectors and checked each event for signs of such 'lensing.' Two models were used for how often a beam hits a lens (optical depth), and two approaches to describing the black hole population — one with a rigid mathematical form and one without. For no merger did the probability of being lensed exceed 1%.
Even a null result proved useful. Accounting for the lensing hypothesis allowed narrowing the uncertainty in the redshift (distance converted to epoch) at which the merger rate peaks by 10%. And for the most distant outskirts, where ground-based detectors usually cannot reach, the upper limit on the merger rate dropped by a factor of ten. These numbers are a modest but first step toward seeing the invisible: the epoch beyond the peak of star formation, when the universe was young and full of massive stars.
If a gravitational lens creates multiple images of a single merger, they arrive at Earth with differences ranging from minutes to months, and detectors might mistake them for separate events.
In the movie 'Interstellar,' the supermassive black hole Gargantua bends starlight, creating multiple images — gravitational waves experience a similar effect, except instead of light, it's the ripple of spacetime itself.