Gravitational wave echos from physical black holes

TL;DR

This work argues that gravitational wave echoes are an inevitable consequence of standard general relativity for physically formed black holes, not necessarily signals of new physics. By modeling a neutral, non-spinning collapsar with a moving Dirichlet boundary and solving the perturbation equation with a centrifugal potential plus a Gaussian surface barrier, the authors show that echoes arise from scattering of a gravitational wave burst and exhibit nonuniform time delays and significant Doppler redshift. The study compares these physical BH echoes to those from exotic compact objects, finding notable differences in spacing and spectrum, while noting that the late-time horizon dynamics naturally shape the echo signal. The results suggest that observed echoes could help distinguish physical black holes from mathematical black holes and ECO mimickers, and motivate considering echo phenomena during all phases of binary coalescence.

Abstract

Gravitational wave echos from the coalescence of black hole binaries are often viewed as signals beyond general relativity or standard model. In this work, we show that these echos are inevitable in the black holes coalescence described by standard general relativity. This is because it is the physical black holes formed through gravitational collapse serve as the true description of astronomical black holes. For physical black holes, only their asymptotic structure before the horizon forms are detectible to the outside probes. Here, by investigating the scattering of a gravitational wave burst on a physical black hole and pay special attention to the echos in the waveform, we uncover distinct features of the echos both in the time and frequency domains.

Figures (4)

• Figure 1: The spacetime diagram of a gravitational wave's scattering on a physical black hole. The left solid curve represents the star surface while the right vertical dashed line represents the photon sphere.
• Figure 2: The echos from a Gaussian wavepacket scattering off a physical black hole. Two echos can be seen in this waveform, one around $150-240$ and another one around $290-380$.
• Figure 3: The echos from a Gaussian wavepacket scattering off an ECO. Three echos can be seen in this waveform, the first lies around $190-300$, the second lies around $315-425$ and the last one starting from $440$. The three echos are equally spaced with interval of $125$.
• Figure 4: Subdiagrams (a) and (b) display the absolute value of the Fourier spectrum of the first echo and second echo from physical black holes. Those from ECO are displayed in subdiagrams (c) and (d) respectively.