# Is the expansion of the Universe limited to voids?

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I do not think of a centre.

We can consider gravitationally bound systems as stable and the regions between them as expanding.

We can also consider gravitationally bound systems and everything within them as contracting and the regions between them as stable.

The Universe looks the same in these cases.

That is not so easy to avoid a centre.

If you want to scale an object, you can use for example its center of mass. Or you can use any other random point anywhere. The result of the scaling will be the same, but the position in space of the scaled object (its coordinates) will be different. I am not aware of the possibility of scaling an object without a reference point, at least not in Euclidian geometry.

Say object A is scaled 1/2, and object B is scaled 1/2 too. Now, if you want objects A & B to remain in proportion, as you expected, both objects A & B must share a same scaling center. If center of scaling for A is in its center of mass, and B in its center of mass too, after scaling, objects A & B will not be under the same proportions anymore: they will feel like getting away from each other.

Inversely, if scaling is expanding, objects A & B scaled with respective centers of mass will eventually collide.

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Note; I am playing devil's advocate since I share your point of vue. you can answer to my objection saying that "scaling happens everywhere, there is no centre" the same way cosmologists explain that "expansion happens everywhere, there is no centre". But i would prefer a geometrical answer.

Edited by michel123456
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I think you would have to differentiate between scaling and expansion.Scale does require a point of reference,while expansion does not.When an object is scaled it is always proportional in all directions.Expansion does not follow this rule,in fact expansion usually takes the easiest path possible all the time.In the case of the expansion of the universe we can't really determine where those paths are.

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That is not so easy to avoid a centre.

If you want to scale an object, you can use for example its center of mass. Or you can use any other random point anywhere. The result of the scaling will be the same, but the position in space of the scaled object (its coordinates) will be different. I am not aware of the possibility of scaling an object without a reference point, at least not in Euclidian geometry.

Say object A is scaled 1/2, and object B is scaled 1/2 too. Now, if you want objects A & B to remain in proportion, as you expected, both objects A & B must share a same scaling center. If center of scaling for A is in its center of mass, and B in its center of mass too, after scaling, objects A & B will not be under the same proportions anymore: they will feel like getting away from each other.

Inversely, if scaling is expanding, objects A & B scaled with respective centers of mass will eventually collide.

----------------

Note; I am playing devil's advocate since I share your point of vue. you can answer to my objection saying that "scaling happens everywhere, there is no centre" the same way cosmologists explain that "expansion happens everywhere, there is no centre". But i would prefer a geometrical answer.

You are basically right, but the problem exists in both alternatives (as you seem to imply yourself).

In the contraction alternative, proportionality within gravitationally bound systems is not the problem, but in order to calculate the distances between galaxies that are not gravitationally bound to each other, some points have to be considered as central.

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I think you would have to differentiate between scaling and expansion.Scale does require a point of reference,while expansion does not.When an object is scaled it is always proportional in all directions.Expansion does not follow this rule,in fact expansion usually takes the easiest path possible all the time.In the case of the expansion of the universe we can't really determine where those paths are.

I guess one could use the balloon analogy inversed (in contraction) in order to solve the problem.

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I like that Iggy - but surely that only works when you can assume isotropy and homogeneity. Of course on a small scale this is not a valid assumption and one cannot use shell theorem and others to simplify matters.

I think you make a good point.

But, I believe homogeneity is a typical simplifying assumption with this type of thing -- even at quite small scales. The Jeans length for example, is often applied to molecular clouds and other relatively small things.

The large sphere that fits between andromeda and the milky way may well have a calculated critical density far greater than the measured mass/volume - but it ain't gonna expand because of the milky way and andromeda

I agree. That is a good counterexample.

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But, I believe homogeneity is a typical simplifying assumption with this type of thing -- even at quite small scales. The Jeans length for example, is often applied to molecular clouds and other relatively small things.

Is there a critical radius greater than which you can assume both homogeneity and isotropy in the universe ( the i i radius?)

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Is there a critical radius greater than which you can assume both homogeneity and isotropy in the universe ( the i i radius?)

Not that I've heard of.

At larger and larger scales, homogeneity is a better and better approximation. When it becomes a 'good' approximation would very much depend on how good of an approximation you need... which would be situation specific and not something that could be declared in general.

Edited by Iggy
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IMHO, the universe is part of an infinite chain of higher entities. Just youtube Michio Kaku, on aliens. He feels that the universe may actually be part of some multi-verse, which contains many universes. And it goes on. So, for example, Earth<Solar System<Orion Arm<Milky Way<The Local Group<The Local Supercluster<Our Universe<Our Multi-verse<our xxx-verse and so on. So what I think is that expansion of the Universe is constantly being limited by the size of our Multi-verse and the growth of other Universes within our multi-verse. Note that simpler logic and no curving of spacetime is used as I simply am too dumb to actually apply them. Others are free to attack my thoughts, but that is just my opinion.

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Since space has no observable boundary, but the fabric of space-time cannot occupy infinite space because it was created only a limited time ago and expands at a limited speed, doesn't that mean the universe is expanding into an infinite void since we can't find a boundary of where it stops?

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