Why can the universe be expanding inward away from point of singularity (POS)?

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HZY
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Much has been said about the universe expanding from a point of singularity (POS). But why can't the POS be a shell of singularity (SOS) such that the expansion would take place inside the SOS without breaking it. Did anybody thought about that?
 
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"Singularity" does NOT mean point. The universe did not expand from a point in space but from a point in time. In terms of space, it happened "everywhere at once" but not at a point.

This is probably THE most common misconception in cosmology and you hear it on every TV show and popularization there is. You do NOT see it in physics books.
 
I have thought about this an I believe what you are trying to imagine translates to a closed or inward curvature of spacetime, meaning our universe exists in a bubble. The logical problem with that line of thought is that there would be an obvious midpoint or "center" of the universe for which there is no observational evidence. What seemed to be a straight line through space would wrap around like the old asteroid video game!

An example of your "sos" is the event horizon of a black hole... the point of singularity may or may not be a dot in the middle of the "shell" from which nothing can escape but no one knows because math doesn't work anymore to explain what happens in there. I prefer to think of "point" as a reference to a location on a graph (our sun on a graph at a moment of time is a point) and a singularity is when all the forces of nature unite (difficult to imagine).
 
jerromyjon said:
I have thought about this an I believe what you are trying to imagine translates to a closed or inward curvature of spacetime, meaning our universe exists in a bubble. The logical problem with that line of thought is that there would be an obvious midpoint or "center" of the universe for which there is no observational evidence. What seemed to be a straight line through space would wrap around like the old asteroid video game!

An example of your "sos" is the event horizon of a black hole... the point of singularity may or may not be a dot in the middle of the "shell" from which nothing can escape but no one knows because math doesn't work anymore to explain what happens in there. I prefer to think of "point" as a reference to a location on a graph (our sun on a graph at a moment of time is a point) and a singularity is when all the forces of nature unite (difficult to imagine).
phinds said:
"Singularity" does NOT mean point. The universe did not expand from a point in space but from a point in time. In terms of space, it happened "everywhere at once" but not at a point.

This is probably THE most common misconception in cosmology and you hear it on every TV show and popularization there is. You do NOT see it in physics books.

If the space on the outside of the universe is trillion times bigger, then the universe itself becomes a point, won't you say?
 
jerromyjon said:
I have thought about this an I believe what you are trying to imagine translates to a closed or inward curvature of spacetime, meaning our universe exists in a bubble. The logical problem with that line of thought is that there would be an obvious midpoint or "center" of the universe for which there is no observational evidence. What seemed to be a straight line through space would wrap around like the old asteroid video game!

An example of your "sos" is the event horizon of a black hole... the point of singularity may or may not be a dot in the middle of the "shell" from which nothing can escape but no one knows because math doesn't work anymore to explain what happens in there. I prefer to think of "point" as a reference to a location on a graph (our sun on a graph at a moment of time is a point) and a singularity is when all the forces of nature unite (difficult to imagine).

Are you saying that when all the forces in nature have united, they ended up cancel each out (conserved) so that we are backed to the original zero-state?
 
mfb said:
No. And it is so far away from actual science that you cannot even point to one aspect and say "there is the problem".

Well, but that seems to be what Alan Guth was saying in his "Alan Guth - How Vast is the Cosmos?" interview on Youtube.
 
HZY said:
Are you saying that when all the forces in nature have united, they ended up cancel each out (conserved) so that we are backed to the original zero-state?
No they do not, they just combine to one interaction that looks different from the interactions we see today.

HZY said:
Well, but that seems to be what Alan Guth was saying in his "Alan Guth - How Vast is the Cosmos?" interview on Youtube.
Where?
 
HZY said:
If the space on the outside of the universe is trillion times bigger, then the universe itself becomes a point, won't you say?

I might say that if there were any such thing as "the space outside the universe" but since there isn't your question has no answer.
 
HZY said:
If the space on the outside of the universe is trillion times bigger, then the universe itself becomes a point, won't you say?

No. I think what you are calling the universe is the observable universe and outside of the universe is the rest of the universe we cannot see. Why would you think the big bang happened only in our observable universe? Is that what you mean? If not, please clarify.

HZY said:
Well, but that seems to be what Alan Guth was saying in his "Alan Guth - How Vast is the Cosmos?" interview on Youtube.

At what point in the video did he say this?
 
Jimmy, I don't get the impression he's referring to the observable universe because even though we can't see anything outside this boundary where light hasn't reached us doesn't mean it doesn't interact with what we can see... he originally referred to an isolated "sos", not that he realizes the implications.
 
A singularity is a mathematical point, not a point in 'space'. It merely tells us the boundary beyond which our math fails to yield meaningful predictions.
 
Chronos said:
A singularity is a mathematical point, not a point in 'space'. It merely tells us the boundary beyond which our math fails to yield meaningful predictions.

Well, that makes sense. I stand corrected.

But still, does the concept of point/shell applicable when referring to the universe as a whole, physically in 3D? It seems to me that when physicists talk about the universe expanded from a SINGLE POINT 13 billion years ago (i.e. inflation), they are referring to a point in space. No? I mean, why would Prof. Alan Guth refer the size of the universe after 64 e-foldings as being the size of a grape fruit if not physical size he is referring to.
 
HZY said:
It seems to me that when physicists talk about the universe expanded from a SINGLE POINT 13 billion years ago (i.e. inflation), they are referring to a point in space. No?
No.

I mean, why would Prof. Alan Guth refer the size of the universe after 64 e-foldings as being the size of a grape fruit if not physical size he is referring to.
Two reasons. First, he's speaking to a lay audience, so he can't go on about four-dimensional curved manifolds and metric expansions... he needs some plain English words that kinda sort of convey the right idea, and that's the best he could come up with.

Second, it is possible to expand a spherical surface to any size you want without the expansion proceeding from any single point in the spherical surface. It is true that if you draw a picture or a make a model (such as the surface of a grapefruit) of that expanding spherical surface, you will end up with a three-dimensional ball that has a central point - but that's just an artifact of the picture/model. The spherical surface can be described and analyzed without any reference to any point outside itself.
 
Nugatory said:
No.Two reasons. First, he's speaking to a lay audience, so he can't go on about four-dimensional curved manifolds and metric expansions... he needs some plain English words that kinda sort of convey the right idea, and that's the best he could come up with.

Second, it is possible to expand a spherical surface to any size you want without the expansion proceeding from any single point in the spherical surface. It is true that if you draw a picture or a make a model (such as the surface of a grapefruit) of that expanding spherical surface, you will end up with a three-dimensional ball that has a central point - but that's just an artifact of the picture/model. The spherical surface can be described and analyzed without any reference to any point outside itself.

What about distance galaxies? Haven't they been observed VISUALLY as being receding away from us? I am sure you would agree that visual observation is physical 3D by default. If so, then projecting backward 13 billion years will end up with the universe to be observed VISUALLY, if some physical being had been alive then to oberve it with their eyes and a MICROSCOPE, as being the size of a single PHYSICAL point, wouldn't you say?
 
Scientists are in the habit of meaning the observable universe when talking about the universe. The size of the entire universe is unknown, probably unknowable, and possibly infinite. It wouldn't make a whole lot of sense to talk about the unobservable universe to a lay audience.
 
HZY said:
What about distance galaxies? Haven't they been observed VISUALLY as being receding away from us? I am sure you would agree that visual observation is physical 3D by default. If so, then projecting backward 13 billion years will end up with the universe to be observed VISUALLY, if some physical being had been alive then to oberve it with their eyes and a MICROSCOPE, as being the size of a single PHYSICAL point, wouldn't you say?
No, I would not say that at all, since it did not start at a point. Think about it this way ... if it HAD started from a point then we would be able to find out WHERE the point was and there would be a preferred direction for the expansion of space, but neither of those is supported by empirical observations.
 
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Or think of it like this, perhaps... since observers are part of the universe there is no outside "microscope view". Putative observers were even smaller scale than the universe.
 
phinds said:
No, I would not say that at all, since it did not start at a point. Think about it this way ... if it HAD started from a point then we would be able to find out WHERE the point was and there would be a preferred direction for the expansion of space, but neither of those is supported by empirical observations.

Sounds like the universe is not expanding but oozing. It is oozing space out of itself everywhere at once.
 
Chronos said:
Scientists are in the habit of meaning the observable universe when talking about the universe. The size of the entire universe is unknown, probably unknowable, and possibly infinite. It wouldn't make a whole lot of sense to talk about the unobservable universe to a lay audience.

Well, could the observable (finite) universe be treated as a point relative to the entire (infinite) universe?
 
No. Were that the case expansion of the universe would exhibit a preferred direction. It does not.
 
HZY said:
Sounds like the universe is not expanding but oozing. It is oozing space out of itself everywhere at once.
I can't tell if you are joking or just being ridiculous.
 
HZY said:
Well, could the observable (finite) universe be treated as a point relative to the entire (infinite) universe?
To what end ?

Also, it is not known that the entire universe is infinite in extent. It could be finite but unbounded.
 
A 'point' on an unbounded, or infinite, manifold, is undefinable. Apologies for reintroducing geometry into this discussion, but, this is exactly how Einstein formulated relativity. His idea remains wildly popular.
 
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phinds said:
I can't tell if you are joking or just being ridiculous.

Sorry about that. I was trying to go beyond the word expansion, which connotes change in physical geometry in some dynamic way (motion/acceleration). But what seems to be really happening is that more space is emerging from existing space. But how does this emerging take place? My guess is that it takes place homogeneously, isotropically, and motionlessly. In other words, there is no pushing aside existing space to accommodate for emerging space, it is just there! more space. This seems to counter the intuitive notion of expansion which should involve some process of becoming or traversing. But No, there is no becoming or traversing here, only there! it is; only, it was smaller before but bigger now. So I have this doubt, which is: is the universe really expanding, or could it be that nothing is really happening other than perhaps we are just in the midst of an appearing/disappearing quantum fluctuation, with all the accompanying manifestations that should soon fade away as the fluctuation subside.
 
Chronos said:
A 'point' on an unbounded, or infinite, manifold, is undefinable. Apologies for reintroducing geometry into this discussion, but, this is exactly how Einstein formulated relativity. His idea remains wildly popular.

Yet we are living in this undefinable point, aren't we not?
 
phinds said:
To what end ?

If the physical size of something is infinitely large, then whatever is contained within it would be infinitely small, wouldn't you say? So the question is could ∞-1 be contained within ∞ and still be visible in terms of physical size. If the observable part of the universe is contained within an infinitely large sized universe, then it automatically becomes infinitely small relative to it.

phinds said:
Also, it is not known that the entire universe is infinite in extent. It could be finite but unbounded.

You mean it curves back in a loop.
 
HZY said:
is the universe really expanding

Yes, that has been the accepted fact for over a century and it has been better and better tested over and over.

HZY said:
or could it be that nothing is really happening other than perhaps we are just in the midst of an appearing/disappearing quantum fluctuation, with all the accompanying manifestations that should soon fade away as the fluctuation subside.

That would be the Boltzmann Brain idea, I think. It is an extraordinary claim (because we don't know if the universe is infinite) despite being possible, and has zero evidence.

HZY said:
Yet we are living in this undefinable point

Now you are trying to use word descriptions where they no longer suffice. A point is an abstraction. Chronos was describing how spacetime volumes may be a better description in a spacetime continuum. E.g. Earth is not a point particle.

To make the point approximation valid, you would have to quantify your description and its intention. If you are asking for a valid mass approximation for looking at Earth in relation to most distant objects affected by it, you can interchange it with its mass center. However, for the other cases (like the Moon), you would need other approximations.

HZY said:
If the physical size of something is infinitely large, then whatever is contained within it would be infinitely small, wouldn't you say?

And here is where your lack of quantification breaks down. Spacetime has a metric. That means we can, and have to in many cases of its physics, keep track of a scale as the universe expands.

One of the cases of physics were we have to keep track of a scale is, obviously, cosmology in describing the universe and its behavior.
 
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