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Hawking radiation is supposed to very slowly evaporate a black hole (terms and conditions apply :] ).

Apart from Hawking radiation, is there any mechanism or effect that can make a black hole cease to exist? Or once they are formed are they expected to exist in this form "forever"?

PS: I posted the same question to Physics but I think it applies well to both SEs. So I hope it is okay to try here as wwell.

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It can merge with another black hole, if that counts. A simulation of that is illustrated here

That aside, no black hole with decay more quickly, as far as I can find out, than the "normal" decay of a non-rotating, uncharged black hole into which nothing is falling. Let's call this the "standard rate of decay".

"Feeding it" (allowing any matter or energy to fall into it), will reduce the decay below the standard rate, or even reverse it. Since the CMB pervades space and will fall into the black hole, blocking this (for instance by surrounding the hole by an opaque refrigerated shell) will be necessary to allow it to decay at the standard rate. Indeed for a black hole of mass over roughly $10^{22} kg$ the CMB would feed it faster than it decays, if not blocked.

Energy from the spin of the black hole can be extracted using the Penrose process. Since this answer shows that rotation also reduces the emission of Hawking radiation, you can clearly speed up the decay by extracting energy from as much of the spin as possible, but the fastest decay you get is, once again, the standard rate.

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  • $\begingroup$ Thanks! I didn't know about the Penrose process and it sounds kind of fascinating. If you allow me three clarifying questions I'll write them in three separate comments: By isolating from CMB etc. do you mean that the CMB will become cool enough by that time that the temperature of the Hawking radiation will be hotter than CMB, thus allowed to go away? Or do you mean something else? $\endgroup$ – Helen Sep 30 '18 at 10:56
  • $\begingroup$ Second clarification: If "feeding it" "lengthens its life", then is it implied that its life can shorten by not feeding it? $\endgroup$ – Helen Sep 30 '18 at 10:57
  • $\begingroup$ And, if we exclude these two slow and theoretical mechanisms (and merging), I understand that there are no other known "palpable" mechanisms - BH are otherwise really eternal as far as we know. Do I understand correctly that this is the current state of affairs? $\endgroup$ – Helen Sep 30 '18 at 10:59
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    $\begingroup$ Have moved clarifications into the answer $\endgroup$ – Steve Linton Sep 30 '18 at 13:05

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