What Does Gravity Have To Do WithThe Expanding Universe?

[polymath257]

In the standard BB description, there are no 'outer limits'. The universe is always spatially homogeneous: the same in all directions. Even if space is finite in extent, there are no boundaries in the way you seem to be asking.

If space extends beyond the universe, into which it is supposed to be expanding, then it must have caused that space in the first instance, or even while it is currently expanding.

Once again, you have an image of some boundary expanding outwards. That is NOT what the BB model says. There is no 'beyond the universe' spatially. Instead, wherever you are, the basic 'look and feel' is the same: galaxies in all directions.

The universe isn't 'expanding into' some space beyond the universe (even in models where space is finite). Instead, the distances between galaxies *within* the universe are getting larger over time.

In a technical sense, the universe is literally expanding into the future, not into more space.

[vulcanlogician]

If space extends beyond the universe, into which it is supposed to be expanding, then it must have caused that space in the first instance, or even while it is currently expanding.

Once again, you have an image of some boundary expanding outwards. That is NOT what the BB model says. There is no 'beyond the universe' spatially. Instead, wherever you are, the basic 'look and feel' is the same: galaxies in all directions.

The universe isn't 'expanding into' some space beyond the universe (even in models where space is finite). Instead, the distances between galaxies *within* the universe are getting larger over time.

In a technical sense, the universe is literally expanding into the future, not into more space.

I think I get what you mean. The universe expands, but not into something. But I'm curious, according to the big bang model (and assuming the universe is finite), is there a place "on the edge" of the universe where one could stand and see the universe in front of him, but if he turns 180 degrees he will see nothing but an abyss? Or does that question even have an answer?

[Anomalocaris]

Once again, you have an image of some boundary expanding outwards. That is NOT what the BB model says. There is no 'beyond the universe' spatially. Instead, wherever you are, the basic 'look and feel' is the same: galaxies in all directions.

The universe isn't 'expanding into' some space beyond the universe (even in models where space is finite). Instead, the distances between galaxies *within* the universe are getting larger over time.

In a technical sense, the universe is literally expanding into the future, not into more space.

I think I get what you mean. The universe expands, but not into something. But I'm curious, according to the big bang model (and assuming the universe is finite), is there a place "on the edge" of the universe where one could stand and see the universe in front of him, but if he turns 180 degrees he will see nothing but an abyss? Or does that question even have an answer?

That depends on your definition of “universe”. If universe is taken to mean the set of all space and time, then obviously no where can you see what is not in the set that includes all by definition. But if by universe, you mean only a particular subset of space matching certain criteria, then the answer depends on the criteria.

[polymath257]

Once again, you have an image of some boundary expanding outwards. That is NOT what the BB model says. There is no 'beyond the universe' spatially. Instead, wherever you are, the basic 'look and feel' is the same: galaxies in all directions.

The universe isn't 'expanding into' some space beyond the universe (even in models where space is finite). Instead, the distances between galaxies *within* the universe are getting larger over time.

In a technical sense, the universe is literally expanding into the future, not into more space.

I think I get what you mean. The universe expands, but not into something. But I'm curious, according to the big bang model (and assuming the universe is finite), is there a place "on the edge" of the universe where one could stand and see the universe in front of him, but if he turns 180 degrees he will see nothing but an abyss? Or does that question even have an answer?

The answer to that is NO. yes, the question makes sense. NO, there is no such place.

In this case, a finite universe would have the property that no matter which direction you decide to go (up, down, left, right, forward, backward, etc), if you keep on going, you will eventually circle back to your initial position. Think of the surface of the Earth on this. No matter which direction you go, if you keep going that direction, you will eventually go all the way around the Earth. In a universe with a finite amount of space, the same thing happens only in all 3 dimensional directions.

There is no boundary in the BB model.

[vulcanlogician]

I think I get what you mean. The universe expands, but not into something. But I'm curious, according to the big bang model (and assuming the universe is finite), is there a place "on the edge" of the universe where one could stand and see the universe in front of him, but if he turns 180 degrees he will see nothing but an abyss? Or does that question even have an answer?

That depends on your definition of “universe”.   If universe is taken to mean the set of all space and time, then obviously no where can you see what is not in the set that includes all by definition.  But if by universe, you mean only a particular subset of space matching certain criteria, then the answer depends on the criteria.

I meant a particular spot in space in the known universe at a particular time.

[Anomalocaris]

think I get what you mean. The universe expands, but not into something. But I'm curious, according to the big bang model (and assuming the universe is finite), is there a place "on the edge" of the universe where one could stand and see the universe in front of him, but if he turns 180 degrees he will see nothing but an abyss? Or does that question even have an answer?

That depends on your definition of “universe”.   If universe is taken to mean the set of all space and time, then obviously no where can you see what is not in the set that includes all by definition.  But if by universe, you mean only a particular subset of space matching certain criteria, then the answer depends on the criteria.

I meant a particular spot in space in the known universe at a particular time.

I know.  But for that spot to be “on the edge”, presumably spatially as you mention the edge be be characterized by rotational asymmetry in the sense rotate one way and there is universe in front of you, rotate 180 degrees and there is not, then there must be something on the other side of the edge that is not part of the universe.   Thus the issue comes back to how do you define universe in such a way that there can be, for the lack of a better term, regions, that are not part of the universe.

A commonly used definition of universe is all of space spatially and all of time temporally. In such a case spatial edge of universe appear to have no meaning.

You may say is there somewhere that, if you rotate one way, then there are familiar stars, galaxies, and galaxy clusters in front of you, and rotate another and there will by no stars, galaxies and galaxies no matter how far you look. In this case, you are not really at the edge of the universe. You are at the edge of a particular region of a edgeless but non-homogenous universe. We can not really exclude the possibility that the universe may be nonhomogenous in this way. However, we are pretty sure if it is nonhomogenous this way, then the region of the universe that resembles our familiar region is likely to be much larger than the observable part of the universe.

[vulcanlogician]

think I get what you mean. The universe expands, but not into something. But I'm curious, according to the big bang model (and assuming the universe is finite), is there a place "on the edge" of the universe where one could stand and see the universe in front of him, but if he turns 180 degrees he will see nothing but an abyss? Or does that question even have an answer?

I meant a particular spot in space in the known universe at a particular time.

I know.  But for that spot to be “on the edge”, then there must be something on the other side of the edge that is not part of the universe.   Thus the issue comes back to how do you define universe in such a way that there can be, for the lack of a better term, regions, that are not part of the universe.

Right, I get it. It's like walking on the sphere of the Earth like Polymath said... except you can't leave the surface of the sphere. It's hard to wrap your mind around.

[Banned]

If space extends beyond the universe, into which it is supposed to be expanding, then it must have caused that space in the first instance, or even while it is currently expanding.

Once again, you have an image of some boundary expanding outwards. That is NOT what the BB model says. There is no 'beyond the universe' spatially. Instead, wherever you are, the basic 'look and feel' is the same: galaxies in all directions.

The universe isn't 'expanding into' some space beyond the universe (even in models where space is finite). Instead, the distances between galaxies *within* the universe are getting larger over time.

In a technical sense, the universe is literally expanding into the future, not into more space.

The relation between space and time that you've bought to view rings true.
You explain things well.

So if the universe is expanding into the future - do you think this feature gives space an apparent infinity?

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...We can not really exclude the possibility that the universe may be nonhomogenous in this way. However, we are pretty sure if it is nonhomogenous this way, then the region of the universe that resembles our familiar region is likely to be much larger than the observable part of the universe.

I think that the universe is heterogeneous in its space time properties, which the variety of space time curves seem to indicate. Partly because energy is typically expressed by a distribution of highs and lows, whether in wave or spacial form.

Do you think that if the universe has an edge, that it may be impossible to reach that point, because it has properties which cannot be perceived or experienced by anything that "resembles our familiar region." ?

[polymath257]

think I get what you mean. The universe expands, but not into something. But I'm curious, according to the big bang model (and assuming the universe is finite), is there a place "on the edge" of the universe where one could stand and see the universe in front of him, but if he turns 180 degrees he will see nothing but an abyss? Or does that question even have an answer?

I meant a particular spot in space in the known universe at a particular time.

I know.  But for that spot to be “on the edge”, presumably spatially as you mention the edge be be characterized by rotational asymmetry in the sense rotate one way and there is universe in front of you, rotate 180 degrees and there is not, then there must be something on the other side of the edge that is not part of the universe.   Thus the issue comes back to how do you define universe in such a way that there can be, for the lack of a better term, regions, that are not part of the universe.

A commonly used definition of universe is all of space spatially and all of time temporally.  In such a case spatial edge of universe appear to have no meaning.

You may say is there somewhere that, if you rotate one way, then there are familiar stars, galaxies, and galaxy clusters in front of you, and rotate another and there will by no stars, galaxies and galaxies no matter how far you look.   In this case, you are not really at the edge of the universe.  You are at the edge of a particular region of a edgeless but non-homogenous universe.   We can not really exclude the possibility that the universe may be nonhomogenous in this way.  However, we are pretty sure if it is nonhomogenous this way, then the region of the universe that resembles our familiar region is likely to be much larger than the observable part of the universe.

Exactly. Almost all models from the basic BB viewpoint assume spatial homogeneity except for some perturbations. Small variances from complete homogeneity certainly happen and those variances do, in fact, show themselves on the background radiation.

But, as far as we have been able to observe, there is no 'edge' in the sense of all matter behind me and nothing in front. And, if there is such an edge, it is several times the current size of the observable universe (otherwise the gravitational effects during the early universe would have left signals).

[Rhondazvous]

In the standard BB description, there are no 'outer limits'. The universe is always spatially homogeneous: the same in all directions. Even if space is finite in extent, there are no boundaries in the way you seem to be asking.

This..

Though I can how understand someone with a passing familiarity with cosmology might misunderstand what the observational evidence tells us.

The observational evidence can hardly begin to help us understand the universe. If we could pinpoint the center of the universe from which it all began to expand, we'd understand a lot more. But shifting time/space has obliterated the center and ergo the boundary.