Hot Gravitons and Gravitational Waves From Kerr Black Holes in the Early
  Universe
  Any abundance of black holes that was present in the early universe will
evolve as matter, making up an increasingly large fraction of the total energy
density as space expands. This motivates us to consider scenarios in which the
early universe included an era that was dominated by low-mass ($M < 5\times
10^8$ g) black holes which evaporate prior to primordial nucleosynthesis. In
significant regions of parameter space, these black holes will become
gravitationally bound within binary systems, and undergo mergers before
evaporating. Such mergers result in three potentially observable signatures.
First, any black holes that have undergone one or more mergers will possess
substantial angular momentum, causing their Hawking evaporation to produce
significant quantities of high-energy gravitons. These products of Hawking
evaporation are predicted to constitute a background of hot ($\sim$eV-keV)
gravitons today, with an energy density corresponding to $\Delta N_{\rm eff}
\sim 0.01-0.03$. Second, these mergers will produce a stochastic background of
high-frequency gravitational waves. And third, the energy density of these
gravitational waves can be as large as $\Delta N_{\rm eff} \sim 0.3$, depending
on the length of time between the mergers and evaporation. These signals are
each potentially within the reach of future measurements.
