Dark radiation constraints on light primordial black holes
  Light black holes could have formed in the very early universe through the
collapse of large primordial density fluctuations. These primordial black holes
(PBHs), if light enough, would have evaporated by now because of the emission
of Hawking radiation; thus they could not represent a sizable fraction of dark
matter today. However, they could have left imprints in the early cosmological
epochs. We will discuss the impact of massless graviton emission by (rotating)
PBHs before the onset of big bang nucleosynthesis (BBN) and conclude that this
contribution to dark radiation is constrained by the cosmic microwave
background (CMB) (with the future CMB Stage 4) and BBN in the lighter portion
of the PBH mass range, under the hypothesis that they dominated the energy
density of the universe.
