Gravitational Wave Echoes

Gravitational waves (GWs) are predicted by general relativity (GR) as freelypropagating disturbances of spacetime. However, tentative detections of postmerger“echoes” by LIGO/Virgo suggest additional structure. Here we show thatthe framework naturally predicts a harmonic modulation of GWs linked to theuniversal timescale T , producing echo signals with characteristic delays of order10􀀀2 s. This effect requires no modification of black hole horizons, but followsdirectly from the harmonic nature of spacetime in .