Evaporating black holes: constraints on anomalous emission mechanisms

Hawking radiation of astrophysical black holes is minute and thought to be unobservable. However, different mechanisms could contribute to an anomalously high emission rate: extra dimensions, new "dark" families of bosons or fermions, or a lower fundamental Planck scale. Do black holes flood the Universe with gravitational waves via mass loss? Here, we show that the formation of black hole binaries and the absence of a stochastic background of gravitational waves can limit the emission rate to $|\dot{M}|\lesssim 10^{-15}M_{\odot}/{\rm yr}$, seven orders of magnitude more stringent than bounds from resolvable inspiralling binaries.