Why The Big Bang banged and didn’t collapsed into a black hole?

[Moon Shine]

TL;DR Summary

The Big Band question. Why don’t we live in a black hole? Or do we?

Summary: The Big Band question. Why don’t we live in a black hole? Or do we?

That actually might be a stupid question... so don’t hesitate to tell me that.

If all mass of our universe was squeezed into something incredible small (compared the universe size) like maybe a tennis ball or maybe even small. It means there was enormously huge density. If my assuming is correct then why this small ”ball” did collapse into a black hole and become singularity? Here we see a lot of examples when much less density (again, compared with the density of the whole universe squeezed into a small ball) was created a black hol. Why our universe didn’t become a black hole?

Thank you in advance for any answer:)

 

[Ibix]

If all mass of our universe was squeezed into something incredible small (compared the universe size) like maybe a tennis ball or maybe even small

All the matter in the observable universe, yes. Our current best models say that the universe is infinite in spatial extent, in which case it was always infinite.

If my assuming is correct then why this small ”ball” did collapse into a black hole and become singularity? Here we see a lot of examples when much less density (again, compared with the density of the whole universe squeezed into a small ball) was created a black hol. Why our universe didn’t become a black hole?

A black hole is formed from a dense chunk of matter surrounded by (near) vacuum. This is a very different situation from more or less uniform high density. The latter can't really collapse towards a point - which point would it collapse towards? There's nothing to choose between any of them, whereas there's a centre to a star.

Furthermore, matter in the early universe was rapidly expanding, so it would have to be stopped before it could collapse.

 

[PAllen]

For more on big band theory, you can start here:
https://en.wikipedia.org/wiki/Big_band

 

[Moon Shine]

A black hole is formed from a dense chunk of matter surrounded by (near) vacuum. This is a very different situation from more or less uniform high density.

Thank you for your answer.
Although... could you please tell me... Was there no vacuum in the universe before The Big Bang?

 

[Moon Shine]

For more on big band theory, you can start here:
https://en.wikipedia.org/wiki/Big_band

Thank you, it was helpful :)

 

[Ibix]

Thank you for your answer.
Although... could you please tell me... Was there no vacuum in the universe before The Big Bang?

No. It was full of high density matter (and more exotic stuff early on). It didn't even become transparent until a few hundred thousand years had passed. You don't get sufficient inhomogeneity for the (now normal) stars floating around in empty space for quite a while after that - a couple of hundred million years, if memory serves.

To be fair, some theories do predict microscopic "primordial black holes" that form from random density fluctuations. They evaporate very quickly. As far as I am aware, the search for the signatures of such things is ongoing.

 

[Ibix]

For more on big band theory,

It's related to the rubber sheet model.

 

[Moon Shine]

It's related to the rubber sheet model.

Was that hight density matter also changing time?

 

[Ibix]

How would you tell? You can't (even in principle) put a clock at the big bang and a clock now and compare their tick rates. There isn't really any physical meaning to the question.

 

[Moon Shine]

How would you tell? You can't (even in principle) put a clock at the big bang and a clock now and compare their tick rates. There isn't really any physical meaning to the question.

I meant more like wrinkles in space-time

 

[Ibix]

I think you may be taking "fabric of spacetime" a bit too literally.

Clocks close to a primordial black hole would tick slowly compared to those further away, as with any black hole (caveat: general relativity may not be correct for very small black holes, so there's a big assumption in my claim). But uniform homogeneous matter yields a uniform homogeneous spacetime (minor irregularities like microscopic black holes aside).

Again, I believe some models predict primordial gravitational waves from small inhomogeneities, which might be (poetically and not particularly precisely) referred to as ripples in spacetime. I don't believe our current generation of detectors is capable of detecting them even if they exist..

 

[Moon Shine]

You’re right actually. Among all the “cosmos things“ spacetime fabric is something that I don’t understand well enough. My mind just can’t connect space and time into one thing. So yes, I do have problems with understanding this.
Thank you again for the explanations and for wasting your time on someone like me:) Your answers we’re really interesting and usefu!

 

[PeroK]

My mind just can’t connect space and time into one thing.

What about a railway timetable, as a spacetime diagram?

 

[Moon Shine]

What about a railway timetable, as a spacetime diagram?

View attachment 253222

Hmmm...still not quite there. For example... I’m traveling for point A to point B (which are 5 light years away from each other). But if I travel faster than light or slower than light it means the journey would take less/more time. So what’s the point of the time in space time fabric at all, if time always depends on my speed? (I do know I’m not the most smartest person in the world. But that’s exactly why I’m here. So please try not to laugh :) ).

 

[PeroK]

Hmmm...still not quite there. For example... I’m traveling for point A to point B (which are 5 light years away from each other). But if I travel faster than light or slower than light it means the journey would take less/more time. So what’s the point of the time in space time fabric at all, if time always depends on my speed? (I do know I’m not the most smartest person in the world. But that’s exactly why I’m here. So please try not to laugh :) ).

Time is important! If I say to you there is a train leaving Central Station, then your next question is "what time does it leave?".

Any "event" must have not only a position in space, but a time.

The insight that the theory of relativity brings is that, more fundamentally than previously thought, space and time are interconnected in four-dimensional spacetime. Whereas, classical physics is more 3+1 dimensional.

Anyway, perhaps starting with some of the basic concepts of SR is better than diving in with questions about black holes and the early universe.

 

[PeterDonis]

Was there no vacuum in the universe before The Big Bang?

According to inflationary models, which appear to be our best current ones, before the Big Bang the universe was filled with an "inflaton" field in a "false vacuum" state. In other words, there was no ordinary matter or radiation present, but there was a very large energy density stored in something like a "vacuum". At the end of inflation, this very large energy density got transformed into ordinary matter and radiation, creating the hot, dense, rapidly expanding state that we call the "Big Bang".

No. It was full of high density matter

No, it wasn't. He asked about before the Big Bang. See above.

 

[timmdeeg]

At the end of inflation, this very large energy density got transformed into ordinary matter and radiation, creating the hot, dense, rapidly expanding state that we call the "Big Bang.”

It seems to be a matter of definition. If the “Big Bang” is identified with the “birth” of our universe it happened before inflation, see the coresponding Wikipedia article.

 

[PeterDonis]

It seems to be a matter of definition.

Yes, and the definition I gave is (a) the one used most often in actual textbooks and peer-reviewed papers, and (b) the only one that makes sense if you ask what happened before the Big Bang, as the post I was responding to does, since if the Big Bang means the "birth" of our universe then by definition there was nothing before it.

see the coresponding Wikipedia article

You've been around here long enough to know that Wikipedia is not a valid source.

 

[Ibix]

No, it wasn't. He asked about before the Big Bang. See above.

Ok - terminology confusion on my part.

 

[timmdeeg]

Yes, and the definition I gave is (a) the one used most often in actual textbooks and peer-reviewed papers, and (b) the only one that makes sense if you ask what happened before the Big Bang, as the post I was responding to does, since if the Big Bang means the "birth" of our universe then by definition there was nothing before it.

You've been around here long enough to know that Wikipedia is not a valid source.

Yes I'm aware of that but am still a bit confused. Other though also not peer-reviewed references are:

Ned Wright What came before the big bang?
The standard Big Bang model is singular at the time of the Big Bang, t = 0. This means that one cannot even define time, since spacetime is singular.

cornell.edu "Ask an Astronomer"
Today, most astronomers believe in the theory of inflation (and there are pieces of evidence supporting this). According to this theory, the Universe underwent exponential expansion about 10-30 seconds after the Big Bang.

I remember various discussion regarding this issue. Now from your post #18 I understand that the textbook definition is the correct one implying that the Big Bang coincides with the reheating epoch. Thanks for clarifying.

 

[PeterDonis]

I'm aware of that but am still a bit confused.

Think about what each of the references you give means by the term "Big Bang", in order for what they are saying to make sense.

 

[PeterDonis]

The standard Big Bang model is singular at the time of the Big Bang, t = 0.

It's worth noting that this is not true of all models. For example, in inflation models, there is nothing that guarantees that an initial singularity is present--the singularity theorems that ground that conclusion for what Wright calls the "standard" model (by which he means a matter or radiation dominated FRW spacetime) do not apply to inflation models because those models violate the energy conditions that are an assumption of the theorems. In the "eternal inflation" models that Wright mentions in passing, there is no singularity anywhere.

 

[PAllen]

Look, terminology has undergone evolution, and there is no universal consensus on terminology. Clearly, as of Hawking and Ellis, 1973, the Big bang model had an initial singularity, and the big bang referred to earliest moments after this. Today, inflation is certainly not universally supported - at least one of its initial leading lights now totally disavows it. Within inflationary models, you can choose to call the Big Bang the start of inflation or the moment after inflation - just terminology not physics. Also, I don’t see anything in the OP asking about before the Big Bang. Maybe that’s how you interpret it, not how I would. I would interpret it as asking about the state shortly after the big bang, and that similar states at present, surrounded by vacuum, would collapse into a black hole. This is a good question which was dealt with well early in this thread.

Anyway, @PeterDonis, i agree the terminology you advocate is the most common in recent years, but I actually prefer the notion that Big Bang starts from the earliest moment within a model causally connected to our observable universe. This means at the start of inflation(for our bubble), even for eternal inflation models.

 

[PeterDonis]

I don’t see anything in the OP asking about before the Big Bang

He didn't ask about that in the OP. He asked about it in post #4, which was the post I originally responded to.

 

[PAllen]

He didn't ask about that in the OP. He asked about it in post #4, which was the post I originally responded to.

Ok, but that just strikes me as the standard misunderstanding of the Big Bang being as an explosion into something (e.g. vacuum), which is perfectly well answered with or without inflation, and this common misunderstanding really has nothing to do with what you define as the beginning of the Big Bang in various models.

 

[PeterDonis]

that just strikes me as the standard misunderstanding of the Big Bang being as an explosion into something (e.g. vacuum)

Yes, that's quite possible.

 

[timmdeeg]

Think about what each of the references you give means by the term "Big Bang", in order for what they are saying to make sense.

My personal preference is the Big Bang at $t = 0$ independent of any model. Which related to the LambdaCDM model means before inflation as stated by the Cornell Astronomer. I thought this is the modern definition which replaced to old pre-inflation definition. But according to your explanations that doesn't seem to be the case.

 

[PeterDonis]

My personal preference is the Big Bang at $t = 0$ independent of any model.

But in models with no initial singularity, there is no natural $t = 0$, so this usage cannot be independent of any model.

Whereas we know that there was a hot, dense, rapidly expanding state that is the earliest one for which we have good evidence; the question is what came before it, inflation or something else. Calling that earliest known state the "Big Bang" is the only use of that term that is model independent.

 

[PeterDonis]

Which related to the LambdaCDM model means before inflation as stated by the Cornell Astronomer.

But the Cornell astronomer is not even mentioning the fact that there are inflation models with no initial singularity. So, as is unfortunately the case very often with pop science articles, he is misleading you by leaving out important factors that could not be left out if he were trying to make similar claims in a peer-reviewed paper.

The astronomer is also failing to mention that the time he quotes for inflation, "about $10^{-30}$ seconds after the Big Bang", is not an actual time in the actual inflation model--since, as I have already noted, there are inflation models that don't have an initial singularity at all. Instead, as far as I can tell, it's a time that basically corresponds to taking the hot, dense, rapidly expanding state that is the earliest one for which we have good evidence, pretending that that state occurs in a standard FRW spacetime without inflation, figuring out at what time after the initial singularity in the standard FRW spacetime without inflation that state would occur and calling that the time of that state, and then extrapolating times backwards from that into the inflation epoch in the actual inflation model.

 

[PeterDonis]

I actually prefer the notion that Big Bang starts from the earliest moment within a model causally connected to our observable universe. This means at the start of inflation(for our bubble), even for eternal inflation models.

I'm not sure I understand. In eternal inflation models, there is no "start of inflation" anywhere. What creates our "bubble" is the end of inflation within that bubble--inflation ending is what "separates" our universe from the rest of the eternally inflating spacetime.

Actually, even in inflation models without eternal inflation, I'm not clear about what "the start of inflation" means. The spacetime geometry during the inflation epoch is basically de Sitter (since the inflaton field in this epoch has the same stress-energy tensor as a cosmological constant), and de Sitter spacetime has no initial singularity.

 

[PAllen]

I'm not sure I understand. In eternal inflation models, there is no "start of inflation" anywhere. What creates our "bubble" is the end of inflation within that bubble--inflation ending is what "separates" our universe from the rest of the eternally inflating spacetime.

Actually, even in inflation models without eternal inflation, I'm not clear about what "the start of inflation" means. The spacetime geometry during the inflation epoch is basically de Sitter (since the inflaton field in this epoch has the same stress-energy tensor as a cosmological constant), and de Sitter spacetime has no initial singularity.

It doesn't matter whether there is a singularity or not. We could even consider Hawking's no boundary universe. In any model (manifold) that includes the current observable universe, there are a set of events causally connected to any part of the currently observable universe. FLRW solution provides a standard time slicing back to the end of inflation. One can then pick some isotropic slicing before then to cover all that is causally connected to our the observable universe. That may mean it is better to think of the beginning as t -> -∞, rather than t=0. So what?

 

[PeterDonis]

We could even consider Hawking's no boundary universe.

And what, in your view, would the term "Big Bang" signify in such a universe?

That may mean it is better to think of the beginning as t -> -∞, rather than t=0. So what?

If the "beginning" is $t \rightarrow - \infty$ (for example, if everything prior to the end of inflation is a de Sitter spacetime region), what, in your view, would the term "Big Bang" signify in such a universe?

 

[PAllen]

And what, in your view, would the term "Big Bang" signify in such a universe?
If the "beginning" is $t \rightarrow - \infty$ (for example, if everything prior to the end of inflation is a de Sitter spacetime region), what, in your view, would the term "Big Bang" signify in such a universe?

The earliest history covered by the model, whatever its coordinate value is in some standard coordinates. For de sitter, negative infinite is the most natural, but of course it is trivial to introduce coordinates that scale this to a finite value (at the cost of hiding some of hiding natural time symmetry).

 

[PeterDonis]

The earliest history covered by the model

I don't understand. Isn't the model supposed to cover everything? Isn't that what we mean when we say the "beginning" is $t \rightarrow - \infty$? Or in the Hawking no boundary case, isn't the model supposed to include the no boundary region?

 

[PAllen]

I don't understand. Isn't the model supposed to cover everything? Isn't that what we mean when we say the "beginning" is $t \rightarrow - \infty$? Or in the Hawking no boundary case, isn't the model supposed to include the no boundary region?

Not sure what our miscommunication is. I am including everything. I said so very explicitly. Earliest doesn’t have to be a particular moment, as I mean it. I literally mean all that is past causally connected to the observable universe. That would exclude other bubbles in an eternal inflation, but that is all. In standard coordinates, the Big Bang would be asymptotically minus infinity in a de sitter universe. In other coordinates you could make it whatever finite value you wanted, i.e. t‘ > 5 would equivalent to t > -∞ .