  It has recently been suggested that the presence of a plenitude of light
axions, an Axiverse, is evidence for the extra dimensions of string theory. We
discuss the observational consequences of these axions on astrophysical black
holes through the Penrose superradiance process. When an axion Compton
wavelength is comparable to the size of a black hole, the axion binds to the
black hole "nucleus" forming a gravitational atom in the sky. The occupation
number of superradiant atomic levels, fed by the energy and angular momentum of
the black hole, grows exponentially. The black hole spins down and an axion
Bose-Einstein condensate cloud forms around it. When the attractive axion
self-interactions become stronger than the gravitational binding energy, the
axion cloud collapses, a phenomenon known in condensed matter physics as
"Bosenova". The existence of axions is first diagnosed by gaps in the mass vs
spin plot of astrophysical black holes. For young black holes the allowed
values of spin are quantized, giving rise to "Regge trajectories" inside the
gap region. The axion cloud can also be observed directly either through
precision mapping of the near horizon geometry or through gravitational waves
coming from the Bosenova explosion, as well as axion transitions and
annihilations in the gravitational atom. Our estimates suggest that these
signals are detectable in upcoming experiments, such as Advanced LIGO, AGIS,
and LISA. Current black hole spin measurements imply an upper bound on the QCD
axion decay constant of 2 x 10^17 GeV, while Advanced LIGO can detect signals
from a QCD axion cloud with a decay constant as low as the GUT scale. We
finally discuss the possibility of observing the gamma-rays associated with the
Bosenova explosion and, perhaps, the radio waves from axion-to-photon
conversion for the QCD axion.
