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">Jérémy 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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