The ringdown-quantum gravity connection in real and analogue black holes

Due to quantum gravity, a black hole horizon should feature quantum fluctuations. We explore the possibility that these fluctuations radiate Hawking radiation dynamically. By studying the observed ringdowns after black hole mergers, we find that the amplitude of the horizon fluctuations should be on the order of the Planck length, in order to emit the quantity of gravitons corresponding to Hawking radiation. If the horizon fluctuations themselves are quasinormal modes, then the Hawking graviton spectrum is much more accurate than the usual blackbody spectrum. Furthermore, it would only take one quasinormal mode, with its broad, damped spectrum, to emit the entire Hawking spectrum of gravitons. We check the dynamical Hawking radiation hypothesis by means of a sonic black hole; we induce additional fluctuations in the horizon, and observe enhanced Hawking radiation. As expected, we see that the spontaneous Hawking radiation is entangled, while the enhanced Hawking radiation is not, since we induce it deterministically.