A black hole merger is not a silent impact. The newborn hole quivers and decays, emitting gravitational waves — ripples in space itself. This signal is called the 'ringdown,' and it resembles the fading chime of a bell: it has a fundamental tone and several overtones. In a black hole, the tone is set by so-called quasinormal modes — frequencies at which the curved spacetime oscillates.
If gravity is slightly more complex than general relativity teaches, new fields may exist in nature — for example, a light scalar field. The authors of the study took one such extended theory (shift-symmetric Horndeski theory) and examined how such a field would affect a black hole's ring. It turns out the effect is twofold: first, the frequencies themselves shift slightly; second, 'foreign' notes appear in the signal — frequencies of the scalar field. And here's the key: if the scalar field is weak, both effects, the shift and the admixture, are roughly equal in strength. But if the field is not so weak, the foreign frequencies can ring louder than the shift in the fundamental tone.
So, hunting for new physics by looking for 'contamination' of the signal with foreign frequencies may be more advantageous than chasing tiny shifts. This changes the strategy for seeking deviations from Einstein's theory: perhaps gravitational wave detectors are already 'hearing' the whisper of new fields: we just don't yet know how to recognize it.
The term 'quasinormal modes' comes from black hole physics, but today it is used even to describe oscillations of liquid droplets and optical resonators — anywhere there are damped waves.
In Carl Sagan's novel 'Contact,' the alien signal contained 'extra' frequencies in which a message was encoded — much like the possible 'foreign' ring of a black hole.