It is well known how Black hole forms. But once it is formed are there any circumstances which can lead a black hole to becoming a normal mass again?
Hawking radiation leads to a complete evaporation of black holes, whether there are remnants and the problems this creates is a matter of ongoing discussion.For example see here for a review.
Forty years after the discovery of Hawking radiation, its exact nature remains elusive. If Hawking radiation does not carry any information out from the ever shrinking black hole, it seems that unitarity is violated once the black hole completely evaporates. On the other hand, attempts to recover information via quantum entanglement lead to the firewall controversy. Amid the confusions, the possibility that black hole evaporation stops with a "remnant" has remained unpopular and is often dismissed due to some "undesired properties" of such an object. Nevertheless, as in any scientific debate, the pros and cons of any proposal must be carefully scrutinized. We fill in the void of the literature by providing a timely review of various types of black hole remnants, and provide some new thoughts regarding the challenges that black hole remnants face in the context of the information loss paradox and its latest incarnation, namely the firewall controversy. The importance of understanding the role of curvature singularity is also emphasized, after all there remains a possibility that the singularity cannot be cured even by quantum gravity. In this context a black hole remnant conveniently serves as a cosmic censor. We conclude that a remnant remains a possible end state of Hawking evaporation, and if it contains large interior geometry, may help to ameliorate the information loss paradox and the firewall controversy. We hope that this will raise some interests in the community to investigate remnants more critically but also more thoroughly.`
Apart from the possibility of BH relics as outlined above, if it were possible to remove or ‘destroy’ a BH event horizon, then that might qualify. Of course, to avoid the cosmic censorship hypothesis (no naked singularities please) you need:
- a BH that doesn’t have a singularity, i.e. something like a Bardeen black hole, also known as a ‘regular black hole’. BBH’s are thought to have a de Sitter core. Some people think astrophysical black holes are ‘regular’ i.e. predicted singularities just show where general relativity fails.
The event horizon of the no-singularity BH is still specified by mass $M$, spin $J$ and charge $Q$ (as per Kerr-Newman):
If you violate Eqn (1) then – no more event horizon. So theoretically, if the BBH can absorb enough angular momentum and charge, and re-stabilise in a steady state, then the event horizon disappears and – maybe – you have a ‘normal mass’ again.
There is a 2013 paper that thinks the accretion process from a BH disk might be able to achieve this, and suggest a test of the hypothesis via study of the energy of BH mergers. Not sure whether this has been falsified or not yet.