expanding balloon model question

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ajiko - 27 Mar 2006 02:09 GMT

The expanding balloon model of the universe gives a technique to imagine the
homogeneity of the sky.

Question: Is this a solution of GR?

Using whatever model we like, we can do the following thought experiment.
Pick a direction and find a far away quasar in that general direction. What
we see is the beginning of a galaxy billions of years ago. Imagine an
astonomer billions of years later, looking out from that galaxy. If he
looks in our direction he sees our galaxy as it is forming billions of years
ago. If he looks in the opposite direction he also sees galaxies most of
which are beyond the our sight. The light of their beginnings hasn't yet
reached us. However, the homogeneous nature of the sky implies that they
exist. The thought experiment gets repeated again. An astronomer (billions
of years later) on that doubly removed galaxy looks out in the opposite
direction and finds yet another far away quasar and so on.

These are the main possibilities that I can imagine:

1) The universe can be modeled as some kind of expanding balloon and the
universe closes around on itself . This means that a far away astronomer in
the thought experiment sequence looks out and sees our galaxy from the
opposite side as our first look.

2) The universe doesn't actually close around itself. This has a finite and
an infinite variation:
1) The infinite variation is implied if the homogeneity is really
complete.
2) The finite variation implies that the homogeneity ends, either
abruptly or gradually.

In all cases, it seems, the bulk of the mass of the universe lies beyond
what we can see. Also, the bulk of the mass that becomes galaxies lies
beyond what we can see.

The current mainstream cosmology include a total mass of the universe. How
is the mass beyond our viewing horizon accounted?

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oriel36 - 27 Mar 2006 19:00 GMT

In 1920 they had no idea that things such as galaxies existed,as far as
they were concerned there was no such structures and they were
sprinkled hither and thither.

Albert thought it was a shame that light leaving stars would go to
waste if light just went in straight lines so he bent,yes he bent, the
universe to stop that from happening.

"This conception is in itself not very satisfactory. It is still less
satisfactory because it leads to the result that the light emitted by
the stars and also individual stars of the stellar system are
perpetually passing out into infinite space, never to return, and
without ever again coming into interaction with other objects of
nature. Such a finite material universe would be destined to become
gradually but systematically impoverished."

http://www.bartleby.com/173/30.html

This is genuinely the funniest thing I have ever read and it is so dumb
that I dare not try to convince people least they attempt to justify
the 1920 relativistic reasoning.

It is just that funny on its own notwithstanding that the poor guy
excludes the notion of stellar island structures we now call galaxies -

"This view is not in harmony with the theory of Newton. The latter
theory rather requires that the universe should have a kind of centre
in which the density of the stars is a maximum, and that as we proceed
outwards from this centre the group-density of the stars should
diminish, until finally, at great distances, it is succeeded by an
infinite region of emptiness. The stellar universe ought to be a finite
island in the infinite ocean of space."

Poor Albert was using Isaac too much for Newton's idea of the rest of
the Universe says nothing of centers and does even require one -

"Cor. 2. And since these stars are liable to no sensible parallax from
the annual motion of the earth, they can have no force, because of
their immense distance, to produce any sensible effect in our system.
Not to mention that the fixed stars, every where promiscuously
dispersed in the heavens, by their contrary actions destroy their
mutual actions, by Prop. LXX, Book I." NEWTON

You can enjoy the charade behind relativity up to a point but
ultimately it all goes back to Isaac and his stupid attempt to fit
astronomy into his ballistic agenda otherwise known as the 'universal
law of gravitation' [all bow].

I love living in the astronomical freedom of the 21st century .

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Ben Rudiak-Gould - 31 Mar 2006 14:40 GMT

> These are the main possibilities that I can imagine:
>
[quoted text clipped - 9 lines]
> 2) The finite variation implies that the homogeneity ends, either
> abruptly or gradually.

All correct.

> In all cases, it seems, the bulk of the mass of the universe lies beyond
> what we can see. Also, the bulk of the mass that becomes galaxies lies
> beyond what we can see.

Not necessarily; the universe might be smaller than the visible universe, in
which case we're seeing several copies of the whole universe. But recent
analysis of the CMBR does seem to rule this out.

> The current mainstream cosmology include a total mass of the universe. How
> is the mass beyond our viewing horizon accounted?

Any "mass of the universe" figure you've seen is really the mass of/in the
visible universe. Popular articles very often incorrectly say "the universe"
when they mean "the visible universe".

The density of the universe is a more sensible measure.

-- Ben

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Hexenmeister - 31 Mar 2006 14:47 GMT

| > These are the main possibilities that I can imagine:
| >
[quoted text clipped - 11 lines]
|
| All correct.

"Can be" is not in question. Elephants can be camels.

| > In all cases, it seems, the bulk of the mass of the universe lies beyond
| > what we can see. Also, the bulk of the mass that becomes galaxies lies
[quoted text clipped - 12 lines]
|
| The density of the universe is a more sensible measure.

Yeah... zero. Very sensible.
Androcles.

| -- Ben

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Natural Science Forum / Physics / Relativity / March 2006

expanding balloon model question