### Why did the big bang not just produce a big black hole?

• 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?

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.

• 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.

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.

You say everything was not contracted to a single point. Again, flies in the face of everything I've ever been told, but even assuming the universe was once the size of a grapefruit (they say it was), that should be enough to produce a black hole. If you cram the sun down to grapefruit size that would produce one, so why not the whole universe?

@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.

@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.

@zibadawatimmy, if "blah" requires assumptions which are not satisfied at big bang, then when does the big bang end, and the "non big bang" begin? If you answer that, I can rephrase my question.

@yadda Cosmologists don't even all define the big bang the same way. For some it is the end of inflation, others that it is the entirety of inflation, yet others say it comes before (at scale factor zero). In either case the end of inflation is a clear borderline. You seem to want the latter, in which case the big bang is instant and ends when it begins. It happens everywhere, then what will expand into our visible universe achieves thermal equilibrium, and then inflation hits.

On further reflection, concentrating on the implications of uniformity as you've started to do here is probably the best way to address this.

@zibadawatimmy, all I hear is "blah blah" and my eyes glaze over. Just throw me a time when it ends, i.e. "after 10 seconds" or "after inflation" or "after universe has achieved volume of moon" etc. If you say nobody agrees, then throw a common figure out, or throw out something for the sake of argument.

@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.

yadda - you are asking for info that isn't really relevant. I think you need to understand some of the underlying physics first, so following zibadawa's links will be useful for you.

To be fair the three quotes answer the question in a plainly self-referrential way. You cannot prove something by positing the thing. Flatness of the universe has no bearing on the event horizon existence. If manufacturing a flat region defeated the confinement, a carefully arranged BH collision could be made to provide an escape route. But it does not. Same goes for the outward motion, it makes no difference. The only requirement for a black hole to form is mass enclosed in a small enough radius. If it is, BH appear whether you move, stand or get it surrounded by angry flying bees.

@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.

I wonder if you're overestimating the hairiness of BH formation @zibadawatimmy — mass creates gravity, gravity creates event horizon. At which point can you spot them 'many more reasons' in this process? If you've got a sphere radius R with mass X, it will have a well-defined gravity pull. Whether it collapses or not is defined by physical forces, momentum and such. Stan's key point is that universe beyond my light speed horizon defines events in my locality, which is at odds with modern physics.

• 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.

You said nothing demarcates a black hole's event horizon - is there a possibility that the observable universe is actually black hole and the 'big bang' was its creation?

@TracyCramer nothing *local*, as in there is no experiment over small extent in space and small duration in time that will detect anything special there. Some closed Big Bang cosmologies can be thought of as the interiors of black holes, since one can 'glue' them together to a black hole exterior, but not in general. There'd be no evidence for any such thing anyway.

Most if not all of what you said flies in the face of everything I've ever read or been told. Not determined by a particular density? Sure they are - packing enough mass into a small enough region is supposed to produce one. So all the mass in the universe packed into something the size of a grapefruit or the moon should produce one. I don't have space to rebut every point you've made but suffice to say it flies in the face of everything I've ever learned.

@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.

I think you are assuming the black hole starts at the event horizon, and calculating density from that. I'm not doing that, and I don't care about the event horizon. I'm saying if you pack enough mass into a small enough space you are supposed to get a black hole. So I want to know why "all matter/energy in universe" packed into "grapefruit/moon/whatever" volume doesn't produce one. I don't care where the event horizon ends up being after this is done.

@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.

Well if "whatever" is violated at big bang conditions, when does the big bang stop, and "normal" conditions start, either in seconds, or size of universe, after start of big bang? For example if you say "big bang stops at 10 seconds in" I'll ask why the mass/volume at that juncture doesn't produce a black hole. Unless the universe is pretty big by that point, it seems we would have a black hole. Might want to answer this in an actual "answer" as these comments aren't a good format for back and forth.

@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.

@stan liou, I'm talking about why didn't a black hole form given X conditions. Clearly, an event horizon has nothing to do with why it did or didn't form, it only has to do with after the thing has formed.

@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.

@Stan Liou, 1) Event horizons don't determine whether black holes form, black holes determine whether event horizons form. 2) You don't understand my question, and you are not helpful, so I'll wait for someone else to post - thanks.

@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.

If you fall into a BH, you can clearly notice when you cross event horizon: at that point signals from the part of your body that has already crossed the horizon will not reach your brain anymore.

@OlegMihailik Not quite that simple. Your statement would be correct if you're hovering just above the horizon (using rocket power or whatever), so that if you reach out with any part of your body, it will be cut off. But that case is where you're accelerated by the thrust keeping you hovering, so you can equivalently say it was torn away by the thrust (since the force necessary to keep stationary diverges to infinity as one gets closer). ... But if you're in a local inertial frame, i.e. freefalling and small compared to the BH, you can't detect the horizon.

Wrong. Event horizon **is** that simple: you cannot cross it back full stop. A foot of free-falling man crossing EH would not be able to send signals to the head that's still outside. Unless you're claiming to have invented a plain simple escape route of ants crawling on a surface of infalling body :-)

@OlegMihailik It *is* simple, but you're just mistaken. The foot will be able to communicate with the rest of the body of a free-falling man because the rest of the body will shortly be crossing the horizon--otherwise, the man would not be free-falling. ... I suggest you look the trajectories of two infalling point-particles at a Penrose diagram of Schwarzschild spacetime; it becomes very obvious that the one that falls first can still send signals the one following it, precisely because the one following it falls in rather than staying out of the black hole.

The head is following the feet at a distance, which as short as it may well be, takes time for a signal to cover. Assume a body 2 light second long, feet sent a signal 1 second into the BH. The light will take at least 1 second to reach the event horizon. The head will take **more** thsn 1 second to reach the horizon. That means the signal will be interrupted or delayed — which is detectable, thus EH is very much noticeable.

@OlegMihailik That there is no *interruption* is dead obvious from the Penrose diagram. There's also no delay except in as much as the tidal forces stretch the body--but the tidal forces can be arbitrarily low if the black hole is very large compared to the body. Either the tidal forces are negligible, in which case there is no delay, or they're not, in which case there is nothing special about the horizon because a similar delay will occur at everywhere else the tidal forces are significant. ... I suggest you open a new question regarding this or related topics.

• 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.

• 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.

I don't think the scientists would say this. They say this.

At least, thats what I understood from GUT

• 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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