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Old 12th November 2017, 10:54 AM   #1
HansMustermann
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Big Bang, act 2

So it just dawned upon me that there's a question I should have asked the last time: so... err... why didn't it turn into a black hole anyway?

I'll take the roundabout way here because I'm lazy. Last I heard the mass of the visible universe is somewhere around 6x1052. Or thereabouts. Plus/minus one order of magnitude. The mass of the sun is around 2x1030, so that would be about 3x1022 solar masses. A 1 solar mass black hole would have a Schwarzschild radius of about 3km, so that brings us to approximately 10[sup]23[/km] or about 1010 light years for the Schwarzschild radius of the visible universe. That's about 10 billion light years radius, unless I messed up the mental maths something fierce. (Which wouldn't surprise me.)

We're not even talking about immediately after it blew up. We only need to go back a couple billion years to go back to where it should have been a black hole. Or if that plus/minus goes about 50% on the plus side, it would STILL be a black hole, and we're inside one.

So, what am i missing? If the argument against an explosion in flat space is that it would become a black hole before it can explode, how didn't it turn into a black hole anyway?
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Old 12th November 2017, 11:45 AM   #2
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Originally Posted by HansMustermann View Post
So it just dawned upon me that there's a question I should have asked the last time: so... err... why didn't it turn into a black hole anyway?

I'll take the roundabout way here because I'm lazy. Last I heard the mass of the visible universe is somewhere around 6x1052. Or thereabouts. Plus/minus one order of magnitude. The mass of the sun is around 2x1030, so that would be about 3x1022 solar masses. A 1 solar mass black hole would have a Schwarzschild radius of about 3km, so that brings us to approximately 10[sup]23[/km] or about 1010 light years for the Schwarzschild radius of the visible universe. That's about 10 billion light years radius, unless I messed up the mental maths something fierce. (Which wouldn't surprise me.)

We're not even talking about immediately after it blew up. We only need to go back a couple billion years to go back to where it should have been a black hole. Or if that plus/minus goes about 50% on the plus side, it would STILL be a black hole, and we're inside one.

So, what am i missing? If the argument against an explosion in flat space is that it would become a black hole before it can explode, how didn't it turn into a black hole anyway?
The problem seems to be in considering it like explosion in the first place.

https://en.wikipedia.org/wiki/Initial_singularity

Quote:
The initial singularity was a singularity of infinite density thought to have contained all of the mass and space-time of the Universe[1] before quantum fluctuations caused it to rapidly expand in the Big Bang and subsequent inflation, creating the present-day Universe.
So the argument against an explosion is simply that there was nothing (no space-time) for an explosion to, well, explode into.


Similarly this answers your primary question, why no black hole. As in the big bang it is also space-time that's, well, bang'n.

https://en.wikipedia.org/wiki/Big_Bang

Quote:
Despite being extremely dense at this time—far denser than is usually required to form a black hole—the universe did not re-collapse into a black hole. This may be explained by considering that commonly-used calculations and limits for gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as the Big Bang.

As noted in the singularity link some theories like loop quantum gravity postulate a big bounce rather than a big bang.

https://en.wikipedia.org/wiki/Big_Bounce
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Old 12th November 2017, 02:48 PM   #3
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Nevertheless, that doesn't answer my actual question. The fact that space expands doesn't currently get matter out of black holes, nor for that matter get the Earth farther from the Sun, because basically they're too gravitationally bound. I'm asking why that would work differently for that initial singularity.
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Old 12th November 2017, 03:23 PM   #4
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Originally Posted by HansMustermann View Post
Nevertheless, that doesn't answer my actual question. The fact that space expands doesn't currently get matter out of black holes, nor for that matter get the Earth farther from the Sun, because basically they're too gravitationally bound. I'm asking why that would work differently for that initial singularity.


Same answer as already given...


Originally Posted by The Man View Post
Similarly this answers your primary question, why no black hole. As in the big bang it is also space-time that's, well, bang'n.
Just as thinking of an explosion into existent space-time you're thinking of a singularity and black holes in already existent space-time. For that initial singularity there is no place that is outside of it. The general conditions are significantly different.

Similarly, as already noted, the expansion of space simply has to prevent the formation of a black hole (prevent gravitational recollapse for some time) and has nothing to do with the fact that "space expands doesn't currently get matter out of black holes, nor for that matter get the Earth farther from the Sun". With no black hole forming back then there is no black hole to get out of back then and even now there is no "out of" the universe then or now. Regardless of its current and previous size(s) the universe still encompasses, well, all of the universe. As quoted before "thought to have contained all of the mass and space-time of the Universe".





While the current expansion is not enough separate gravitationally bound states, other levels of expansion have been postulated.

https://en.wikipedia.org/wiki/Big_Rip
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Old 12th November 2017, 05:33 PM   #5
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I get it that it was the whole space. I really do. No need to keep stating the obvious.

But when you're packed within a r=2MG/c2 sphere, it doesn't MATTER if there's any space outside the event horizon. Hell, it doesn't even matter if there's any space outside your current radius. The universe could be ending one planck length from you, or be infinite, or something in between, and it wouldn't matter one bit. Because you're not getting even one planck length away from the singularity without violating causality. Your 45 degree line on the Penrose diagram can only go towards one place, and that place is the singularity.

Basically if the mass of the currently visible universe were packed within a sphere with a radius of 10 billion light years and you were somewhere in there -- and yes, you were, because there wasn't any space for you to be farther than that -- you shouldn't have been able to go outward at all. Even light from there wouldn't be going anywhere except towards the singularity. Matter wouldn't even bounce and crunch back, it could only crunch without violating causality.

The Big Rip, well, that's interesting, but as far as I understand it (which, granted, may not be much) it describes something in the future. The more the universe has expanded, the closer we are to a big rip. The farther back in time you put t0, the more time you have until the big rip, because the density of the universe is higher at that point, and the square root of (1 - density) under the fraction line gets smaller.

Not sure how it would even apply to a universe that's not yet dominated by dark energy, to be honest, which should only be about 5 billion years ago. I mean, even the equation, go back far enough and pack the matter densely enough, and the time to big rip becomes infinite, and further back that square root becomes imaginary. The conditions in that universe can't produce a big rip. The universe has to change to one dominated by dark energy before it can.

It's not clear to me how that would apply to the initial moment. Please explain.
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Old 12th November 2017, 06:05 PM   #6
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Originally Posted by HansMustermann View Post
I get it that it was the whole space. I really do. No need to keep stating the obvious.
I don't think you understand the implications of that, because your very next sentence says:

Originally Posted by HansMustermann View Post
But when you're packed within a r=2MG/c2 sphere, it doesn't MATTER if there's any space outside the event horizon.
The r=2MG/c2 equation comes from one of Schwarzschild's solutions to Einstein's field equations. You're familiar with it because you've seen it in connection with what we now refer to as the event horizon surrounding the central point mass of a black hole, where there is no matter at or beyond that event horizon.

Schwarzschild, however, did not contemplate black holes. He thought of his equation as appropriate for modeling an isolated star. He was right about that, for distances outside the surface of the star, which is much farther than the r of your equation. So you might as well be asking yourself why there is no black hole at the center of ordinary stars; the situation is basically the same.

Schwarzschild also produced a second solution that holds when there is matter at or beyond the r of your equation. That second solution applies to the interior of ordinary stars, where there is no black hole. It would get you started toward a better understanding of the big bang if you thought of that second solution as being more relevant to the big bang...

...but of course Schwarzschild's second solution isn't the appropriate solution either. I'm mentioning it just to encourage you to let go of your determination to confuse yourself by using an inappropriate equation and thinking it implies a black hole.
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Old 12th November 2017, 06:47 PM   #7
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Consider that the mass of the universe was equally distributed through the whole thing. In which direction would it collapse? Gravity wasn't pulling in any specific direction.

For a black hole you need a difference in density to cause a gravitational gradient.
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Old 12th November 2017, 06:51 PM   #8
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@W.D.Clinger
I don't ask myself why there's no black hole at the centre of ordinary stars, because of Birkhoff's theorem. I'm not sure how that applies to the early universe, but I hope someone will explain.

That said, the whole question was the how and why. Telling me that something is not the solution, is, well, useful information, but still leaves me just as clueless as to what IS the solution.

Or to put it otherwise, it's pointless for me to try a game of 20 questions to get to the solution, when I don't know what questions to ask. Might as well just tell me what the asnwer is, and we can all be done with it
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Old 12th November 2017, 06:58 PM   #9
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Originally Posted by phunk View Post
Consider that the mass of the universe was equally distributed through the whole thing. In which direction would it collapse? Gravity wasn't pulling in any specific direction.

For a black hole you need a difference in density to cause a gravitational gradient.
Well, pulling towards each other? I mean, we can calculate if there's enough matter density in the universe right now for it gravity to make it go crunch or not. I would assume that one can do the same maths back when it did have a LOT of density. In fact, infinite density at time zero.

Plus, far as I know, you don't really need a centre of the universe for things to be gravitationally bound. The sun and Earth can still hold onto each other gravitationally, even if the space they're in is expanding. Our local supercluster can still accelerate towards the great attractor, by just gravity, even if the space is expanding and there is no centre of the universe and so on.

I'm not sure what would keep that early ball of photons or the earliest stars from just attracting each other faster than the space was expanding, if they were close enough to each other.

It seems to me like the early universe should have been more gravitationally bound than anything we see today, and the universe wasn't even dominated by dark energy until some 5 billion years ago. Yet things moved apart anyway.

And that confuzzles me. So I'd like someone to explain to me how that happened.
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Old 12th November 2017, 08:25 PM   #10
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I read your first thread with interest, and wondered when you’d be back for more; no more wondering!

Originally Posted by HansMustermann View Post
So it just dawned upon me that there's a question I should have asked the last time: so... err... why didn't it turn into a black hole anyway?
I’m sure someone addressed this before, but you know that GR and QM (shorthands) are mutually inconsistent, at a fundamental level, right?

So in physical regimes where one needs a theory of quantum gravity, there can be no answers today.

Quote:
I'll take the roundabout way here because I'm lazy. Last I heard the mass of the visible universe is somewhere around 6x1052. Or thereabouts. Plus/minus one order of magnitude. The mass of the sun is around 2x1030, so that would be about 3x1022 solar masses. A 1 solar mass black hole would have a Schwarzschild radius of about 3km, so that brings us to approximately 10[sup]23[/km] or about 1010 light years for the Schwarzschild radius of the visible universe. That's about 10 billion light years radius, unless I messed up the mental maths something fierce. (Which wouldn't surprise me.)

We're not even talking about immediately after it blew up. We only need to go back a couple billion years to go back to where it should have been a black hole. Or if that plus/minus goes about 50% on the plus side, it would STILL be a black hole, and we're inside one.
I don’t get what you’re saying here, I really don’t. For starters, you need to be much clearer with terms like “radius” ...distances in GR are not like the Newtonian ones your intuition is comfortable with. And then there’s what “visible universe” is ...

Quote:
So, what am i missing? If the argument against an explosion in flat space is that it would become a black hole before it can explode, how didn't it turn into a black hole anyway?
Apart from the above, the BBT is not about “an explosion in flat space”!
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Old 12th November 2017, 08:38 PM   #11
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Originally Posted by HansMustermann View Post
Nevertheless, that doesn't answer my actual question. The fact that space expands doesn't currently get matter out of black holes, nor for that matter get the Earth farther from the Sun, because basically they're too gravitationally bound. I'm asking why that would work differently for that initial singularity.
In the first discussion, I don’t think anyone tried to set you straight about this, space expanding thing.

Assume GR rules, OK?

“Spacetime tells matter how to move, matter tells spacetime how to curve.” - John Wheeler.

The space expanding idea comes from a spherical cow ... the assumption that the universe is homogeneous and isotropic. Clearly, the universe is not, especially on small scales (say, < 100 Mpc).

So what does space do on scales where the cow is not spherical?

The answer isn’t accessible using Newtonian intuition
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Old 12th November 2017, 08:45 PM   #12
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Originally Posted by HansMustermann View Post
Well, pulling towards each other? I mean, we can calculate if there's enough matter density in the universe right now for it gravity to make it go crunch or not. I would assume that one can do the same maths back when it did have a LOT of density. In fact, infinite density at time zero.

Plus, far as I know, you don't really need a centre of the universe for things to be gravitationally bound. The sun and Earth can still hold onto each other gravitationally, even if the space they're in is expanding. Our local supercluster can still accelerate towards the great attractor, by just gravity, even if the space is expanding and there is no centre of the universe and so on.

I'm not sure what would keep that early ball of photons or the earliest stars from just attracting each other faster than the space was expanding, if they were close enough to each other.

It seems to me like the early universe should have been more gravitationally bound than anything we see today, and the universe wasn't even dominated by dark energy until some 5 billion years ago. Yet things moved apart anyway.

And that confuzzles me. So I'd like someone to explain to me how that happened.
Words, words, words ...

Assume GR rules, OK?

Write down some equations, guess some values (well, see what astronomers have to say), plug them into the equations, turn the handle ... and the rate of expansion as a function of time is ...
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Old 12th November 2017, 09:34 PM   #13
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Does this make sense?:
Immediately after the BB space is expanding and all of the matter/energy is moving away from all the other matter/energy, expanding in every direction along with space itself.

To form a black hole, at least some of this matter/energy would have to be converging rather than expanding, wouldn't it?
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Old 12th November 2017, 11:13 PM   #14
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Originally Posted by phunk View Post
Consider that the mass of the universe was equally distributed through the whole thing. In which direction would it collapse? Gravity wasn't pulling in any specific direction.
OK. That is really interesting. It's way beyond my skill level.

I think what you are saying is : If the initial singularity didn't have specific directions and created its own space...then where was the lower gravity compared to anywhere else. ( I think that's what you are saying)

I'm now confused if there could be a big universal crunch, at all, in the future and how this would work.

Even worse, if there was a big crunch, in theory, would the same sort of singularity arise?
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Old 13th November 2017, 12:07 AM   #15
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Originally Posted by JeanTate View Post
Apart from the above, the BBT is not about “an explosion in flat space”!
Never said it was. That was a reference to a previous discussion.
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Old 13th November 2017, 12:14 AM   #16
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Originally Posted by Brainache View Post
Does this make sense?:
Immediately after the BB space is expanding and all of the matter/energy is moving away from all the other matter/energy, expanding in every direction along with space itself.

To form a black hole, at least some of this matter/energy would have to be converging rather than expanding, wouldn't it?
It doesn't really have to form, as such. All the matter (well, energy at that point, but that obeys gravity too) was already in that initial singularity. There's no place for it to fall further into.

Basically for all intents and purposes, the universe was already a black hole.

And there's this thing about moving inside a black hole. The space is curved. No matter in which direction you're moving, without exceeding c, all directions lead to the singularity. That's what I meant by that 45 degree line on a Penrose diagram. You can fire a photon in whatever direction you wish, its path ultimately still leads to the singularity.
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Old 13th November 2017, 01:12 AM   #17
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Originally Posted by JeanTate View Post
Words, words, words ...

Assume GR rules, OK?
Trust in GR is not an issue. I wouldn't be asking it for an answer, if i thought it had none.

I mean, I suppose some people do play "Secret Socrates", as in ask the same thing until hopefully everyone agrees with them, but it's not my style. I'm way too arrogant for that. If I think I know better, I say so.

Originally Posted by JeanTate View Post
Write down some equations, guess some values (well, see what astronomers have to say), plug them into the equations, turn the handle ... and the rate of expansion as a function of time is ...
Yea, let's try that for a simpler problem. Well, really the same problem, but avoiding the black hole discussion. And indeed let's start from the fact that it wasn't a spherical cow.

The first proto-galaxies, really, clouds of gas capable of forming stars, are estimated to be 100,000 to 1,000,000 solar masses. They also formed as early as 100 million and 250 million years after the big bang, so they were really really close to each other. Some may have been a mere 100 light years from each other.

Take two of them next to each other, and that's a very gravitationally bound system. Indeed, bound enough for most of them to coalesce with each other into what we together call galaxies. But they in turn were still darn near other such gas clouds, and gravitationally bound to them.

So take two of those that didn't merge. How did universe expansion break that kind of a tightly bound system apart, when currently it doesn't break us apart from the great attractor?
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Old 13th November 2017, 05:29 AM   #18
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Originally Posted by HansMustermann View Post
That said, the whole question was the how and why. Telling me that something is not the solution, is, well, useful information, but still leaves me just as clueless as to what IS the solution.

Or to put it otherwise, it's pointless for me to try a game of 20 questions to get to the solution, when I don't know what questions to ask. Might as well just tell me what the asnwer is, and we can all be done with it
The FLRW solutions are the relevant solutions you need to understand.

I once had occasion to post the mathematical details of one special case of that solution here in order to rebut one of Farsight's claims. You might find it useful. On the other hand, you might not.

At any rate, you would be better off being confused by a relevant solution to the field equations than being confused by an irrelevant solution, as you are now.
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Old 13th November 2017, 06:34 AM   #19
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Originally Posted by W.D.Clinger View Post
The FLRW solutions are the relevant solutions you need to understand.

I once had occasion to post the mathematical details of one special case of that solution here in order to rebut one of Farsight's claims. You might find it useful. On the other hand, you might not.
Thanks, I'll have a look. Much appreciated.

Originally Posted by W.D.Clinger View Post
At any rate, you would be better off being confused by a relevant solution to the field equations than being confused by an irrelevant solution, as you are now.
Well, indeed. But that's why I was asking that someone point me at the relevant one. I mean, if I already knew what that is, I'd already be confused at that one
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Old 13th November 2017, 06:50 AM   #20
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Originally Posted by HansMustermann View Post
I get it that it was the whole space. I really do. No need to keep stating the obvious.
Evidently there is, as the answer to your question was directly given in the quoted links before.

Originally Posted by HansMustermann View Post
But when you're packed within a r=2MG/c2 sphere, it doesn't MATTER if there's any space outside the event horizon. Hell, it doesn't even matter if there's any space outside your current radius. The universe could be ending one planck length from you, or be infinite, or something in between, and it wouldn't matter one bit. Because you're not getting even one planck length away from the singularity without violating causality. Your 45 degree line on the Penrose diagram can only go towards one place, and that place is the singularity.
"it doesn't MATTER if there's any space outside the event horizon."?! Again there isn't an "event horizon" without space "outside" it. That's what the event horizon is a particular space-time geometry where world lines can enter (thus come in from outside) but not leave. Simply having a horizon doesn't make it an event horizon of a black hole.

https://en.wikipedia.org/wiki/Horizo...ral_relativity)

Originally Posted by HansMustermann View Post


Basically if the mass of the currently visible universe were packed within a sphere with a radius of 10 billion light years and you were somewhere in there -- and yes, you were, because there wasn't any space for you to be farther than that -- you shouldn't have been able to go outward at all. Even light from there wouldn't be going anywhere except towards the singularity. Matter wouldn't even bounce and crunch back, it could only crunch without violating causality.
A singularity that expands ain't a singularity no more. What is "causality" without time?

Originally Posted by HansMustermann View Post
The Big Rip, well, that's interesting, but as far as I understand it (which, granted, may not be much) it describes something in the future. The more the universe has expanded, the closer we are to a big rip. The farther back in time you put t0, the more time you have until the big rip, because the density of the universe is higher at that point, and the square root of (1 - density) under the fraction line gets smaller.

Not sure how it would even apply to a universe that's not yet dominated by dark energy, to be honest, which should only be about 5 billion years ago. I mean, even the equation, go back far enough and pack the matter densely enough, and the time to big rip becomes infinite, and further back that square root becomes imaginary. The conditions in that universe can't produce a big rip. The universe has to change to one dominated by dark energy before it can.

It's not clear to me how that would apply to the initial moment. Please explain.
That wasn't about "initial moment" but your assertion, as specifically referred to, of the current expansion not pulling the Earth and Sun apart.
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Old 13th November 2017, 06:55 AM   #21
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Originally Posted by HansMustermann View Post
It doesn't really have to form, as such. All the matter (well, energy at that point, but that obeys gravity too) was already in that initial singularity. There's no place for it to fall further into.

Basically for all intents and purposes, the universe was already a black hole.
Nope, that a black hole has a singularity (at least theoretically) doesn't make a singularity "for all intents and purposes," a black hole.

Originally Posted by HansMustermann View Post
And there's this thing about moving inside a black hole. The space is curved. No matter in which direction you're moving, without exceeding c, all directions lead to the singularity. That's what I meant by that 45 degree line on a Penrose diagram. You can fire a photon in whatever direction you wish, its path ultimately still leads to the singularity.
Right, that's at least one difference "all directions lead to the singularity" that requires space-time outside the singularity.
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Old 13th November 2017, 07:00 AM   #22
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Originally Posted by HansMustermann View Post
It's not clear to me how that would apply to the initial moment. Please explain.
Coming back to this for a moment, that's one of the problems as mentioned by others already. We don't have a workable quantum gravity theory to say exactly what happens at the initial moment, the Planck epoch. We do however know that we don't apparently find ourselves in a situation where all directions lead to a singularity, so the universe isn't and wasn't a black hole.
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Old 13th November 2017, 08:00 AM   #23
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Hans, I think maybe inflation pushed the universe past the Schwarzschild radius?
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Old 13th November 2017, 08:25 AM   #24
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Originally Posted by HansMustermann View Post
So it just dawned upon me that there's a question I should have asked the last time: so... err... why didn't it turn into a black hole anyway?

I'll take the roundabout way here because I'm lazy. Last I heard the mass of the visible universe is somewhere around 6x1052. Or thereabouts. Plus/minus one order of magnitude. The mass of the sun is around 2x1030, so that would be about 3x1022 solar masses. A 1 solar mass black hole would have a Schwarzschild radius of about 3km, so that brings us to approximately 10[sup]23[/km] or about 1010 light years for the Schwarzschild radius of the visible universe. That's about 10 billion light years radius, unless I messed up the mental maths something fierce. (Which wouldn't surprise me.)

We're not even talking about immediately after it blew up. We only need to go back a couple billion years to go back to where it should have been a black hole. Or if that plus/minus goes about 50% on the plus side, it would STILL be a black hole, and we're inside one.

So, what am i missing? If the argument against an explosion in flat space is that it would become a black hole before it can explode, how didn't it turn into a black hole anyway?
I guess space expansion was too fast, once gravity became a thing, for the thing to collapse back on itself.
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Old 13th November 2017, 08:40 AM   #25
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As I was saying already, just telling me that this ain't the answer or that one ain't the answer, is sorta useful, but wouldn't it take less time to just tell me what the answer is, instead of expecting me to arrive to it by ellimination?
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Old 13th November 2017, 08:54 AM   #26
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From what I can see, the universe was already 10 ly across after one second. That's pretty damned fast for an expansion; perhaps too fast for anything like a black hole to form.
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Old 13th November 2017, 09:41 AM   #27
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Originally Posted by HansMustermann View Post
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As I was saying already, just telling me that this ain't the answer or that one ain't the answer, is sorta useful, but wouldn't it take less time to just tell me what the answer is, instead of expecting me to arrive to it by ellimination?
Again, I quoted it in the my first post. Since I have repeated that several times now, whomever you think is saying "that this ain't the answer or that one ain't the answer," without citing the answer already given and linked certainly ain't me.


Here it is again...

Originally Posted by The Man View Post


Similarly this answers your primary question, why no black hole. As in the big bang it is also space-time that's, well, bang'n.

https://en.wikipedia.org/wiki/Big_Bang

Quote:
Despite being extremely dense at this time—far denser than is usually required to form a black hole—the universe did not re-collapse into a black hole. This may be explained by considering that commonly-used calculations and limits for gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as the Big Bang.
Being quoted, linked and going explicitly to your question of why no black hole, what requires you "to arrive to it by ellimination"?

Having been given that answer, discussing it or debating its validity are other matters altogether than you not having been given that answer.
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Old 13th November 2017, 09:50 AM   #28
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Originally Posted by Cheetah View Post
Hans, I think maybe inflation pushed the universe past the Schwarzschild radius?


Originally Posted by Argumemnon View Post
From what I can see, the universe was already 10 ly across after one second. That's pretty damned fast for an expansion; perhaps too fast for anything like a black hole to form.
As expansion during that period was exponentially grater than the current rate of expansion trying to use the current rate of expansion as exemplary of that period is ineffective. The point I was trying to make to HansMustermann earlier in mentioning other rates of expansion and the big rip as an example of such.
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Old 13th November 2017, 11:29 AM   #29
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Perhaps you will find the answer in this video.

Before the Big Bang 4 : Eternal Inflation & The Multiverse

Quote:
Interviews with Alan Guth & other leading cosmologists going into far more depth than average tv show but still accessible to the interested layperson.
What happened before the big bang? In this series of films we explore competing models of the early universe, with this episode focusing on the dominant paradigm for early universe cosmology: inflation. The father of inflationary theory Alan Guth takes us on a tour of inflationary theory and explains why cosmologists find it so compelling and why it leads to a multiverse. Fellow theorists Anthony Aguirre and Yasunori Nomura help us explore more of the details of the theory and try to answer many of its challenges. George Efstathiou, one of the world’s leading observational cosmologists (who gave the press conference for the European Space Agency’s Planck cosmology results ) helps us assess the empirical status of the theory.


YouTube Video This video is not hosted by the ISF. The ISF can not be held responsible for the suitability or legality of this material. By clicking the link below you agree to view content from an external website.
I AGREE
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Old 13th November 2017, 06:45 PM   #30
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Originally Posted by HansMustermann View Post
Never said it was. That was a reference to a previous discussion.
Fair enough.... but why did you mention it in the first place?
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Old 13th November 2017, 06:51 PM   #31
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Originally Posted by Hercules Rockefeller View Post
Perhaps you will find the answer in this video.

Before the Big Bang 4 : Eternal Inflation & The Multiverse

YouTube Video This video is not hosted by the ISF. The ISF can not be held responsible for the suitability or legality of this material. By clicking the link below you agree to view content from an external website.
I AGREE
Ugh.

Let’s leave non-science (the multiverse) out of this, shall we?

HM’s serious questions have nothing to do with inflation either, so let’s leave that alone too, eh?

For those interested in reading up on why both inflation and the multiverse are, today, not good science, I recommend Sabine Hossenfleder’s blog, BackReAction ... http://backreaction.blogspot.com
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Old 13th November 2017, 07:09 PM   #32
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Originally Posted by HansMustermann View Post
<snip>

Yea, let's try that for a simpler problem. Well, really the same problem, but avoiding the black hole discussion. And indeed let's start from the fact that it wasn't a spherical cow.

The first proto-galaxies, really, clouds of gas capable of forming stars, are estimated to be 100,000 to 1,000,000 solar masses. They also formed as early as 100 million and 250 million years after the big bang, so they were really really close to each other. Some may have been a mere 100 light years from each other.

Take two of them next to each other, and that's a very gravitationally bound system. Indeed, bound enough for most of them to coalesce with each other into what we together call galaxies. But they in turn were still darn near other such gas clouds, and gravitationally bound to them.

So take two of those that didn't merge. How did universe expansion break that kind of a tightly bound system apart, when currently it doesn't break us apart from the great attractor?
First, do you have a source for your numbers? Not that I necessarily think they’re wrong, but I like to be sure we’re both working with reliable inputs.

Second, where does the “break that ... apart” part come from?

Third, somewhat unrelated, coalescence.

The Sun is in no danger of becoming a black hole, right? And even after it runs out of H, it still won’t become a black hole, right? Why not? A globular cluster can have a million stars, and can be very old (if you’d like to learn more about these objects, just holler, I’ll compile some links). Yet no such has yet coalesced, to form a ~million sol mass black hole, as far as we know. Why not? Extend this to a rich galaxy cluster ... maybe there is some physics in here which could address your question (in the OP)?
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Old 14th November 2017, 01:54 AM   #33
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Originally Posted by JeanTate View Post
Ugh.

Let’s leave non-science (the multiverse) out of this, shall we?

HM’s serious questions have nothing to do with inflation either, so let’s leave that alone too, eh?

For those interested in reading up on why both inflation and the multiverse are, today, not good science, I recommend Sabine Hossenfleder’s blog, BackReAction ... http://backreaction.blogspot.com
Utter nonsense.
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Old 14th November 2017, 03:09 AM   #34
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Originally Posted by JeanTate View Post
Fair enough.... but why did you mention it in the first place?
Because it's a continuation of that discussion. Where the conclusion was that an explosion in flat space CAN'T create a universe, since all that mass would instantly form a black hole. So the part 2 of that question was: ok, but why doesn't it in an expanding space?

Originally Posted by JeanTate View Post
The Sun is in no danger of becoming a black hole, right? And even after it runs out of H, it still won’t become a black hole, right? Why not? A globular cluster can have a million stars, and can be very old (if you’d like to learn more about these objects, just holler, I’ll compile some links). Yet no such has yet coalesced, to form a ~million sol mass black hole, as far as we know. Why not? Extend this to a rich galaxy cluster ... maybe there is some physics in here which could address your question (in the OP)?
Because they're not within a small enough space. You know, shell theorem. hoop conjecture, and all that. Whereas the early universe WAS in a small enough space.

It's not obvious at all why an example which boils down to "matter is OUTSIDE the hoop" is answering a question which was basically "but wasn't all that matter INSIDE the hoop?" Or if you have a good reason why the former is the answer to the latter, the question was exactly what is that reason. Just postulating it is, well, isn't very enlightening.

No offense, but it seems to me like you're not even answering my messages, but posting some general stuff that I wasn't asking about. While I do appreciate taking the time to answer, it's not much use to me if it's not even trying to understand the question, much less answer it.

Especially if you still have to ask...

Originally Posted by JeanTate View Post
Second, where does the “break that ... apart” part come from?
... that tells me you're definitely not on the same page with my qe´uestion
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Old 14th November 2017, 08:01 AM   #35
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Originally Posted by HansMustermann View Post
just telling me that this ain't the answer or that one ain't the answer, is sorta useful, but wouldn't it take less time to just tell me what the answer is, instead of expecting me to arrive to it by ellimination?
The problem is that nobody really knows the answer... maybe not to exactly this specific question, which could be answered in a dry-humor sort of way, but at least to the obvious next one waiting right behind it. It's one of the remaining mysteries of current physics.

We can say, entirely accurately but rather trivially, that the force of expansion simply exceeded the force of contraction (gravity), but that doesn't explain what was driving the expansion so hard back then. So, what was that force/energy?

We don't even really know what's driving it today, whether they're the same phenomenon or not, or what made the expansion rate change at least a couple of times in the past. The only current mainstream theory that even attempted to explain it and come up with a quantity for the energy behind it, the idea that it was driven by vacuum energy AKA zero-point energy, leads to literally the worst numerical prediction in the history of science, off by a factor of 1,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000,00 0,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000 (10¹²⁰). That's a bit of a subtle hint that we might not have this quite sorted out yet.

Originally Posted by HansMustermann View Post
It doesn't really have to form, as such. All the matter (well, energy at that point, but that obeys gravity too) was already in that initial singularity. There's no place for it to fall further into.

Basically for all intents and purposes, the universe was already a black hole.
And how would we know whether it is or isn't, even now? If it wouldn't have collapsed to a radius of zero, which we already know black holes don't do, what else would it do that's different from what we already see the universe doing? The bigger a black hole is, the less sign of it being a black hole at all you would have from the inside. You could cross the event horizon of a supermassive one with no particular sign that you were doing so and no particular sign of there being a looming singularity somewhere around for you to fall toward. A curved geodesic path that's long enough can't be told apart from a straight one. You could carry on for ages without observing a substantial difference between your environment and perfectly flat background space.

This is not to endorse the idea that our universe actually is a black hole (although some have seriously suggested that), but the similarities to a big enough one are at least enough to show that the right question isn't why the universe isn't a black hole; it's whether, and how, making it a black hole or a not-a-black-hole would yield any differences we could identify. And you did suggest a potential one in this next quote, but...

Originally Posted by HansMustermann View Post
And there's this thing about moving inside a black hole. The space is curved. No matter in which direction you're moving, without exceeding c, all directions lead to the singularity. That's what I meant by that 45 degree line on a Penrose diagram. You can fire a photon in whatever direction you wish, its path ultimately still leads to the singularity.
In a big enough black hole, the singularity could be so far away that you wouldn't really know about it, even in the most purely simple sense, that of looking at it as a location, a place in space. But there's also another reason you might not see any sign of such a "place". Inside a black hole, the roles & behavior of time and space mix, so the singularity is not a place you move toward but an event in your future timeline. And the future is something we're all being inescapably pulled toward, so, again, how would we know the difference between "the future as a distant singularity as seen from inside the black hole" and "the future as... something else but not that"?
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Old 14th November 2017, 08:30 AM   #36
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Originally Posted by The Man View Post
Being quoted, linked and going explicitly to your question of why no black hole, what requires you "to arrive to it by ellimination"?

Having been given that answer, discussing it or debating its validity are other matters altogether than you not having been given that answer.
Well, that citation also just said that current calculations don't apply. But my question was basically "what DOES apply?" Removing one calculation, as per that quote, still lives potentially N to go before I arrive to the one that does apply. It's a process of elimination, since it just eliminated one. Problem is, I don't even know the other N to go, so it's not even going nowhere fast. It's going nowhere slow
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Old 14th November 2017, 08:32 AM   #37
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Thanks, man. That makes a lot of sense. (Well, I think it does. It might be my Dunning-Kruger speaking.)
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Old 14th November 2017, 10:31 AM   #38
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Originally Posted by Delvo View Post
The problem is that nobody really knows the answer... maybe not to exactly this specific question, which could be answered in a dry-humor sort of way, but at least to the obvious next one waiting right behind it. It's one of the remaining mysteries of current physics.

We can say, entirely accurately but rather trivially, that the force of expansion simply exceeded the force of contraction (gravity), but that doesn't explain what was driving the expansion so hard back then. So, what was that force/energy?

We don't even really know what's driving it today, whether they're the same phenomenon or not, or what made the expansion rate change at least a couple of times in the past. The only current mainstream theory that even attempted to explain it and come up with a quantity for the energy behind it, the idea that it was driven by vacuum energy AKA zero-point energy, leads to literally the worst numerical prediction in the history of science, off by a factor of 1,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000,00 0,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000 (10¹²⁰). That's a bit of a subtle hint that we might not have this quite sorted out yet.

And how would we know whether it is or isn't, even now? If it wouldn't have collapsed to a radius of zero, which we already know black holes don't do, what else would it do that's different from what we already see the universe doing? The bigger a black hole is, the less sign of it being a black hole at all you would have from the inside. You could cross the event horizon of a supermassive one with no particular sign that you were doing so and no particular sign of there being a looming singularity somewhere around for you to fall toward. A curved geodesic path that's long enough can't be told apart from a straight one. You could carry on for ages without observing a substantial difference between your environment and perfectly flat background space.

This is not to endorse the idea that our universe actually is a black hole (although some have seriously suggested that), but the similarities to a big enough one are at least enough to show that the right question isn't why the universe isn't a black hole; it's whether, and how, making it a black hole or a not-a-black-hole would yield any differences we could identify. And you did suggest a potential one in this next quote, but...

In a big enough black hole, the singularity could be so far away that you wouldn't really know about it, even in the most purely simple sense, that of looking at it as a location, a place in space. But there's also another reason you might not see any sign of such a "place". Inside a black hole, the roles & behavior of time and space mix, so the singularity is not a place you move toward but an event in your future timeline. And the future is something we're all being inescapably pulled toward, so, again, how would we know the difference between "the future as a distant singularity as seen from inside the black hole" and "the future as... something else but not that"?
Lots of good stuff here, good job Delvo!

Some nitpicks:

1) in an isotropic and homogeneous universe where GR rules, you (can) get expansion with or without Dark Energy ... it’s just that the rate of expansion, by time, will be different. HM said he’d read W.D.Clinger’s derivation, but I’m not sure if he’s done so, and if he has, did he understand it, in respect of his questions?

2) HM has, in the course of this thread, asked at least two, quite distinct questions (though, confusingly, I get the impression that he thinks it’s just the one question). The less interesting/not serious question (to me anyway) is about what happened waaaaay before the surface of last scattering (when photons streamed free), where/when “the beginning”, multiverse, inflation, and so on may be relevant; the other is about what happened afterwards, about galaxies, stars, etc. This second question can be addressed more robustly, if only because the relevant physics and astronomical observations are themselves more robust.

3) not really a nitpick ... confusing, to me at least, is HM’s mixing of a dog’s eye view of the universe vs what we can actually observe, from here on Earth. So your emphasis on the latter is a good contribution to this thread, IMHO.
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Old 14th November 2017, 10:54 AM   #39
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Originally Posted by HansMustermann View Post
Because it's a continuation of that discussion. Where the conclusion was that an explosion in flat space CAN'T create a universe, since all that mass would instantly form a black hole. So the part 2 of that question was: ok, but why doesn't it in an expanding space?



Because they're not within a small enough space. You know, shell theorem. hoop conjecture, and all that. Whereas the early universe WAS in a small enough space.

It's not obvious at all why an example which boils down to "matter is OUTSIDE the hoop" is answering a question which was basically "but wasn't all that matter INSIDE the hoop?" Or if you have a good reason why the former is the answer to the latter, the question was exactly what is that reason. Just postulating it is, well, isn't very enlightening.

No offense, but it seems to me like you're not even answering my messages, but posting some general stuff that I wasn't asking about. While I do appreciate taking the time to answer, it's not much use to me if it's not even trying to understand the question, much less answer it.

Especially if you still have to ask...



... that tells me you're definitely not on the same page with my qe´uestion
Ya know, when someone seems to have not understood what I’m saying/writing, my first instinct is to blame myself, for not communicating my ideas etc clearly enough. When almost everyone seems to have misunderstood - as seems to be the case in this thread, re your core question/idea - I know I have failed, not the listeners/readers.

Anyway, let me try this: have you heard of at least one of the “universe simulations”, such as Bolshoi, Millennium, Illustris (there are, I think, dozens now published)? While they vary somewhat in their input assumptions, most (almost all?) include code for an approximation of GR, and a distribution of mass/energy at “CMB time” that is consistent with the CMB observations (most recent ones, Planck; older ones, WMAP). Once started, the simulations proceed without interruption. It’s true that nearly all of them fail, often in more than one way, to simulate what we actually observe today (far more dwarf/satellite galaxies of the MW than we have found to date, for example); however, for most, the “big picture” aspects are pretty close to what we see (well, what astronomers report they see).

In particular, galaxy groups, clusters, and superclusters in the sims look very much like what you see in SDSS images, for example. This may be relevant to your “break apart” idea (or maybe not, as I said, I’d appreciate you clarifying it).

So however it is that you do not understand “GR applied to the universe”, serious attempts to test just that seem to work very well (at least at the level of galaxy lusters, etc).

Does that help?
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Old 14th November 2017, 11:12 AM   #40
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I'm perfectly trusting that GR works and is adequately tested. After all, my GPS wouldn't work if it didn't. Whether GR is right was never the question.

As such, I'm also not very surprised that such simulations work.

I was just asking that someone gives me a simplified version of how/why that works for the early universe. Mostly because I can't seem to find a good explanation of what drove it apart so fast and hard, against some gravity that was much stronger then, on account of everything being closer to everything else.

The thing is, I don't have an alternate crackpot theory to propose. I really believe that the real thing works. I'm not even asking whether it does. I'm just asking how, which would probably also clarify what I got wrong about it.

So just telling me to trust that it works, and there are some simulations that work... well, that still doesn't really answer my question.

I mean, take this as an exact analogy: let's say I asked how something simpler works, let's say a Diesel engine or something. Just telling me that it does work, or to trust in thermodynamics, or that there are very good simulations of Diesel engines, isn't answering the question of how it works, does it? In fact, it doesn't even tell me anything I don't already know. I KNOW that my car's engine works (not as well as VW claimed back then, mind you, but it does work), I have total faith in thermodynamics, etc. But none of that answers the question of how it works.


That said, I'm also perfectly content to leave it at "we don't know", since that's what Delvo tells me. I appreciate a good "we don't know", really. It saves me from trying to figure it out.
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Which part of "Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn" don't you understand?

Last edited by HansMustermann; 14th November 2017 at 11:14 AM.
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