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Big Bang Singularity

by Alan McDougal
Tags: bang, singularity
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yuiop
#19
Jun19-08, 03:47 AM
P: 3,967
Quote Quote by Alan McDougal View Post
The "bounce" you mentioned would make the universe finite as entropy must eventually stop this cyclic phenomenon.
I assume you are talking about finite time. Thermodynamics does not forbid reversible processes as long as the net entropy change is zero. Perhaps that is the case with the universe as a whole. Although "time's arrow" goes in the direction of increasing entropy, it just might be possible that the net change in entropy of the universe as it expands is zero. If that was the case, thermodynamics would not forbid the universe expanding and collapsing repeatedly forever. An example of a reversable process is a black hole forming and then evaporating again. No has seen that happen yet, but no one has said it can not happen in principle on thermodynaic grounds.
gendou2
#20
Jun19-08, 12:54 PM
P: 235
Quote Quote by Alan McDougal View Post
gendoo2,

Maybe all universes are black holes some existing for one blindingly brief moment and others like our own for almost an eternity. The absolute density of our universe might be considered a void or empty space in another. universe

Just wild speculation, but man!! it can be fun.

Alan
This is not the sort of attitude one may bring to a scientific debate!
1. Many-universe theory is not a valid scientific theory.
2. You are not using proper punctuation.
3. Wild speculation may be fun for you, but it can insult other people's intelligence.

Quote Quote by Alan McDougal
I would not like to see this forum descend into wild conjecture and speculation. We should state what is known by present modalities and then try to move further on. We should attempt original thought within the confines of common sense,
I agree, and caution you to avoid being hypocritical.
Allday
#21
Jun20-08, 01:01 AM
P: 163
Without even getting into the realm of quantum GR, It is unclear to me that a BH would form in the early dense universe. Here is my thinking. One of the major misconceptions about the Big Bang is that there was this super dense nugget of stuff early on. What happens in a flat Universe is that the density is the same everywhere, there is not ball of matter. This density increased as you run time backwards from the present.

Now there is a critical density for collapse into a blackhole. You can do a simple classical derivation by setting the escape velocity of a parcel of matter equal to the speed of light. Even if the density of the Universe crosses this threshold at some point, it has this density everywhere and so the curvature is still flat. I would guess that even a non-expanding, but infinite Universe would not form a black hole.

Of course there are quantum effects that need to be take into account in an exact theory, but simplistically from a smooth classical GR perspective I wouldnt expect a black hole to form. If you have pertubations in this density field then individual black holes could form at the overdense sites, but this would still not mean the whole Universe becoming a black hole.
yuiop
#22
Jun20-08, 02:03 AM
P: 3,967
Quote Quote by Allday View Post
Without even getting into the realm of quantum GR, It is unclear to me that a BH would form in the early dense universe. Here is my thinking. One of the major misconceptions about the Big Bang is that there was this super dense nugget of stuff early on. What happens in a flat Universe is that the density is the same everywhere, there is not ball of matter. This density increased as you run time backwards from the present.

Now there is a critical density for collapse into a blackhole. You can do a simple classical derivation by setting the escape velocity of a parcel of matter equal to the speed of light. Even if the density of the Universe crosses this threshold at some point, it has this density everywhere and so the curvature is still flat. I would guess that even a non-expanding, but infinite Universe would not form a black hole.

Of course there are quantum effects that need to be take into account in an exact theory, but simplistically from a smooth classical GR perspective I wouldnt expect a black hole to form. If you have pertubations in this density field then individual black holes could form at the overdense sites, but this would still not mean the whole Universe becoming a black hole.
Assuming an universe of infinite mass would indeed prevent the universe being a black hole or nugget at any time. However, the concept of infinite mass is not one to be used lightly. When you evaluate the true meaning of infinite, the concept of infinite mass is very hard to take seriously. The concept of a universe with infinite volume is not too hard to imagine, but if the density of the universe is isotropic and homogenous everywhere on large scales then even finite density implies infinite mass in a universe of infinite volume.

A second difficulty is this. It is known that in an earlier epoch of the universe, the expansion of the universe actully slowed down, as if it was going to collapse but later the cosmological constant kicked in and started accelerating the expansion. Now the slow down in expansion in the earlier epoch was put down to gravitational collapse. That would not be possible in a universe with infinite mass and volume for the same reasons that it would not be possible to have a black hole at the beginning of the universe in a universe that was always infinite.

That means we cannot explain the initial slow down of expansion by gravity, so we would have to postulate that the cosmological constant changes over time taking on negative values early on in the history of the universe and changing to positive later on. I am not sure a pulsating cosmological constant is the current accepted view. That means an infinite universe is ruled out by the accepted view.
Alan McDougal
#23
Jun20-08, 02:13 AM
P: 42
Gendou2,

This is not the sort of attitude one may bring to a scientific debate!
1. Many-universe theory is not a valid scientific theory.
2. You are not using proper punctuation.
3. Wild speculation may be fun for you, but it can insult other people's intelligence.
So the forum needs protection from my exotic mind! By wild speculation I meant "scientific wild speculation" of the kind Einstein (imagined himself riding on a beam of light) or Newton, (what is gravity).I do not equate myself with these great minds ,but please allow me to think
in my own unique manner which is outside apparently of your "boxed"" in mind

If great minds like this did not indulge in speculation that would be considered "wild" in their time frame we might still be living in the stone age. Some of them were even burned at the stake. Heck I hope you don't want the forum to give me the same punishment!!


I don't like your rhetoric and will punctuate just like I want to. I do not like admonishment << personal insult removed by berkeman >>, and "just one more unfortunate comment like this will see me leaving this forum"

Alan
Allday
#24
Jun20-08, 02:17 AM
P: 163
Quote Quote by kev View Post
Assuming an universe of infinite mass would indeed prevent the universe being a black hole or nugget at any time. However, the concept of infinite mass is not one to be used lightly. When you evaluate the true meaning of infinite, the concept of infinite mass is very hard to take seriously. The concept of a universe with infinite volume is not too hard to imagine, but if the density of the universe is isotropic and homogenous everywhere on large scales then even finite density implies infinite mass in a universe of infinite volume.

A second difficulty is this. It is known that in an earlier epoch of the universe, the expansion of the universe actully slowed down, as if it was going to collapse but later the cosmological constant kicked in and started accelerating the expansion. Now the slow down in expansion in the earlier epoch was put down to gravitational collapse. That would not be possible in a universe with infinite mass and volume for the same reasons that it would not be possible to have a black hole at the beginning of the universe in a universe that was always infinite.

That means we cannot explain the initial slow down of expansion by gravity, so we would have to postulate that the cosmological constant changes over time taking on negative values early on in the history of the universe and changing to positive later on. I am not sure a pulsating cosmological constant is the current accepted view. That means an infinite universe is ruled out by the accepted view.
The concept of infinite mass and infinite volume is exactly what the Friedman Robertson Walker metric implies for a flat Universe. This is a well accepted model and observations from WMAP have been narrowing in on a flat Universe each time new results come out.

It is indeed possible for an infinite Universe with infinite mass to go through a period of deceleration and then acceleration in expansion. The effect comes from the change in the ratio of the density of the components of the Universe. Specifically the matter density drops as a^-3 and the energy density of the cosmological constant remains ... well ... constant (a is the scale factor). The matter causes contraction the cosmological constant causes expansion and eventually takes over.
Wallace
#25
Jun20-08, 02:35 AM
Sci Advisor
P: 1,253
Quote Quote by kev View Post
That means we cannot explain the initial slow down of expansion by gravity, so we would have to postulate that the cosmological constant changes over time taking on negative values early on in the history of the universe and changing to positive later on. I am not sure a pulsating cosmological constant is the current accepted view. That means an infinite universe is ruled out by the accepted view.
Allday is correct, there is nothing in the current 'deceleration followed by acceleration' model of the Universe that prohibits an infinite Universe. In an infinite homogeneous Universe, within any spherical sub section we can observe that the gravitational effect of matter outside that sphere cancels and we need only consider the matter within the sphere. Mathematically this works fine. You can derive the Friedman equations (that come formally from the Einstein Field Equations) simply by considering a finite sphere of matter and working out how its radius changes with time. As the initial radius of the ball is taken to infinity you get an infinite Universe.
yuiop
#26
Jun20-08, 02:45 AM
P: 3,967
Quote Quote by Allday View Post
The concept of infinite mass and infinite volume is exactly what the Friedman Robertson Walker metric implies for a flat Universe. This is a well accepted model and observations from WMAP have been narrowing in on a flat Universe each time new results come out.

It is indeed possible for an infinite Universe with infinite mass to go through a period of deceleration and then acceleration in expansion. The effect comes from the change in the ratio of the density of the components of the Universe. Specifically the matter density drops as a^-3 and the energy density of the cosmological constant remains ... well ... constant (a is the scale factor). The matter causes contraction the cosmological constant causes expansion and eventually takes over.
Quote Quote by Wallace View Post
Allday is correct, there is nothing in the current 'deceleration followed by acceleration' model of the Universe that prohibits an infinite Universe. In an infinite homogeneous Universe, within any spherical sub section we can observe that the gravitational effect of matter outside that sphere cancels and we need only consider the matter within the sphere. Mathematically this works fine. You can derive the Friedman equations (that come formally from the Einstein Field Equations) simply by considering a finite sphere of matter and working out how its radius changes with time. As the initial radius of the ball is taken to infinity you get an infinite Universe.
In that case, if matter density can contract the expansion in a inifinite universe then the fact that the universe is infinite cannot prevent the universe being a black hole when the density was sufficient at the early stages. As Wallace says, "gravitational effect of matter outside that sphere cancels and we need only consider the matter within the sphere"
Wallace
#27
Jun20-08, 05:39 AM
Sci Advisor
P: 1,253
Right, that's why the Universe required some as yet unknown mechanism that inflated the early Universe. As marcus detailed earlier in this thread (and others) the search for the mechanism of inflation if one of the hot research topics of today.
yuiop
#28
Jun20-08, 05:47 AM
P: 3,967
Quote Quote by Wallace View Post
Right, that's why the Universe required some as yet unknown mechanism that inflated the early Universe. As marcus detailed earlier in this thread (and others) the search for the mechanism of inflation if one of the hot research topics of today.

Perhaps they need look no further than GR and the Schwarzschild solution to see how a finite mass confined to considerably less than the Schwarzschild radius of the mass would inflate rapidly at superluminal velocities as detailed in this post: http://www.physicsforums.com/showpos...&postcount=137
gendou2
#29
Jun20-08, 01:35 PM
P: 235
Quote Quote by Alan McDougal View Post
... Please allow me to think in my own unique manner which is outside apparently of your "boxed-in" mind.
If pointing out gaping holes in an attempt at logical argument is having a "boxed-in" mind, then I am guilty of this. I am perfectly willing to hear any radical idea regarding the fascinating field of cosmology. Please don't blame me for your failure to back up your ideas with a coherent argument.

You are free to think whatever you want. Your are not free to harass members of this forum who disagree with your unsupported theories by accusing them of being closed minded. This is abusive behavior and will be reported.

Still, if I have seemed unfriendly to you, I apologize. This was not my intention.
Alex48674
#30
Jun24-08, 01:37 AM
P: 70
Quote Quote by Wallace View Post
Right, that's why the Universe required some as yet unknown mechanism that inflated the early Universe. As marcus detailed earlier in this thread (and others) the search for the mechanism of inflation if one of the hot research topics of today.
I thought that mechanism was a supercooled higg's field causing a negative pressure. Is this wrong, or do you mean what caused it to supercool?
Wallace
#31
Jun24-08, 01:41 AM
Sci Advisor
P: 1,253
Quote Quote by Alex48674 View Post
I thought that mechanism was a supercooled higg's field causing a negative pressure. Is this wrong, or do you mean what caused it to supercool?
I think that is just one of many proposed mechanisms. There is a long way to go before we have a coherent theory of inflation with all the details worked out.
Alex48674
#32
Jun24-08, 11:38 AM
P: 70
Quote Quote by Wallace View Post
I think that is just one of many proposed mechanisms. There is a long way to go before we have a coherent theory of inflation with all the details worked out.
Ohh alright, so what are some of the other bigs ones to look into? And how mainstream is the one I mentioned in comparison?
Wallace
#33
Jun24-08, 06:23 PM
Sci Advisor
P: 1,253
Not sure. Too be honest I haven't heard of that idea. Where did you hear it from?

There are too many theories of inflation, or some process that isn't inflation but solves the same problems, to list. Some extend the standard model of particle physics (which the Higgs idea you refer to would be an example of) while some suggest new mechanisms entirely. This is a very dynamic field, not completely unconstrained by data, but none the less with plenty of freedom for new ideas that have the same effects as other ideas but with a vastly different cause.


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