# Evolution of primordial black hole spin due to Hawking radiation

## TL;DR

This paper investigates how Hawking radiation affects the spin evolution of primordial black holes, suggesting that high-spin black holes could indicate primordial origins or exotic physics.

## Contribution

It derives a lower mass limit for primordial black holes where Hawking radiation slows enough to preserve high spin, challenging the thermodynamic spin limit.

## Key findings

- High-spin primordial black holes can exist above a certain mass threshold.
- Hawking radiation can allow primordial black holes to evade the Thorne limit.
- Observation of high-spin black holes may indicate primordial origin or new physics.

## Abstract

Near extremal Kerr black holes are subject to the Thorne limit $a<a^*_{\rm lim}=0.998$ in the case of thin disc accretion, or some generalized version of this in other disc geometries. However any limit that differs from the thermodynamics limit $a^* < 1$ can in principle be evaded in other astrophysical configurations, and in particular if the near extremal black holes are primordial and subject to evaporation by Hawking radiation only. We derive the lower mass limit above which Hawking radiation is slow enough so that a primordial black hole with a spin initially above some generalized Thorne limit can still be above this limit today. Thus, we point out that the observation of Kerr black holes with extremely high spin should be a hint of either exotic astrophysical mechanisms or primordial origin.