Could the universe be collapsing?

[Cosmologist]

Imagine the universe collapsing inwards under its own gravitational force. The central region would be relatively unaffected at first. Gravity pulling from all directions would cancel it out. The outer edges of the universe would be different. Gravity is pulling in one direction only for the farthest galaxies. Inwards. So the collapse begins with the outermost galaxies slowing and then slowly falling inwards. As this continues over billions of years the density of the outer universe increases. This increase in universal density on the fringes would exert an increasing gravitational force on the hitherto static central region. The core is pulled outwards in all directions to meet this inward falling outer universe. If the milky way resides in the central region of the universe then we would observe an accelerating expansion of the viewable universe. Ofcourse this is what astronomers have observed.

Is this theory plausible? If not please explain why. If it is possible that the universe is collapsing in this way then could we calculate the end of the universe from the rate of expansion?

[Strange]

A couple of immediate problems occur to me.

This idea assumes that there is a centre and an edge to the universe. That doesn't seem to be the case.

Secondly, and more importantly, the mass on the outside of a sphere does not pull on the material inside. This was proved by Isaac Newton some time ago and is known as the Shell Theorem.

[Hornblower]

A couple of immediate problems occur to me.

This idea assumes that there is a centre and an edge to the universe. That doesn't seem to be the case.

Secondly, and more importantly, the mass on the outside of a sphere does not pull on the material inside. This was proved by Isaac Newton some time ago and is known as the Shell Theorem.

Let me add to that. The gravitational action of the outer layer remains at zero everywhere inside its radius, regardless of how massive we make it. For interior locations away from the center, objects will gravitate toward the center as if all the mass inside their radius is concentrated at the center. Thus in an initially uniform spherical system, all parts of it will fall toward the center at rates proportional to their distance from the center. A hypothetical observer would see everything approaching at a velocity proportional to its distance, which is just the opposite of what we actually observe.

[wd40]

In a collapsing universe observable Blue-Shift as opposed to Red-Shift would be everywhere?

[Hornblower]

In a collapsing universe observable Blue-Shift as opposed to Red-Shift would be everywhere?

Yes.

[Cosmologist]

I'm confused. How can the universe not have an edge or centre if it originated from a single point and spread outwards? Assuming the big bang theory is correct and I don't know if it is. How could it theoretically be possible for a big crunch if the universe is infinite?

The mass on the outside of a sphere does not pull on the material inside? I thought that all matter posessed gravity and every object in the universe was attracted to every other object.

We don't know that the universe is perfectly spherical. Does that make any difference?

In a collapsing universe observable Blue-Shift as opposed to Red-Shift would be everywhere? We can only observe part of the universe. It could be a trillion times larger than what we can see and everything we see is out of date. At the limits of our radio telescopes we are looking at the universe over 10 billion years ago. Yes, it was redshifting then.

How do we know the shell theorem applies at any scale? According to theory the universe shouldn't be expanding at an accelerating rate. Rules tend to change at different scales.

Thanks for the interesting replies.

[Strange]

I'm confused. How can the universe not have an edge or centre if it originated from a single point and spread outwards?

I'm sure someone else can answer thois one better than me ... but, basically, the big bang was not an explosion from a point. The entire universe was smaller/denser/hotter in the past. The "bang" happened everywhere and everywhere is still moving apart from everywhere else.

The mass on the outside of a sphere does not pull on the material inside? I thought that all matter posessed gravity and every object in the universe was attracted to every other object.

The thing is, it all cancels out. Imagine you are at the center of the earth; you will be pulled equally in all directions and so not feel any "pull" at all - effectively, no gravity. Now imagine the mass of the earth is spread out in a thing spherical shell all around you. It might be quite intuitive that when you are at the center of the sphere you will feel zero net gravity (because you are pulled equally in all directions). But the surprising thing is that everywhere inside the shell, there is zero gravity: if you are close to one side, then there will be more pull from the material ou are close to. But there is much more material on the other side (that you are further away from). It just turns out that these two things exactly cancel out and so there is not net gravity anywhere inside.

How do we know the shell theorem applies at any scale?

Everything we observe suggests that gravity behaves the same at all scales.

[John Mendenhall]

Let me add to that. The gravitational action of the outer layer remains at zero everywhere inside its radius, regardless of how massive we make it. For interior locations away from the center, objects will gravitate toward the center as if all the mass inside their radius is concentrated at the center. Thus in an initially uniform spherical system, all parts of it will fall toward the center at rates proportional to their distance from the center. A hypothetical observer would see everything approaching at a velocity proportional to its distance, which is just the opposite of what we actually observe.

IIRC, all the interior volume sees zero g, not just the center. See post above by Strange..

[Cougar]

IIRC, all the interior volume sees zero g, not just the center.

Any point with radius r interior to the Earth's volume will "feel" zero g only from the Earth's mass that has radius > r.

[Cougar]

Assuming the big bang theory is correct and I don't know if it is. How could it theoretically be possible for a big crunch if the universe is infinite?

It couldn't. But the big bang theory doesn't say the universe is infinite. We don't know whether it is or isn't.

The mass on the outside of a sphere does not pull on the material inside? I thought that all matter posessed gravity and every object in the universe was attracted to every other object.

Yes, all matter does possess gravity, but as Strange said, if you've got mass on both sides of you, it can cancel out. And again, as Strange said, Newton proved this geometrically back in the 1600s. If you're offset from the center of a uniform sphere, some mass on "your side" of the sphere, with greater radii than your radius, will be closer to you and thus have greater effect. However, since you're offset, there will be more mass on the other side, even though it's farther from you. Remarkably, the more mass at greater distance exactly cancels out the less, closer mass. Hence, no gravitational effect.

We don't know that the universe is perfectly spherical. Does that make any difference?

Not in your scenario where you imagine "density on the fringes would exert an increasing gravitational force on the hitherto static central region." Besides, the visible universe is observationally uniform beyond a certain scale. And though efforts have been made to detect it, to date there is no indication of any birefringence.

We can only observe part of the universe.

Yes, but we can also look into its past. This allows us to map out its evolutionary history. It used to be thought that all the mass in the universe would be slowing the expansion. Whether it would slow down enough to stop and start contracting was the big question. Pretty quickly, though, it was found that there was not nearly enough mass to halt the expansion. And now, to add insult to injury, so to speak, we find that the expansion isn't even slowing down, nor is it coasting at a constant rate. Somehow, it's slowly speeding up. As it turns out, this acceleration of the expansion is pretty well described by setting Einstein's cosmological constant to -1. However, the cause of this acceleration remains a mystery.

[caveman1917]

It couldn't.

Why not? If you accept that we could have an infinite universe that started with the big bang, what is wrong with running the movie backwards?

As it turns out, this acceleration of the expansion is pretty well described by setting Einstein's cosmological constant to -1.

You probably mean introducing a perfect fluid with equation of state parameter , that's not the same as setting the cosmological constant to -1.

[Cougar]

Why not? If you accept that we could have an infinite universe that started with the big bang, what is wrong with running the movie backwards?

Because it would be infinite in extent. Which means it doesn't stop expanding. It always adds 1 to wherever it was. This places the condition on the situation such that there's no running the movie backwards. [exclamation points suppressed.]

You probably mean introducing a perfect fluid with equation of state parameter , that's not the same as setting the cosmological constant to -1.

That's it, thanks. . : )

[caveman1917]

Because it would be infinite in extent. Which means it doesn't stop expanding. It always adds 1 to wherever it was. This places the condition on the situation such that there's no running the movie backwards. [exclamation points suppressed.]

Still not following you. What do you mean by "it doesn't stop expanding"? The time reversal of expanding is contracting. If it can expand it can also contract, no?
If you mean that because at one point in time it's infinite and at another point it is still infinite then it cannot contract, then by the same reasoning it cannot expand either. So if you accept an infinite expanding universe, what's wrong with an infinite contracting one?

What exactly do you mean by "it always add 1 to wherever it was"?

[primummobile]

Still not following you. What do you mean by "it doesn't stop expanding"? The time reversal of expanding is contracting. If it can expand it can also contract, no?
If you mean that because at one point in time it's infinite and at another point it is still infinite then it cannot contract, then by the same reasoning it cannot expand either. So if you accept an infinite expanding universe, what's wrong with an infinite contracting one?

What exactly do you mean by "it always add 1 to wherever it was"?

Cougar is applying a different definition of infinity from what you are. Cougar is saying that a universe that is expanding would have no boundary because no matter how far or fast we were able to travel, the boundary would be beyond us. I would add that there would be an event horizon between us and the boundary because at some distance from us space is going to be expanding faster than light.

It's difficult to imagine a static, infinite universe. I'm not even sure how that would work. But a universe in which you could never reach, or get any information about, the boundary would still be considered by some to be infinite.

I'm not clear why you couldn't run the clock backwards on that, but I'm probably not looking at the whole picture.

I'm more inclined to believe we live in a finite but unbounded universe.

[speedfreek]

I think I know what Cougar was referring to.

Can a closed (or positively curved) universe be infinite?

Can an open (or flat) universe contract?

[Cosmologist]

I'm sure someone else can answer thois one better than me ... but, basically, the big bang was not an explosion from a point. The entire universe was smaller/denser/hotter in the past. The "bang" happened everywhere and everywhere is still moving apart from everywhere else.

If it was smaller then it had scale and shape.... erm... didn't it?

The thing is, it all cancels out. Imagine you are at the center of the earth; you will be pulled equally in all directions and so not feel any "pull" at all - effectively, no gravity.

I based the idea of the internal universe remaining initially static on that very idea.

Now imagine the mass of the earth is spread out in a thing spherical shell all around you. It might be quite intuitive that when you are at the center of the sphere you will feel zero net gravity (because you are pulled equally in all directions).

With you so far.

But the surprising thing is that everywhere inside the shell, there is zero gravity: if you are close to one side, then there will be more pull from the material ou are close to. But there is much more material on the other side (that you are further away from). It just turns out that these two things exactly cancel out and so there is not net gravity anywhere inside.

I'm having trouble understanding this. Surely the mass is about the same on all sides and you would be pulled towards the closest side.

The Oort cloud circles the sun. Not a sphere but the best example I can come up with in reality. Sounds like you are saying I would feel no gravitational effects from Oort cloud objects so long as I'm inside the solar system.

Everything we observe suggests that gravity behaves the same at all scales.

According to the laws of gravity the Galaxies should be flying apart. We had to invent 'Dark Energy' to explain it. I say 'invent' because its nothing more than a term. We have no idea what is holding Galaxies together. To say that 90% of the universe is invisible says we dont know anything. Maybe the laws of gravity aren't as simple as we thought. Just an observation.

I believe you about the shell theorem. just having trouble getting my head around it.

It couldn't. But the big bang theory doesn't say the universe is infinite. We don't know whether it is or isn't.

If the universe isn't infinite then its finite. That means it has edges. Now I'm really confused.

Yes, all matter does possess gravity, but as Strange said, if you've got mass on both sides of you, it can cancel out. And again, as Strange said, Newton proved this geometrically back in the 1600s. If you're offset from the center of a uniform sphere, some mass on "your side" of the sphere, with greater radii than your radius, will be closer to you and thus have greater effect. However, since you're offset, there will be more mass on the other side, even though it's farther from you. Remarkably, the more mass at greater distance exactly cancels out the less, closer mass. Hence, no gravitational effect.

I think I'm starting to get it. My head hurts.

Pretty quickly, though, it was found that there was not nearly enough mass to halt the expansion.

How can we know the total mass of the universe when we have no clue how big the universe is? that was my theory. That it might be much bigger and denser beyond the range of our telescopes. Perhaps that mass is pulling the innards of the universe outwards. Maybe the outermost part of the universe is orbiting the rest of the universe. Like the Oort cloud around the sun and planets. Imagine if the Oort cloud began to shrink. Wouldn't the planets would be pulled outwards one by one? Seemingly expanding the solar system? Ok. Its a two dimensional example I know.

I would add that there would be an event horizon between us and the boundary because at some distance from us space is going to be expanding faster than light.

now I understand what you mean by a finite but unbounded universe. Isn't that impossible according to special relativity? Is it adding space-time or stretching itself faster than light? Wouldn't peter out at some point. Where does it derive the energy to infinitely keep growing? Everything dies eventually.

[cjameshuff]

If it was smaller then it had scale and shape.... erm... didn't it?

It means lengths were shorter, nothing more. It's perfectly valid to have an infinite universe that is expanding, where all lengths reach zero and density becomes infinite at a finite time in the past. (It is generally presumed that unknown high energy physics intervene before this point is reached, but current physics can get very close in human terms without becoming invalid.)

I'm having trouble understanding this. Surely the mass is about the same on all sides and you would be pulled towards the closest side.

It's not the same on all sides. The closer you are to one side, the less mass you have toward that side, and the more toward the opposite side. With a hollow sphere, this exactly counteracts the increased distance from the mass on the opposite side.

The Oort cloud circles the sun. Not a sphere but the best example I can come up with in reality. Sounds like you are saying I would feel no gravitational effects from Oort cloud objects so long as I'm inside the solar system.

If they were uniformly distributed in a sphere and you are far enough from any Oort cloud objects that the difference from a continuous shell is insignificant, yes, that is correct. An object within a hollow sphere is not attracted in any direction.

According to the laws of gravity the Galaxies should be flying apart. We had to invent 'Dark Energy' to explain it. I say 'invent' because its nothing more than a term. We have no idea what is holding Galaxies together. To say that 90% of the universe is invisible says we dont know anything. Maybe the laws of gravity aren't as simple as we thought. Just an observation.

Dark energy is the energy needed to account for the accelerating expansion, dark matter is what's used to explain the rotation rates of galaxies. Two completely different concepts.

And modified gravity theories keep being tried and never work as well as dark matter. They particularly fail in cases such as the famous Bullet Cluster.

If the universe isn't infinite then its finite. That means it has edges. Now I'm really confused.

No, it doesn't. The surface of a sphere is finite and doesn't have any edges. The same can apply to the universe.

How can we know the total mass of the universe when we have no clue how big the universe is? that was my theory. That it might be much bigger and denser beyond the range of our telescopes. Perhaps that mass is pulling the innards of the universe outwards. Maybe the outermost part of the universe is orbiting the rest of the universe. Like the Oort cloud around the sun and planets. Imagine if the Oort cloud began to shrink. Wouldn't the planets would be pulled outwards one by one? Seemingly expanding the solar system? Ok. Its a two dimensional example I know.

We only need to know the density, not the total mass (which may be infinite). We have a decent idea of the density, and the default assumption is that we are not in a special location in the universe (the cosmological principle).
And no, the planets wouldn't be pulled anywhere. Again, Shell Theorem.

now I understand what you mean by a finite but unbounded universe. Isn't that impossible according to special relativity? Is it adding space-time or stretching itself faster than light? Wouldn't peter out at some point. Where does it derive the energy to infinitely keep growing? Everything dies eventually.

No, it doesn't violate relativity (general relativity...special relativity doesn't particularly apply). And there's a reason it's called dark energy.

[AGN Fuel]

If it was smaller then it had scale and shape.... erm... didn't it?

I think your confusion may stem from a presumption that the expansion of the universe is occurring in a pre-existing (I want to say space, but that would be confusing!) void - this is the down side to the "explosion" analogy that is commonly used. Because the universe is all there is and each point sees expansion from each other point anywhere you happen to be in the universe (excluding localised gravitation attraction), there is no preferred central location.

I'm having trouble understanding this. Surely the mass is about the same on all sides and you would be pulled towards the closest side.

Gravitational force between any two objects is dependent on two variables - their masses and the distance between them. The only point within a sphere where the mass surrounding it will be equal in all directions is in the exact centre of that sphere.

Now, imagine a point in the sphere that it not in the centre - you can see that by being offset, there must be part of the mass of the sphere that is close, but more of the mass of the sphere that is further away (this must be so or the point wouldn't be non-central). What Newton calculated some 400 odd years ago, was that the less mass (but closer) is precisely balanced by the greater mass (but further away) no matter where the point is inside the sphere.

According to the laws of gravity the Galaxies should be flying apart. We had to invent 'Dark Energy' to explain it. I say 'invent' because its nothing more than a term. We have no idea what is holding Galaxies together. To say that 90% of the universe is invisible says we dont know anything. Maybe the laws of gravity aren't as simple as we thought. Just an observation.

Dark matter, not dark energy (which is more related to your expansion of the universe issues).

We know that the laws of gravity are not 'as simple as we thought' by virtue of the fact we haven't been able to establish how relativity and quantum mechanics mesh. They both work, they have both been demonstrated experimentally countless times. But they don't really 'talk' to each other. Not sure if the answer to dark matter is contained within that solution, but there is no doubt that our understanding is not complete.

[Cosmologist]

An object within a hollow sphere is not attracted in any direction.

I guess that kills off any possibility of a Pellucidar type world anywhere in the universe. Hey wait a minute! What about the Ring World novels by Larry Niven and Dyson Sphere's? People are supposed to be able to walk around the inner surfaces of those. So you are saying gravity becomes zero inside a sphere at a certain distance from its inner surfaces? Oh wait. My mistake. They used centripetal gravity.

We have a decent idea of the density, and the default assumption is that we are not in a special location in the universe (the cosmological principle).

But we have no idea where we are in relation to the entire universe. What we can see is quite varied. Supermassive blackholes have nearly infinite density and intergalactic space is virtually empty. The entire universe could be solid for all we know. What we call the universe might reside inside a bubble. Far fetched I know. It just seems to me that its a very big assumption to expect the whole universe to be exactly like our part. The universe could be so vast that areas of it obey different laws of physics. Its like aliens skimming earths atmosphere, taking a sample and declaring the earth must be made entirely of air.

I think your confusion may stem from a presumption that the expansion of the universe is occurring in a pre-existing (I want to say space, but that would be confusing!) void - this is the down side to the "explosion" analogy that is commonly used. Because the universe is all there is and each point sees expansion from each other point anywhere you happen to be in the universe (excluding localised gravitation attraction), there is no preferred central location.

What about other universes? So it expands like yeast. So the big crunch would appear as an increase in density only? That would make starflight easier wouldn't it?

What Newton calculated some 400 odd years ago, was that the less mass (but closer) is precisely balanced by the greater mass (but further away) no matter where the point is inside the sphere.

Newton doesn't get enough praise nowadays. That man was sharp. Thanks for the explanation. Trying to imagine it.

Thanks for the corrections peeps. Hopefully the new science around the discovery of the Higgs will shed some light on those mysteries like a unifield field theorem.

[Cosmologist]

An object within a hollow sphere is not attracted in any direction.

I guess that kills off any possibility of a Pellucidar type world anywhere in the universe. Hey wait a minute! What about the Ring World novels by Larry Niven and Dyson Sphere's? People are supposed to be able to walk around the inner surfaces of those. So you are saying gravity becomes zero inside a sphere at a certain distance from its inner surfaces? Oh wait. My mistake. They used centripetal gravity.

We have a decent idea of the density, and the default assumption is that we are not in a special location in the universe (the cosmological principle).

But we have no idea where we are in relation to the entire universe. What we can see is quite varied. Supermassive blackholes have nearly infinite density and intergalactic space is virtually empty. The entire universe could be solid for all we know. What we call the universe might reside inside a bubble. Far fetched I know. It just seems to me that its a very big assumption to expect the whole universe to be exactly like our part. The universe could be so vast that areas of it obey different laws of physics. Its like aliens skimming earths atmosphere, taking a sample and declaring the earth must be made entirely of air.

I think your confusion may stem from a presumption that the expansion of the universe is occurring in a pre-existing (I want to say space, but that would be confusing!) void - this is the down side to the "explosion" analogy that is commonly used. Because the universe is all there is and each point sees expansion from each other point anywhere you happen to be in the universe (excluding localised gravitation attraction), there is no preferred central location.

What about other universes? So it expands like yeast. So the big crunch would appear as an increase in density only? That would make starflight easier wouldn't it?

What Newton calculated some 400 odd years ago, was that the less mass (but closer) is precisely balanced by the greater mass (but further away) no matter where the point is inside the sphere.

Newton doesn't get enough praise nowadays. That man was sharp. Thanks for the explanation. Trying to imagine it.

Thanks for the corrections peeps. Hopefully the new science around the discovery of the Higgs will shed some light on those mysteries like a unifield field theorem.

[Strange]

now I understand what you mean by a finite but unbounded universe. Isn't that impossible according to special relativity?

Special relativity is a very simple and limited theory. It is not impossible according to general relativity; in fact, the whole idea is based on general relativity.

Is it adding space-time or stretching itself faster than light?

Note that expansion (or stretching) is not defined by speed but as a percentage increase. Consider a series of equally spaced points:

A...B...C...D...E... ... Z

After some time the space between each point will have increased:

A....B....C....D....E.... ... ... Z

The distance from A to B has increased by 1 unit. The distance from A to C has increased by 2 units. And so on. (And the same is true for every other point.) Therefore C will appear to be receding from A twice as fast as B is. At some point there will be a point that appears to be receding from A faster than the speed of light.

However, this does not contradict relativity because (a) the "speed of light restriction" is purely a local thing, (b) there is no real motion, just an increase in space between things and (c) the whole model is based on relativity. (The expansion of the universe was predicted before it was observed.)

Wouldn't peter out at some point. Where does it derive the energy to infinitely keep growing?

It doesn't require energy to keep going (see Newton's first law of motion, for example). In the past it was thought that gravity would eventually slow the expansion down and cause it to reverse. However, since then we have seen evidence that the rate of expansion is actually increasing. Energy is required to do this and this is called "dark energy" (because we don't know what it is). This may be some sort of energy or it might be that gravity doesn't work as expected on really large scales. Currently we don't know.

[Strange]

That man was sharp.

Very, very true.

[primummobile]

Very, very true.

Newton invented calculus when he found he didn't have the tools to solve some of his problems. At least, he independently invented calculus.

[primummobile]

now I understand what you mean by a finite but unbounded universe.

Actually, I probably shouldn't have mentioned the event horizon in that post. When I say finite but unbounded, I mean that the entirety of the universe may have a finite volume, but you can travel infinitely far in a "straight" line. That would imply some kind of curvature into either a higher dimension or curvature on a scale too large for us to detect. Whether or not the event horizon existed would not be relevent to that.

[cjameshuff]

I guess that kills off any possibility of a Pellucidar type world anywhere in the universe. Hey wait a minute! What about the Ring World novels by Larry Niven and Dyson Sphere's? People are supposed to be able to walk around the inner surfaces of those. So you are saying gravity becomes zero inside a sphere at a certain distance from its inner surfaces? Oh wait. My mistake. They used centripetal gravity.

The Ringworld was also a ring, not a sphere, though in the Ringworld's case that didn't matter very much.

But we have no idea where we are in relation to the entire universe. What we can see is quite varied. Supermassive blackholes have nearly infinite density and intergalactic space is virtually empty. The entire universe could be solid for all we know. What we call the universe might reside inside a bubble. Far fetched I know. It just seems to me that its a very big assumption to expect the whole universe to be exactly like our part. The universe could be so vast that areas of it obey different laws of physics. Its like aliens skimming earths atmosphere, taking a sample and declaring the earth must be made entirely of air.

That's kind of the point. You can come up with any number of "what ifs" for what we can't see, but you can't test them, and assuming that we are in a special location and that conditions are drastically different away from us is equivalent to assuming you can't model them from what you can see, so you may as well give up.

From what we can see, the universe appears to have variations only on relatively small scales, being quite uniform on the largest scales we can see. We have no reason to expect this to change outside of what we can see. If evidence of such variation is found, it'll hopefully narrow down the nature of the variation as well, and explain why our location is "special".

[caveman1917]

I think I know what Cougar was referring to.

Can a closed (or positively curved) universe be infinite?

Can an open (or flat) universe contract?

That would be no and yes respectively.

I think i see the misunderstanding. Cougar is referring to the point that a flat expanding universe starting from a big bang will not stop at some point and start contracting. I was referring to the point that we can just as well model a flat contracting universe that ends in a big crunch if it started out as contracting. Though it would of course not be our universe. We probably both had different interpretations of the question stated as "is it theoretically possible that...".

[WayneFrancis]

Imagine the universe collapsing inwards under its own gravitational force. The central region would be relatively unaffected at first. Gravity pulling from all directions would cancel it out. The outer edges of the universe would be different. Gravity is pulling in one direction only for the farthest galaxies. Inwards. So the collapse begins with the outermost galaxies slowing and then slowly falling inwards. As this continues over billions of years the density of the outer universe increases. This increase in universal density on the fringes would exert an increasing gravitational force on the hitherto static central region. The core is pulled outwards in all directions to meet this inward falling outer universe. If the milky way resides in the central region of the universe then we would observe an accelerating expansion of the viewable universe. Ofcourse this is what astronomers have observed.

Is this theory plausible? If not please explain why. If it is possible that the universe is collapsing in this way then could we calculate the end of the universe from the rate of expansion?

The first problem is that there is no indication that there is a "centre" as you describe. I'll go with the balloon analogy here. If the universe is just the surface of the balloon then the "centre" of the balloon doesn't represent any point in 3D space. At best it represents a point in time.

The second problem I see is that if the universe was collapsing we'd observe a blue shifting of light not red shifting. Acceleration only means the rate is increasing. It is like a car at the top of a hill. It can roll down either side either toward you or away from you but regardless of the direction it rolls in you will find the rate (speed) at which it rolls will increase until something causes the car to start slowing down like trying to climb another hill or just friction with the road.

It is very hard to get the concept of space time across to laypeople. Even harder when you consider that GR doesn't care if the universe is open and infinite in size or closed and finite. In either case we believe the universe is unbounded which is also at odds with what you are describing. The observations don't indicate a change as you describe. The larger the scale you go to the more uniform the universe seems but you're describing something that is very non uniform.

[noncryptic]

Regarding a big crunch in an infinitely large universe:

If space itself can expand, why could it in principal not contract and thus make the universe more dense over time, eventually causing conditions similar to the early epoch of the universe? The universe would remain infinite in expanse but become more dense.
All that aside from the question of whether or not there is or could be a cosmic 'contraction force' (ie negative dark energy or somesuch).

[caveman1917]

Regarding a big crunch in an infinitely large universe:

If space itself can expand, why could it in principal not contract and thus make the universe more dense over time, eventually causing conditions similar to the early epoch of the universe? The universe would remain infinite in expanse but become more dense.
All that aside from the question of whether or not there is or could be a cosmic 'contraction force' (ie negative dark energy or somesuch).

Going from expanding to contracting in a flat universe requires going through a singularity, so you won't get an expanding flat universe to stop and reverse. However you can certainly model a flat infinite universe that starts out as contracting and which would thus go towards a big crunch.

[cjameshuff]

Going from expanding to contracting in a flat universe requires going through a singularity,

I'm not sure this is what you meant to say. I see no reason for an expanding universe to have to go through a singularity to start contracting, though a contracting universe might have to do so to start expanding again. Flatness isn't even a factor.