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It is generally accepted that black hole emits Hawking radiation. I wonder if this radiation is different in case of black hole created by collapsing anti-matter.

Consider a thought experiment. At the beginning we have antimatter which collapses into black hole. Then black hole evaporates as a result of Hawking radiation. What is the end result of this radiation: matter, antimatter or mix of both?

If antimatter, then we have a method to distinguish between black hole and anti-black hole - it contradicts to responses to answers to "Anti-Matter Black Holes" that there is no method to find a difference between black hole and anti-black hole.

If matter or mix - then we have strange alchemic transmutation: antimatter (beginning of the experiment) transforms to plain matter. So maybe anti-black hole emits Hawking radiation consisting of antimatter only.

Another possibility: Hawking radiation consisting only of truly neutral particles (photons, Higgs, etc.). This is strange too because antimatter changed to neutral particles.

What is the end result of anti-black hole evaporation?

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then we have strange alchemic transmutation: antimatter (beginning of the experiment) transforms to plain matter.

Before we even worry about black holes, note that there is no conservation law that prevents production of antimatter from matter. For example, some nuclei undergo beta decay and emit positrons. This is basically because conservation laws are additive. As a simple example, a photon can do pair production, which is a process in which it disappears and produces an electron and an antielectron. This doesn't violate any conservation laws, because although the electron and antielectron differ in charge and lepton number, the sums of these quantities are zero.

The no-hair theorems tell us that a black hole can be characterized by only three numbers: mass, charge, and angular momentum. That isn't enough information to tell very much at all about the type or types of matter that went into its formation by gravitational collapse -- if it even formed by gravitational collapse at all. It's possible that there are primordial black holes that did not form by gravitational collapse, so that no "matter" went into them.

What is strange from the point of view of particle physics is that black hole evaporation doesn't conserve quantities like baryon number and lepton number. The radiation is predicted to be purely thermal, which means that it can't have an imbalance of, e.g., electrons and antielectrons. Note that in practical terms, we will probably never be able to detect Hawking radiation, and in any case nearly all of it would be photons.

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  • $\begingroup$ Ok, so the end result can be all of this: anti-matter, plain matter, and most probably photons. And we don't have any way to infer if black hole was created from matter or anti-matter. Sounds reasonable. $\endgroup$
    – robsosno
    Commented Jul 29, 2017 at 19:10
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    $\begingroup$ One more thought: so regardless if we had matter or anti-matter at the beginning we have at the end equal amount of matter and anti-matter assuming equal probabilities. This means that in the (very) long term we will have more anti-matter in the Universe than now. $\endgroup$
    – robsosno
    Commented Jul 29, 2017 at 19:21
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    $\begingroup$ This means that in the (very) long term we will have more anti-matter in the Universe than now. You may want to look at physics.stackexchange.com/questions/74041/… . Black hole evaporation is actually not a big contributor to the state of the universe in the very distant future. $\endgroup$
    – user4552
    Commented Jul 30, 2017 at 1:21
  • $\begingroup$ Thank you for all your explanatory answers. I understand them. It is just very counterintuitive to allow things like this: p- + e+ --> black hole --> p+ + e- $\endgroup$
    – robsosno
    Commented Jul 30, 2017 at 11:22

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