  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.
