Gravitational wave production by Hawking radiation from rotating
  primordial black holes
  In this paper we analyze in detail a rarely discussed question of gravity
wave production from evaporating primordial black holes. These black holes emit
gravitons which are, at classical level, registered as gravity waves. We use
the latest constraints on their abundance, and calculate the power emitted in
gravitons at the time of their evaporation. We then solve the coupled system of
equations that gives us the evolution of the frequency and amplitude of gravity
waves during the expansion of the universe. The spectrum of gravitational waves
that can be detected today depends on multiple factors: fraction of the total
energy density which was occupied by primordial black holes, the epoch in which
they were formed, and quantities like their mass and angular momentum. We
conclude that very small primordial black holes which evaporate before the
big-bang nucleosynthesis emit gravitons whose spectral energy fraction today
can be as large as $10^{-7.5}$. On the other hand, those which are massive
enough so that they still exist now can yield a signal as high as $10^{-6.5}$.
However, typical frequencies of the gravity waves from primordial black holes
are still too high to be observed with the current and near future gravity wave
observations.
