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So I was looking through some articles about primordial black holes the other day and it said that these black holes can be insanely tiny. Some of the articles state the mass could range from $10^{12}$ all the way to 1 solar mass.

But how could this be possible if when the matter is compressed to a black hole, the matter if less than the Chandrasekhar limit just becomes a white dwarf or if below Tolman-Oppenheimer-Volkoff becomes a neutron star.

Thank you in advance.

My references are below: https://physicsworld.com/a/concerning-primordial-black-holes/ https://www.livescience.com/dark-matter-made-of-black-holes.html

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    $\begingroup$ Perhaps helpful: an educated guess about the mass within the initial horizon in a collapsing star; a question about the order of horizon versus singularity formation. Note that, while electron-degenerate matter is unstable above the Chandrasekhar limit, neutron-degenerate matter is perfectly happy below the Chandrasekhar limit, which is also below the T.O.V. limit. See e.g. this catalog of neutron star masses. Low-mass black holes are likewise allowed. $\endgroup$
    – rob
    Jul 28, 2022 at 2:15
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    $\begingroup$ @rob Neutron star masses are not below the realistic (taking account of neutronisation) Chandrasekhar mass for iron - which is around 1.1 solar masses. $\endgroup$
    – ProfRob
    Jul 28, 2022 at 6:46
  • $\begingroup$ Thanks, I didn't realize there was a chemical correction to the Chandrasekhar limit. I was trying to make the point that, while stellar evolution gives a minimum mass to white dwarves, neutron stars, and black holes produced from stars, the degenerate matter is stable at lower masses and smaller objects could exist if they were formed by some non-stellar process, such as during the Big Bang. Likewise for sub-stellar primordial black holes. For sub-stellar-mass neutron stars, @ProfRob, you might recognize this answer. $\endgroup$
    – rob
    Jul 28, 2022 at 14:27
  • $\begingroup$ @rob Indeed, theoretically, a neutron "star" may exist that is lower than the Chandrasekhar mass for iron. But none have been observed. The presently observed neutron stars are not evidence that they could exist below the Chandrasekhar mass. $\endgroup$
    – ProfRob
    Jul 28, 2022 at 15:03

2 Answers 2

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Primordial black holes were never stars at any stage in their lifecycle, so those mass limits do not apply to them. See this Wikipedia article.

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  • $\begingroup$ Could you explain though why electron degeneracy pressure wouldn't prevent just any clump of matter from collapsing into a black hole? $\endgroup$ Jul 27, 2022 at 22:26
  • $\begingroup$ The wikipedia page doesn't explain how. It just says that it can happen. $\endgroup$ Jul 27, 2022 at 22:27
  • $\begingroup$ Also wouldn't matter have to get into the Schwarzschild Radius to become a black hole. It doesn't matter whether that is a star or a cloud of gas. So what process would make the matter compress then. $\endgroup$ Jul 27, 2022 at 22:31
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    $\begingroup$ @RoghanArun A primordial black holes arises from a density fluctuation in the early universe and starts out dense - it doesn’t have to collapse under gravity. That is why it is primordial. $\endgroup$
    – gandalf61
    Jul 28, 2022 at 5:45
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    $\begingroup$ External pressure in addition to any internal pressure from gravity. This can occur as the result of an explosion of sufficient violence to overcome electron degeneracy pressure. $\endgroup$ Jul 28, 2022 at 17:36
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As you say in the comments to the other answer, it doesn't matter what type of matter it is anyway it is simply a question of density. If there is a region of dense enough matter you get a blackhole.

The early universe was very dense, if small fluctuations of this density are enough to create black holes then you get primordial black holes. These are from matter densely packed in the very early universe even before there were stars or even atoms.

The normal forces you are used to dealing with don't apply to this super dense super hot "primordial soup" of a universe so things like the limits of mass size given by the stellar life cycle were not applicable.

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    $\begingroup$ The dense, super hot primordial soup is not dissimilar to the conditions at the centre of a neutron star. $\endgroup$
    – ProfRob
    Jul 28, 2022 at 6:48
  • $\begingroup$ So is it because there were no electrons back then to provide repulsion and so is that why the Chandrasekar limit didn't exist? $\endgroup$ Jul 28, 2022 at 11:51
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    $\begingroup$ @RoghanArun Again, the Chandrasekar limit deals with gravitational compression of a body of given parameters. It's not a magical black hole limit; it's not precise even for stars. The conditions under which primordial black holes would form are entirely different, and do not depend on gravitational compression at all. We could probably create black holes of our own (heck, according to some, we already did and it happens all the time naturally when cosmic rays impact our atmosphere); the Chandrasekar limit is of no concern there. We don't know if electrons were involved, but it's irrelevant $\endgroup$
    – Luaan
    Jul 28, 2022 at 12:11
  • $\begingroup$ I think it's more that there was enough pressure everywhere to overcome the Chandrasekar limit. Think of it like this; an amount of gas that would just disperse in vacuum would stay together as a bubble when underwater, because the pressure of the water keeps it together. $\endgroup$
    – Anju Maaka
    Jul 28, 2022 at 12:12
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    $\begingroup$ @RoghanArun The early universe was very dense because the early universe was very small. It's not that it was a dense point somehow being held together in empty space, but the actual space available to be in was very small so all the matter and energy was crowded together. Then the big bang happened and space got a lot bigger very quickly. $\endgroup$ Jul 28, 2022 at 16:40

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