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  1. Are there sharp distinctions between primordial black holes and the usual black hole?

  2. Can a primordial black hole smoothly become massive to be a black hole?

  3. Can a black hole smoothly become less massive to be a primordial black hole? Say by evaporation?

Some facts:

  • Primordial black holes masses can be far below stellar mass, thus far less than black hole masses.

  • Primordial black holes are a hypothetical type of black hole that formed soon after the Big Bang. In the early universe, high densities and heterogeneous conditions could have led sufficiently dense regions to undergo gravitational collapse, forming black holes. Yakov Borisovich Zel'dovich and Igor Dmitriyevich Novikov in 1966 first proposed the existence of such black holes. The theory behind their origins was first studied in depth by Stephen Hawking in 1971. Since primordial black holes did not form from stellar gravitational collapse, their masses can be far below stellar mass (c. 4×$10^{30}$ kg).

  • Primordial black holes could be a type of dark matter called MACHOs, which stands for massive compact halo objects, because astronomers think they're found in the halos, or outskirts, of galaxies. Such black holes would be difficult to see if they're simply floating quietly in space and keeping to themselves.

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2 Answers 2

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The only properties a black hole has are its mass and spin (and, theoretically, electric charge, but we don't expect to find electrically charged black holes in Nature).

The only difference between a primordial black hole, and a black hole of stellar origin, is when the black holes are made. Primordial black holes (if they exist) are formed by processes in the early Universe such as during a phase transition. Stellar origin black holes form from stellar collapse. Therefore, a primordial black hole can't become a stellar origin black hole, and vice versa.

However, if you had two black holes, there would be no way to do experiments on the black hole to say which was primordial and which was stellar origin. The way we would distinguish them is based on theory of stellar evolution, which puts a lower bound on the mass of black holes, while primordial black holes can be of lower mass.

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    $\begingroup$ "we don't expect to find electrically charged black holes in Nature" -> why is that? I think researchers also propose both electrically charged or magnetic black hole in Nature? $\endgroup$ Feb 11, 2021 at 0:40
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    $\begingroup$ Basically the same reason you don't see large clumps of charged matter: if you have a positively charged black hole, it would attract electrons until it neutralized. However, as purely theoretical objects, black holes with electric and magnetic monopole moments are fun to think about. $\endgroup$
    – Andrew
    Feb 11, 2021 at 0:42
  • $\begingroup$ @Andrew but that kind of assumes that there are sufficient electrons in the vicinity of the black hole for it to become neutralized, no? $\endgroup$
    – Michael
    Feb 11, 2021 at 20:31
  • $\begingroup$ @Michael This is kind of a glib but also kind of a serious answer. If the black hole was formed with a macroscopic (large) amount of positive charge, we must have gotten those charges from somewhere, since the Universe we observe is electrically neutral to good precision. So whatever process formed a positively charged black hole very likely also formed an equal and opposite cloud of negative charge. $\endgroup$
    – Andrew
    Feb 12, 2021 at 1:07
  • $\begingroup$ Somewhat more seriously... let's look at two cases. One would be a black hole formed in the aftermath of a supernova. Maybe, for whatever reason, positively charged nuclei collapsed into a black hole and electrons were ejected from the supernova. Well if it was a macroscopic amount of charge, that positive charge is going to attract the ejected electrons. As another example, supermassive black holes typically have accretion disks with a lot of matter, which would be a great source of charges. $\endgroup$
    – Andrew
    Feb 12, 2021 at 1:09
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You have stated correctly, in your question, the main difference between primordial black holes and black holes formed by stellar collapse: it is simply a large difference in mass.

The term "primordial" of course refers to when the black hole formed, so you could say that the terminology is just referring to that. But the point is that in the very early universe one can suggest mechanisms whereby black holes of modest mass could form, but those processes are unlikely to gather whole stars-worth of material and form a heavier black hole. Meanwhile, stellar collapse can form black holes of mass say a few times a solar mass (or more), but it is hard to find such a process which can form a less massive black hole. So these two different processes lead to black holes of different mass.

(Actually some estimates suggest a primordial black hole up to a solar mass might be possible; see comments).

A primordial black hole is still a hypothesis, not yet with anything like observational evidence, but likely enough to be taken seriously. I don't know what mass distribution is predicted but the mass might be as high as that of a mountain, say ($10^{12}$ kg). That's still tiny compared to several solar masses. In your question you ask whether such a black hole could grow by absorbing material. It certainly could, but the trouble is the process would be too slow to have had much effect by now because the black hole is tiny: the Schwarzschild radius is around a femtometre (the size of an atomic nucleus)! Such a tiny black hole will never encounter enough material to grow substantially. It is more likely to evaporate. The evaporation would finish in a burst of gamma rays but no burst of the correct form has been detected up till now.

The larger black holes do not evaporate because the evaporation process (which by the way is not quite universally accepted) itself becomes very slow as the mass goes up, and a heavy enough black hole will absorb more mass even just from the cosmic background radiation than it could get rid of by Hawking radiation.

So the net result is that black holes in very different regimes of mass stay in those very different regimes for many billions of years. They don't evolve quickly enough to become like one another by this time.

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    $\begingroup$ This is a great answer, but just a note that the mass distribution of primordial black holes is really very uncertain (unsurprisingly), and there are papers suggesting that you can produce primordial black holes that are approximately or slightly lighter than 1 solar mass, for example: arxiv.org/abs/astro-ph/9708060. $\endgroup$
    – Andrew
    Feb 11, 2021 at 12:39
  • $\begingroup$ @Andrew thanks; I'll adjust the text a little to reflect this $\endgroup$ Feb 11, 2021 at 14:30
  • $\begingroup$ Regardug "the larger black holes do not evaporate", this seem to need a small correction. At some far in the future time the temperature of a black hole's radiation will be greaer than the CBR which constantly gets smaller. $\endgroup$
    – Buzz
    Feb 17, 2021 at 15:51
  • $\begingroup$ @Buzz I meant they do not evaporate on the kinds of timescales which are relevant to answering the question. Your point is about a very different timescale, one which is, indeed, so long that we can't confidently expect our limited knowledge of such things to be adequate. $\endgroup$ Feb 17, 2021 at 18:45
  • $\begingroup$ Hi Andrew - I may have misunderstood the intention of the 3 item question. I did not perceive it to include any time limitation. $\endgroup$
    – Buzz
    Feb 17, 2021 at 21:06

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