Any extremal black holes are primordial. (arXiv:1906.04196v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Arbey_A/0/1/0/all/0/1">Alexandre Arbey</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Auffinger_J/0/1/0/all/0/1">J&#xe9;r&#xe9;my Auffinger</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Silk_J/0/1/0/all/0/1">Joseph Silk</a>

Extremal Kerr black holes, if they exist, cannot have an astrophysical origin
due to the Thorne limit $a a^*_{rm lim}$ should be a proof of its primordial origin.

Extremal Kerr black holes, if they exist, cannot have an astrophysical origin
due to the Thorne limit $a<a^*_{rm lim}=0.998$. However this limit can be
evaded if they are primordial and subject to evaporation by Hawking radiation.
We derive the lower mass limit above which Hawking radiation is slow enough so
that a primordial black hole with a spin initially above the Thorne limit can
still be above this limit today. Thus, the observation of a Kerr black hole
with $a^* > a^*_{rm lim}$ should be a proof of its primordial origin.

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