Is the universe bounded or unbounded?

[otseng]

In the Light, stars, and creationism thread, I proposed a theory to reconcile a young earth with being able to see stars that are billions of light years away. The theory assumes that the Big Bang is true, however, it also assumes that the universe is bounded. In typical cosmology, it is assumed that the universe is unbounded.

Bounded means that the universe has a boundary to it. There exists an "edge" to the universe in which beyond this boundary, our universe does not exist.

In an unbounded universe, there is no "edge". The universe "wraps" around itself. So, if you are to go in any direction in a straight line, you will eventually come back to the starting point.

This is hard to conceptualize, but can be explained like a surface of a sphere. On the surface of a sphere, if you start at any point and then go in a straight line, you will eventually come back to the starting point. Now, instead a 2-D surface on a sphere, the universe is a 3-D topology that curves in on itself.

The ramifications of either of these two assumptions make for drastically different cosmological conclusions.

So, the questions are:
1. Is the universe bounded or unbounded? Why?
2. What are the ramifications of whether it is bounded or unbounded?

[otseng]

But I guess you might want to say that the reason we see galaxies moving away from us in all directions is not because space is expanding (i.e. the raisins in cookie dough model) but because our galaxy was the centre of a big explosion in space.

This is one of those questions that gets raised all the time. Here's a typical response from a physics FAQ

I think it gets raised all the time because it remains unclear.

First problem is that the expanding balloon analogy would only work if the universe was non-Euclidean. If measurements determined that the universe was non-Euclidean, the analogy would then have merit.

Second is what evidence suggests that space is expanding, rather than matter in space?

Another thing. If something is not detectable (either directly or indirectly), of what value would it be in our discussions? And how would it be falsifiable?

Effectively you just asked how we could falsify something that was unfalsifiable! I would say that any idea that isn't, in principle, falsifiable should not be taken seriously. Ideas that, in practice, aren't falsifiable need more work doing on them before they're taken seriously.

Exactly what I was getting at.

[otseng]

If the big bang were an ordinary explosion in an already existing space we would be able to look out and see the expanding edge of the explosion with empty space beyond.

If it was empty space beyond, there would be nothing to see. Further, the Big Bang is no ordinary "explosion". I do not think comparing the Big Bang to any type of explosion on Earth would be appropriate.

Instead we see back towards the big bang itself and detect a faint background glow from the hot primordial gases of the early universe. This Cosmic Microwave Background Radiation (CMBR) is uniform in all directions. This tells us that it is not matter which is expanding outwards from a point but rather, it is space itself which expands evenly.

I do not understand this. How does the CMBR show that space is expanding?

It is important to stress that other observations support the view that there is no centre to the universe, at least in so far as observations can reach. The fact that the universe is expanding uniformly would not rule out the possibility that there is some denser, hotter place that might be called the centre, but careful studies of the distribution and motion of galaxies confirm that it is homogeneous on the largest scales we can see, with no sign of a special point to call the centre.

Why is it so important to stress that there is no center? Perhaps because it would falsify the Copernican principle?

Effectively you just asked how we could falsify something that was unfalsifiable! I would say that any idea that isn't, in principle, falsifiable should not be taken seriously. Ideas that, in practice, aren't falsifiable need more work doing on them before they're taken seriously.

If the universe has a large radius beyond any detection, then we'll need to explore other possibilities. Are there any other models that would be Euclidean and also unbounded?

[otseng]

Two things that come to mind. Faster than light travel is possible. It would just take more energy than is available in our universe. There could then be interesting ideas to come from the question of FTL travel.

If it would take more energy than is available in our universe, then FTL travel would be impossible in our universe.

It is not material and can hardly be said to bound the universe because at a local level we could travel beyond this limit in space. There is nothing preventing it.

True. If an observer goes beyond an existing boundary, the observer would then be the boundary. The boundary would be defined as the edge of the outermost matter/energy, which would also include observers.

[Greatest I Am]

Two things that come to mind. Faster than light travel is possible. It would just take more energy than is available in our universe. There could then be interesting ideas to come from the question of FTL travel.

If it would take more energy than is available in our universe, then FTL travel would be impossible in our universe.

It is not material and can hardly be said to bound the universe because at a local level we could travel beyond this limit in space. There is nothing preventing it.

True. If an observer goes beyond an existing boundary, the observer would then be the boundary. The boundary would be defined as the edge of the outermost matter/energy, which would also include observers.

Where is it writen that some day we will not be able to tap into another universe if we live in a multi universe realm. This has not been proven as yet. This would be the source of energy required for FTL travel.

There is also the hope that someone will best Einsteins theories and allow for FTL.

I hate to give up at this early stage of knowledge of possible FTL travel.

The universe will become restrictive for man if Einstein is the final world.

Regards
DL

[Goat]

Two things that come to mind. Faster than light travel is possible. It would just take more energy than is available in our universe. There could then be interesting ideas to come from the question of FTL travel.

If it would take more energy than is available in our universe, then FTL travel would be impossible in our universe.

It is not material and can hardly be said to bound the universe because at a local level we could travel beyond this limit in space. There is nothing preventing it.

True. If an observer goes beyond an existing boundary, the observer would then be the boundary. The boundary would be defined as the edge of the outermost matter/energy, which would also include observers.

Where is it writen that some day we will not be able to tap into another universe if we live in a multi universe realm. This has not been proven as yet. This would be the source of energy required for FTL travel.

There is also the hope that someone will best Einsteins theories and allow for FTL.

I hate to give up at this early stage of knowledge of possible FTL travel.

The universe will become restrictive for man if Einstein is the final world.

Regards
DL

Actually, the theory is things can't move 'at the speed of light'. It is 'theortically' possible to go faster than light, just not at light speed.

[QED]

But I guess you might want to say that the reason we see galaxies moving away from us in all directions is not because space is expanding (i.e. the raisins in cookie dough model) but because our galaxy was the centre of a big explosion in space.

This is one of those questions that gets raised all the time. Here's a typical response from a physics FAQ

I think it gets raised all the time because it remains unclear.

First problem is that the expanding balloon analogy would only work if the universe was non-Euclidean. If measurements determined that the universe was non-Euclidean, the analogy would then have merit.

I wonder why you say this? It works for a flat sheet of rubber just as well -- only we don't have a familiar model like the balloon for visualizing a sheet that gets uniformly stretched in all directions. Incidentally, I prefer the "cookie dough" model over the balloon because the expansion takes place in all three dimensions.

Second is what evidence suggests that space is expanding, rather than matter in space?

George Smoot and his COBE team provided that just seven years ago:

Taken together, the only theory which coherently explains these phenomena relies on space expanding through a change in metric. Interestingly, it was not until the discovery in the year 2000 of direct observational evidence for the changing temperature of the cosmic microwave background that more bizarre constructions could be ruled out. Until that time, it was based purely on an assumption that the universe did not behave as one with the Milky Way sitting at the middle of a fixed-metric with a universal explosion of galaxies in all directions (as seen in, for example, an early model proposed by Milne).

Additionally, scientists are confident that the theories which rely on the metric expansion of space are correct because they have passed the rigorous standards of the scientific method. In particular, when physics calculations are performed based upon the current theories (including metric expansion), they appear to give results and predictions which, in general, agree extremely closely with both astrophysical and particle physics observations. The spatial and temporal universality of physical laws was until very recently taken as a fundamental philosophical assumption that is now tested to the observational limits of time and space. This evidence is taken very seriously because the level of detail and the sheer quantity of measurements which the theories predict can be shown to precisely and accurately match visible reality. The level of precision is difficult to quantify, but is on the order of the precision seen in the physical constants that govern the physics of the universe.

I'd like to think that you had an appreciation of the care that goes into the science behind these sorts of conclusions. You could get this from reading George's book Wrinkles in Time which we've discussed before.

[QED]

Incidentally otseng, if you're thinking that galactic red-shift is due to a "conventional style explosion" centered on our galaxy, projecting galaxies away from us in all directions, then I'm wondering how you account for Hubble's law relating the degree of redshift to the distance of the galaxies?

[Greatest I Am]

Two things that come to mind. Faster than light travel is possible. It would just take more energy than is available in our universe. There could then be interesting ideas to come from the question of FTL travel.

If it would take more energy than is available in our universe, then FTL travel would be impossible in our universe.

It is not material and can hardly be said to bound the universe because at a local level we could travel beyond this limit in space. There is nothing preventing it.

True. If an observer goes beyond an existing boundary, the observer would then be the boundary. The boundary would be defined as the edge of the outermost matter/energy, which would also include observers.

Where is it writen that some day we will not be able to tap into another universe if we live in a multi universe realm. This has not been proven as yet. This would be the source of energy required for FTL travel.

There is also the hope that someone will best Einsteins theories and allow for FTL.

I hate to give up at this early stage of knowledge of possible FTL travel.

The universe will become restrictive for man if Einstein is the final world.

Regards
DL

Actually, the theory is things can't move 'at the speed of light'. It is 'theortically' possible to go faster than light, just not at light speed.

Strange.

Not to doubt but how do you go FTL if you cannot go through or pass light speed?
I thought that the only requirement was enough energy.

Regards
DL

[Goat]

Two things that come to mind. Faster than light travel is possible. It would just take more energy than is available in our universe. There could then be interesting ideas to come from the question of FTL travel.

If it would take more energy than is available in our universe, then FTL travel would be impossible in our universe.

It is not material and can hardly be said to bound the universe because at a local level we could travel beyond this limit in space. There is nothing preventing it.

True. If an observer goes beyond an existing boundary, the observer would then be the boundary. The boundary would be defined as the edge of the outermost matter/energy, which would also include observers.

Where is it writen that some day we will not be able to tap into another universe if we live in a multi universe realm. This has not been proven as yet. This would be the source of energy required for FTL travel.

There is also the hope that someone will best Einsteins theories and allow for FTL.

I hate to give up at this early stage of knowledge of possible FTL travel.

The universe will become restrictive for man if Einstein is the final world.

Regards
DL

Actually, the theory is things can't move 'at the speed of light'. It is 'theortically' possible to go faster than light, just not at light speed.

Strange.

Not to doubt but how do you go FTL if you cannot go through or pass light speed?
I thought that the only requirement was enough energy.

Regards
DL

There is the theoretical partial of tachyons. I think the work on it had too many assumptions , but it is there never the less.

[otseng]

I wonder why you say this? It works for a flat sheet of rubber just as well -- only we don't have a familiar model like the balloon for visualizing a sheet that gets uniformly stretched in all directions. Incidentally, I prefer the "cookie dough" model over the balloon because the expansion takes place in all three dimensions.

Wouldn't though a flat sheet of rubber be more consistent with an Euclidean bounded geometry? Wouldn't also the cookie dough analogy be bounded?

Additionally, scientists are confident that the theories which rely on the metric expansion of space are correct because they have passed the rigorous standards of the scientific method.

I read through the wiki article, but I still don't see the observational evidence of the metric expansion of space.

Incidentally otseng, if you're thinking that galactic red-shift is due to a "conventional style explosion" centered on our galaxy, projecting galaxies away from us in all directions, then I'm wondering how you account for Hubble's law relating the degree of redshift to the distance of the galaxies?

It could simply be that distant objects are moving faster instead of the space between expanding.