B 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 > -∞ .

 

[PeterDonis]

In standard coordinates, the Big Bang would be asymptotically minus infinity in a de sitter universe.

But that is not part of the universe; there is no event or spacelike hypersurface at "asymptotically minus infinity".

It is true that the initial singularity in non-inflation FRW models is also not part of the universe, but it does have the important property of geodesic incompleteness; timelike and null godesics cannot be extended into the past at or beyond a finite value of their affine parameter. That is what justifies the usual interpretation of the initial singularity as "the beginning of the universe" in these idealized models, and that in turn motivates the use of the term "Big Bang" to refer to it. de Sitter spacetime does not have this property, so I don't think "asymptotically minus infinity" is a reasonable referent for the term "Big Bang" in such a spacetime.

In other coordinates you could make it whatever finite value you wanted

But that finite value would not label part of the actual spacetime; it would label some boundary that is only present in an extension, as the boundaries in Penrose coordinates do.

In any case, I am not concerned with coordinate choices or coordinate-dependent properties; I am assuming that any reasonable interpretation of the term "Big Bang" will refer to something that is picked out by physics, not just a coordinate choice.

 

[PAllen]

But that is not part of the universe; there is no event or spacelike hypersurface at "asymptotically minus infinity".

It is true that the initial singularity in non-inflation FRW models is also not part of the universe, but it does have the important property of geodesic incompleteness; timelike and null godesics cannot be extended into the past at or beyond a finite value of their affine parameter. That is what justifies the usual interpretation of the initial singularity as "the beginning of the universe" in these idealized models, and that in turn motivates the use of the term "Big Bang" to refer to it. de Sitter spacetime does not have this property, so I don't think "asymptotically minus infinity" is a reasonable referent for the term "Big Bang" in such a spacetime.
But that finite value would not label part of the actual spacetime; it would label some boundary that is only present in an extension, as the boundaries in Penrose coordinates do.

In any case, I am not concerned with coordinate choices or coordinate-dependent properties; I am assuming that any reasonable interpretation of the term "Big Bang" will refer to something that is picked out by physics, not just a coordinate choice.

And I have said several times that I don’t view the big bang as being an event or specific hypersurface. I don’t disagree with what you said, I just don’t find it relevant to my definition.

 

[PeterDonis]

I don’t disagree with what you said, I just don’t find it relevant to my definition.

Fair enough. At this point we've given the OP more than enough to chew on, anyway.

 

[timmdeeg]

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.

Ok and as I understand you most cosmologists would agree with that.

We have some indirect evidence that inflation happened but it seems not possible to talk about a "state" of the universe before inflation in terms of physical properties like density, temperature or the like. I would still prefer to identify the Big Bang with the very "begin" (to replace $t = 0$) of the universe but from the above this has caveats. Thanks for your explanations.

 

[PeterDonis]

We have some indirect evidence that inflation happened but it seems not possible to talk about a "state" of the universe before inflation in terms of physical properties like density, temperature or the like.

In the "eternal" inflation models, which seem to me to be the ones mostly favored now, there is no "before" inflation; inflation extends indefinitely into the past ("asymptotically to minus infinity" as @PAllen put it). The state of the universe in any inflation region of spacetime is pretty boring: the inflaton field is in its "false vacuum" state (which is what causes inflation), and nothing else is present.

 

[timmdeeg]

In the "eternal" inflation models, which seem to me to be the ones mostly favored now, there is no "before" inflation; inflation extends indefinitely into the past ("asymptotically to minus infinity" as @PAllen put it).

But you don't say the de Sitter like state of that patch of the universe which created our bubble "extends indefinitely into the past", right? I have a problem with "inflation extends indefinitely ..." because as I understand it inflation means exponential expansion which was extremely short.

Well after all I tend to see the Big Bang after inflation which also fits to the solution of the horizon and flatness problem.

 

[PeterDonis]

you don't say the de Sitter like state of that patch of the universe which created our bubble "extends indefinitely into the past", right?

In eternal inflation models, there is only one de Sitter like patch, the patch that is eternally inflating. As I said before, each "bubble" gets created when inflation ends in a small pocket and that pocket experiences a "Big Bang" (in my preferred meaning of the term--a hot, dense, rapidly expanding state with lots of energy in the Standard Model fields, the energy having just come from a "reheating" event in that small pocket when inflation ended there). The de Sitter like patch extends indefinitely into the past.

I have a problem with "inflation extends indefinitely ..." because as I understand it inflation means exponential expansion which was extremely short.

In eternal inflation models, inflation is eternal, not extremely short.

 

[timmdeeg]

In eternal inflation models, there is only one de Sitter like patch, the patch that is eternally inflating. As I said before, each "bubble" gets created when inflation ends in a small pocket and that pocket experiences a "Big Bang" (in my preferred meaning of the term--a hot, dense, rapidly expanding state with lots of energy in the Standard Model fields, the energy having just come from a "reheating" event in that small pocket when inflation ended there). The de Sitter like patch extends indefinitely into the past.

In eternal inflation models, inflation is eternal, not extremely short.

Thanks, very interesting stuff. I want to deepen this a little more and found 3. THE
INFLATIONARY UNIVERSE
This article written by Alan Guth doesn't seem to be pop-science of the sort which isn't recommendable. But if otherwise please give me a hint.

 

[PeterDonis]

This article written by Alan Guth doesn't seem to be pop-science of the sort which isn't recommendable. But if otherwise please give me a hint.

Here's a hint: is it a textbook or a peer-reviewed paper? If not, it isn't recommendable. That doesn't necessarily mean it's incomplete or misleading; it just means that, unless you yourself are an expert in the subject, you can't tell whether the scientist is (most likely unintentionally) saying something that is incomplete or misleading. Textbooks and peer-reviewed papers have other experts reviewing what is said to catch those things.

 

[timmdeeg]

Here's a hint: is it a textbook or a peer-reviewed paper? If not, it isn't recommendable.

So this rule is set in stone and and famous names and formulae don't change that.

That doesn't necessarily mean it's incomplete or misleading; it just means that, unless you yourself are an expert in the subject, you can't tell whether the scientist is (most likely unintentionally) saying something that is incomplete or misleading.

Or unless I ask an expert in the subject. Is this article incomplete or misleading?

 

[PeterDonis]

So this rule is set in stone and and famous names and formulae don't change that.

If you have a particular source that isn't a textbook or a peer-reviewed paper, and you are sure that it is reliable, you can always reference it, and the moderators can always weigh in if they need to.

But if you have to ask whether a particular source is reliable, that means you don't already know that it is. Then the rule I gave is the best rule I can give you.

Or unless I ask an expert in the subject.

Sure, but then be prepared to be told that no experts have the time to take a detailed look. None of us get paid for this.

Is this article incomplete or misleading?

I have no idea since I haven't had time to read it. If I have time I will take a look and give any feedback that I think is relevant.

 

[PAllen]

After look at this article, I would describe thus:

It is a presentation of Alan Guth‘s own views on cosmology circa 1997 at the reader level and quality of a better than average Scientific American article. Good pop sci by a highly qualified author, but not remotely a professional article. If Guth taught an undergraduate course on this material for astronomy majors, the level would be far more precise and rigorous than this, let alone in one of his many papers.

 

[Brad Watson - Miami]

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

That's false. The BIG Bang(-Bit Bang) is our best model for the beginning of the Universe 13.8 billion years ago. Our Universe is clearly not infinite in space or eternal in time in our past. Perhaps you mean perpetual.

 

[Nugatory]

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

That's false. The BIG Bang(-Bit Bang) is our best model for the beginning of the Universe 13.8 billion years ago. Our Universe is clearly not infinite in space or eternal in time in our past. Perhaps you mean perpetual.

You may be misunderstanding Ibix’s point. Our best models say that the universe is infinite in spatial extent, and always (that is, at the time of and ever since the Big Bang) has been.

 

[Ibix]

That's false. The BIG Bang(-Bit Bang) is our best model for the beginning of the Universe 13.8 billion years ago. Our Universe is clearly not infinite in space or eternal in time in our past. Perhaps you mean perpetual.

As Nugatory has already noted, you appear to be confused about what I was claiming. The universe, according to our current models, is infinite in spatial extent. Exactly what happened 13.9bn years ago is still a topic of research. Naive models can be read as suggesting that the universe started then, but I believe that opinions among more sophisticated models differ on what went on. But whether the universe has a finite or infinite past, if it is currently infinite in spatial extent then it can never have been finite in extent.