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If there were no Hawking radiation (and no black hole radiation of any kind, or any way for black holes to reduce in size or disappear), then it seems a black hole would represent an irreversible end state of any matter or energy that became dense enough. They would only grow over time.

If this were the case, it seems plausible that the high-density environment of the early universe would have created a large number of small primordial black holes, filling space like bubbles in a carbonated beverage. Since these could never evaporate, they would only combine and grow, forming perhaps a large part of the "matter" in the universe.

The question is, does the universe we currently observe require some form of black hole evaporation having "cleaned up" the primordial black holes below a certain size – thus forming indirect confirmation of the phenomenon?

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    $\begingroup$ The physics we know requires that black holes radiate. There's no doubt about that. And only those tiny black holes that seemingly don't exist today radiate significantly. You cannot draw conclusions from their non-existance. $\endgroup$
    – Karl
    Jan 4 at 16:50
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    $\begingroup$ "If this was the case ..." I see no logical connection between the first and second paragraph of your question. $\endgroup$
    – Karl
    Jan 4 at 16:51
  • $\begingroup$ Not only it is not required, but according to the current science, the Hawking radiation does not exits. It has been neither predicted by any confirmed theory nor observed by experiment. It also contradicts fundamental laws of physics through the Information Paradox. $\endgroup$
    – safesphere
    Jan 5 at 10:37

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The problem is that we do not know many microscopic primordial black holes were formed. Indeed we don't know if any primordial black holes were formed. We know too little about the early evolution of the universe to be able to say anything definite.

We can make assumptions about processes that might have occurred, then construct a mathematical model based on these assumptions and see whether the model predicts primordial black holes. But the result is only as good as the assumptions that went into it, and right now we can only make assumptions as we have no hard data.

Right now we have not observed any primordial microscopic black holes so we can only say that either a small density of them was formed or that an evaporation process has destroyed them, but we don't know which is true.

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  • $\begingroup$ I would also add that we don't know much about how quantum gravity works, and quantum gravity's going to strongly shape the both the putative "microscopic" primordial black holes and the early universe. $\endgroup$ Jan 4 at 22:37
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The high-density environment of the early universe is not intrinsically more likely to produce primordial black holes than is the low-density environment today. This is because black hole formation is not connected to density. It is connected to the depth of the gravitational potential.

More precisely, at the density of the universe, the size of a black-hole-forming region is always the same as the size of the cosmological horizon, which grows over time. Black holes form from regions of above-average density. However, variations in the density cannot evolve while they are larger than the horizon, because they are not in causal contact; and after they are smaller than the horizon, it is too late to make a black hole. Thus, cosmological black hole formation is linked to "primordial" density variations set as initial conditions or by something like inflation. In particular, the primordial density contrast must be of order 1 (so the density of the black-hole-forming region is around twice the cosmological average).

We know the amplitudes of primordial density variations at large scales. We can see them directly in the cosmic microwave background. The fractional density contrasts are around one part in $10^4$ to $10^5$, far too weak to make black holes. However, the horizon is smaller in the early universe, so black hole formation in the early universe is linked to the amplitudes of density variations at much smaller scales than we can observe. It is possible that density variations at small scales are really extreme, and then primordial black hole formation is expected. But this is by no means automatic or even a natural thing to expect.

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You may find the answer in this summary of a new cosmological model, posted at the CERN preprint server Zenodo: https://zenodo.org/records/7226521#.ZHbS1XbMKHv

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