I suppose most of you will agree with me that Black Holes (BH) are among the coolest and most mysterious objects in our Universe. Theoretically predicted a century ago by the Einstein's theory of GR, we know nowadays that they (probably) form the "heart" of all big enough galaxies and can potentially reach a humongous mass via merging - a phenomenon that recently got to the top pages of newspapers.

If we set asides the merging of already existing Black Holes, the ordinary way in which BH's "puff" into existence is via gravitational collapse of a large star (above ~20 times the mass of our Sun). This mechanism is relatively well-researched and it gives a lower bound for the mass of a "stellar" Black Hole.

My question is in regard to another natural mechanism for BH's formation, namely through fluctuations during the early stages of the Universe (supposing the Big Bang Model), when pressure and density were very, very high. I'd like to know what is the lower bound for the current mass of the remnants of such "primordial" BH's?

Remark 1: Note that I'm not asking for the theoretical bounds in the size of primordial Black Holes at the moment of their formation - such are given for example here! I'm interested instead in how small the possible survivors from this generation of BH's can be today... After all, if they indeed formed when the Universe was young and dense, they definitely have gained some weight till now.

Remark 2: If you know another natural mechanism - e.g. an accidental massive collision or something else - which could have caused the formation a "small" BH, I'll be glad to hear it.

Remark 3: With a risk to digress from my main question, may I also ask if someone knows how much time is needed for an average stellar BH to evaporate down to a half of its initial mass?

PS: If you wonder why I'm asking such a question, it is because (spinning) BH's may serve very well either as an accelerator for cosmic rockets or, if the worm-hole hypothesis is true, as a medium for superluminous travel through space. In this sense, if we find a small BH that couldn't have appeared naturally, this may be a sign that it was created by a highly-advanced civilization...


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