Spin of Primordial Black Holes
  Primordial black holes, formed from rare peaks in the primordial fluctuations
$\zeta$, are non-rotating at zeroth order in $\zeta_{\rm rms}$. We show that
the spin also vanishes at first order in $\zeta_{\rm rms}$, suggesting the
dimensionless spin parameter $a_{\rm rms} \sim \zeta_{\rm rms}^2$. We identify
one quadratic contribution to the spin by calculating (and extrapolating to the
formation time) the torque on a black hole due to ambient acoustic waves. For a
reasonable density of primordial black holes this implies a percent level spin
parameter.
