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Questions I've often wondered about:

  1. If all the matter and energy were concentrated at a single point at the big bang, why wasn't that a black hole, or why didn't it form one?

  2. If the reason #1 above didn't form a black hole is one of several explanations such as inflation or whatever else, then why didn't all the mass and energy form a big black hole at some finite time after this big bang happened? For instance, I've (possibly incorrectly) heard that inflation made the universe the size of an orange. Well why didn't it form a black hole then? Or once the universe expanded to, say, the size of the moon. Why not then? Just insert whatever reasonable size you want in place of "orange" or "moon." The question is why didn't a black hole form out of all the matter and energy after the Big Bang?

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    $\begingroup$ Yadda - your comments on the two excellent answers below show that there are some fundamentals that you need to understand. Without them you won't be able to understand why this question doesn't really make sense. I would suggest studying behaviour of GR models of spacetime approaching that singularity at time = 0 to see why your assumptions are flawed. $\endgroup$
    – Rory Alsop
    Commented Nov 5, 2014 at 10:33
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    $\begingroup$ Also read astronomy.stackexchange.com/questions/6305 $\endgroup$
    – Rory Alsop
    Commented Nov 5, 2014 at 10:34

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Your problem essentially arises from trying to apply Schwarzschild black hole logic, the assumptions of which are pretty much maximally violated at the big bang.

The following were true at the big bang, and violate the usual black hole formation logic.

The event occurred everywhere in space, not actually a point. In particular, the energy was uniformly distributed everywhere. The net gravitational potential was therefore near zero, and there was no one point to which everything could collapse. Furthermore, since stuff was everywhere, there was no expanse of vacuum (in a flat spacetime, no less) outside the collapsing region. And furthermore, things were moving rapidly, were in a highly excited state, and were not in thermal equilibrium (until inflation hit, and then things were too diluted and causally disconnected to collapse en masse).

As for what we could describe the universe as at time $t=0$, we have no idea. General relativity has a spacelike singularity there, and it subsequently cannot say anything beyond that. It is expected that general relativity is not a correct description of spacetime in the pre-inflation era, in large part because quantum mechanics introduces significant effects in those conditions, and it is well known that the two theories are incompatible.

Edit:

This question has been asked on the physics SE many times.

https://physics.stackexchange.com/q/20394/55483

https://physics.stackexchange.com/q/3294/55483

https://physics.stackexchange.com/q/26435/55483

Perhaps the answers there will be illuminating.

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    $\begingroup$ Both black hole and big bang singularities are spacelike. Though if you insist on applying Newtonian concepts here, then Newton's shell theorem means that everything can collapse around every point, which is morally similar to the big bang in reverse anyway. Of course there's no problem with a 'Newtonianized' big bang because they never form black holes, so I'm not sure whether that addresses the question, but perhaps that could be developed more. $\endgroup$
    – Stan Liou
    Commented Nov 4, 2014 at 0:18
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    $\begingroup$ @yadda Those things are simplifications, which are inevitably misleading. The scale factor goes to zero at $t=0$, but the universe remains infinite in expanse at all other times (unless we assume it is finite, but this is an uncommon assumption to me). Density goes to infinity everywhere, not at a single point. And the whole "really dense makes a black hole" thing is an argument that requires assumptions which are not satisfied at the big bang. $\endgroup$
    – zibadawa timmy
    Commented Nov 4, 2014 at 2:29
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    $\begingroup$ @yadda if you consider the Sun in isolation, or generally the gravitational field on any isolated, spherically symmetric body, then it is described by the Schwarzschild spacetime. If you then somehow shrink it, then obviously you get a Schwarzschild black hole. So what you've learned is true but is highly specific to a particular context, and your mistake is in generalizing from wrong properties of that context. $\endgroup$
    – Stan Liou
    Commented Nov 4, 2014 at 3:43
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    $\begingroup$ @yadda I have added some links to the physics SE, where this question has been asked and answered many times. In short, there are two possibilities: either it works as you want because things are nice enough for it, or they don't because they aren't. In early cosmology we are in the latter case, and no "specific time" is ever going to change this dichotomy. Advance time enough and black holes will form in the way you expect in the small scale ways we are accustomed to. Earlier than that and conditions aren't right. It's like expecting ice cubes in the center of the sun. $\endgroup$
    – zibadawa timmy
    Commented Nov 4, 2014 at 19:14
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    $\begingroup$ @OlegMihailik That's false. There are many more requirements than simply "enough mass within a radius". That is exactly the argument for the formation of a Schwarzchild black hole. It's pretty much correct in the universe as it is today, but it's just not applicable soon after the big bang. GR has a lot more in it than just mass and radii. See also Stan's answer. $\endgroup$
    – zibadawa timmy
    Commented Aug 15, 2015 at 6:40
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A black hole is a region of spacetime separated by an event horizon, which means no signals from the interior can propagate outward, no matter how long one waits. Locally, there is nothing special about the event horizon; if you fall in a black hole, there is nothing marking that you've crossed and no local experiment (short in space and duration) that will tell you that you are already doomed. The most important conceptual observation here is that a 'black hole' means is defined not by the local conditions, but by the structure of spacetime on a larger scale.

That means that thinking of black holes as essentially determined by some particular density is a mistake. This bears out if you look at the density of a simple case of a Schwarzschild black hole: the larger the black hole is, the less dense it is (though for volume, some caveats apply). There is not magical 'density point' for black holes; whether something does or doesn't form a black hole is determined by global conditions of spacetime.

EDIT: @zibadawa timmy's point regarding uniformity is very relevant. Since all points in space are equivalent, there is no special point around which an absolute event horizon could form to enclose it in an observer-independent manner, and thus no black hole. This is the most important difference in which the large-scale structure spacetime in Big Bang solutions is very different from stellar collapse scenarios.

1) If all the matter and energy were concentrated at a single point at the big bang, why wasn't that a black hole, or why didn't it form one?

Matter and energy wasn't necessary concentrated at a single point. There only Big Bang cosmologies for which that is even a workable analogy is those involving a closed universe, which is definitely not all of them. But that's a separate misconception.

But as far as we know, the local density at every point did diverge to infinity in the finite past. So it still makes sense to ask why didn't that cause the formation of a black hole. But the answer to that is simple: it didn't because there is no reason for it to do so, as the magnitude of the local density is not relevant.

The question is why didn't a black hole form out of all the matter and energy after the big bang?

We don't need a special mechanism for preventing it because there's no general reason for it do become a black hole in the first place.

I qualify with 'general' here because there is a sense in which a closed universe cosmology is already like the interior of a black hole, and the universe as a whole could even recollapse as a Big Crunch, mimicking the more ordinary kind of stellar collapse into a black hole. The Big Crunch is empirically ruled out by the discovery that the cosmological expansion is accelerating, though.

Thus, again, whether or not it forms (something like) a black hole depends on the large-scale structure of spacetime, not however large or small the local density becomes.

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    $\begingroup$ @yadda I suspect most of the problem lies in not what you've been read, but in your personal interpretation of what you've read. I have provided my reasons, but you can also check wikipedia on black holes for sentences such as this: "the average density of a $10^8$ solar mass black hole is comparable to that of water", which is correct, entirely trivial to check for anyone, and even sourced for the paranoid. This illustrates the fact that one needs to consider to the structure of spacetime on a larger scale to see whether or not a black hole forms, and not just look at a raw density number. $\endgroup$
    – Stan Liou
    Commented Nov 4, 2014 at 2:49
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    $\begingroup$ @yadda As I've said (and Stan said to you in a comment on my answer), the "pack enough mass/energy into a small enough space..." argument is only valid for Schwarzschild black holes. This assumes you have a mass density gravitating towards a point within a larger (approximately) flat spacetime which is in (approximately) the vacuum state. This is extremely violated at big bang conditions. Singularities in GR do not all have a single characterization. We can only meaningfully describe a few cases out of the many conceivable, and each is meaningfully different from the last. $\endgroup$
    – zibadawa timmy
    Commented Nov 4, 2014 at 13:36
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    $\begingroup$ @yadda As I said in the first sentence of this answer, a black hole is a region enclosed by an event horizon. That's what the term 'black hole' means. If you're "not doing that" and you "don't care about the event horizon", then you're not even talking about black holes at all. It's that simple. $\endgroup$
    – Stan Liou
    Commented Nov 4, 2014 at 15:20
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    $\begingroup$ @yadda An event horizon has everything to do with it because its presence or absence directly determines whether or not the black hole formed, thus directly answering your question. "Black hole forms" means "event horizon forms", because that is the presence of the event horizon is the defining property of black holes. Your insistence that it's irrelevant is a bit like asking to show why someone is a bachelor while not being allowed to refer to their marriage status in any way. It's a completely ludicrous condition because it's part of the essential meaning of the terms we're discussing. $\endgroup$
    – Stan Liou
    Commented Nov 4, 2014 at 16:37
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    $\begingroup$ @yadda 1) neither one physically determines the other; they literally mean the same thing, cf. first page of first sentence wikipedia, or subsequent explanation or subsequent elaborations, such as "defining feature of a black hole is the appearance of an event horizon" 2) ok. $\endgroup$
    – Stan Liou
    Commented Nov 4, 2014 at 16:59
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What scientists says, is that mass didn't exist in the first time it was pure energy, and inflation happened at very high speeds (more than 50 times the speed of light) so even when particles and mass appeared (less than 1 in billion energy was converted to mass, matter and anti matter following: E = mc^2) there was a very high expansion velocity, such that Hydrogen and Helium formed in the rates (75% H, 25% He, and very little amount of Li) but no heavier elements says the scientists, densities rapidly and uniformly decreased (in terms of minutes due to expansion speeds), on the other hand a Blackhole requires very high mass densities to form.

So it happened that initial conditions were very different from a big star/supernova and lead to different results.

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  • $\begingroup$ I don't think the scientists would say this. They say this. $\endgroup$
    – peterh
    Commented Feb 17, 2018 at 21:44
  • $\begingroup$ At least, thats what I understood from GUT $\endgroup$
    – AMA1123
    Commented Feb 19, 2018 at 8:28
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A black hole being created by, say, the collapse of a star has a void on one side and matter moving in one direction (towards the center) on the other getting denser as time progressed.

The Big Bang represents almost the exact opposite - all matter was surrounded by equal amount of equally dense matter and all matter was moving away from each other. In such a uniform universe there is nothing to cause a singular collapse.

Much later, when the density and expansion rate were lower, there is a possibility of random motion gathering enough mass to create a black hole ... however, in this case you are probably talking about billions of black holes - which by now would be innumerable very large black holes.

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Who said it didn't produce a big black hole?

We may very well live inside a big black hole. If you apply the hypothesized mass of our Universe to the Schwarzschild radius equation, the resulting radius will not be too far (in order of magnitude) to the observable radius of the visible Universe. Indeed, what we call "Big Bang" may simply be the formation of our "black-hole-universe" from a previous star in another Universe (thus the theory of a "Multiverse"). That explains at least why our Universe is finite, but light or matter apparently can't escape it.

This was first proposed at least 45 years ago (here), I don't know why isn't it more popular, since it's so fascinating. (If you live in a poor country -- I believe science should be universal, and not just for the rich -- I suggest using sci-hub, like here.)

The answers to this question explain the idea with more detail.

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