Black hole echoes

We consider a very simple model for gravitational wave echoes from black hole merger ringdowns which may arise from local Lorentz symmetry violations that modify graviton dispersion relations. If the corrections are sufficiently soft so they do not remove the horizon, the reflection of the infalling waves which trigger the echoes is very weak. As an example, we look at the dispersion relation of a test scalar field corrected by rotonlike operators depending only on spatial momenta, in Gullstrand-Painlevé coordinates. The near-horizon regions of a black hole do become reflective, but only very weakly. The resulting “bounces” of infalling waves can yield repetitive gravity wave emissions but their power is very small. This implies that to see any echoes from black holes we really need an egregious departure from either standard GR or effective field theory, or both. One possibility to realize such strong echoes is the recently proposed classical firewalls which replace black hole horizons with material shells surrounding timelike singularities.

Figure 1

Left panel: Plot of the numerical evaluation of $k_0^2{r}_0^2|R|/\alpha$, where $R$ is the reflection coefficient, as a function of dimensionless frequency $f$. Right panel: Plot of the real part of the perturbation $k_0^2{r}_0^2{\psi }_1(r)/\alpha$, at fixed frequency $f=2$, as a function of the distance from the horizon $r-r_0$.

Figure 2

Spacetime diagram of the proposed “classical firewall” of [31] completely excising the horizons. The RN singularity is enclosed by a nearby shell with Planckian density.