# The Initial Spin Probability Distribution of Primordial Black Holes

## TL;DR

This paper models the initial spin distribution of primordial black holes formed from early universe inhomogeneities, revealing that spins are typically at percent levels due to first-order tidal effects and shape asymmetries.

## Contribution

It provides a theoretical framework using peak theory to predict the probability distribution of primordial black hole spins at formation, incorporating tidal torques and asphericity effects.

## Key findings

- Typical spin parameter $a_s$ is about a few percent.
- Spin distribution depends on the variance of overdensity and power spectrum width.
- Narrow spectra suppress spin due to velocity shear alignment.

## Abstract

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.