The Initial Spin Probability Distribution of Primordial Black Holes
  We study the spin of primordial black holes produced by the collapse of large
inhomogeneities in the early universe. Since such primordial black holes
originate from peaks, that is, from maxima of the local overdensity, we resort
to peak theory to obtain the probability distribution of the spin at formation.
We show that the spin is a first-order effect in perturbation theory: it
results from the action of first-order tidal gravitational fields generating
first-order torques upon horizon-crossing, and from the asphericity of the
collapsing object. Assuming an ellipsoidal shape, the typical value of the
dimensionless parameter $a_{\rm s}=S/G_N M^2$, where $S$ is the spin and $M$ is
the mass of the primordial black hole, is about
$\sigma_\delta\sqrt{1-\gamma^2}/2\pi$. Here, $\sigma^2_\delta$ is the variance
of the overdensity at horizon crossing and the parameter $\gamma$ is a measure
of the width of the power spectrum giving rise to primordial black holes. One
has $\gamma=1$ for monochromatic spectra. For these narrow spectra, the
suppression arises because the velocity shear, which is strongly correlated
with the inertia tensor, tends to align with the principal axis frame of the
collapsing object. Typical values of $a_{\rm s}$ are at the percent level.
