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In theoretical physics, an extremal black hole is a black hole with the minimum possible mass that is compatible with its charge and angular momentum.[1]
The concept of an extremal black hole is theoretical and none have thusfar been observed in nature. However, many theories are based on their existence.
In supersymmetric theories, extremal black holes are often supersymmetric: they are invariant under several supercharges. This is a consequence of the BPS bound. Such black holes are stable and emit no Hawking radiation. Their black hole entropy[2] can be calculated in string theory.
It has been suggested by Sean Carroll that the entropy of an extremal black hole is equal to zero. Carroll explains the lack of entropy by creating a separate dimension for the black hole to exist within.[3]
The Hawking radiation of extremal black holes is considered non-thermal (non-Planck distributed), with no associated temperature.[4]
The hypothetical black hole electron is super-extremal (having more charge and angular momentum than a black hole of its mass "should").
^Kallosh, Renata; Linde, Andrei; Ortín, Tomás; Peet, Amanda; Van Proeyen, Antoine (1 December 1992). "Supersymmetry as a cosmic censor". Physical Review D. 46 (12): 5278–5302. arXiv:hep-th/9205027. Bibcode:1992PhRvD..46.5278K. doi:10.1103/PhysRevD.46.5278. PMID 10014916. S2CID 15736500.
^Bekenstein, Jacob D. (1973). "Black Holes and Entropy". Phys. Rev. D. 7 (8): 2333–2346. Bibcode:1973PhRvD...7.2333B. doi:10.1103/PhysRevD.7.2333. S2CID 122636624.
^Carroll, Sean M.; Johnson, Matthew C.; Randall, Lisa (2009). "Extremal limits and black hole entropy". Journal of High Energy Physics. 2009 (11): 109. arXiv:0901.0931. Bibcode:2009JHEP...11..109C. doi:10.1088/1126-6708/2009/11/109. S2CID 73604121.
^Good, Michael (2020). "Extremal Hawking Radiation". Physical Review D. 101 (104050): 104050. arXiv:2003.07016. Bibcode:2020PhRvD.101j4050G. doi:10.1103/PhysRevD.101.104050. S2CID 212725670.
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