From discussions that we have been having people have been claiming that the mainstream view is that space expands between galaxies at a constant rate everywhere. But this is not true for gravitaionally bound objects.

Now ... I was thinking and shouldnt there be some gravitational pull everywhere in space? Grant it some places have less than others but all places even in the remote voids of space there is some gravity.

This being said wouldnt that mean that some places are expanding slower than others depending on how much gravity that part of space is exposed to?

Or is it like a binary option ... either expanding at the rate of expansion of the universe or not. I think for the BBers you need this constant to explain the redshift properly.

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I intrude to point out that this isn't what "people have been claiming". They've been pointing out that when the universe expands, gravitationally and electromagnetically bound structures don't expand along with it.
Now I'm gone again.

Grant Hutchison

OK ... again my questions is does gravitationally bound means without effects from gravity? Or just that they are pulling each other together?

If the former then as a follow up doesnt the gravitational effects go on forever?

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Well, I agree it is a "fuzzy" kind of definition, but to be gravitationally bound means gravity keeps objects in orbit around in each other (the minimum distance between them is not growing with the expansion, but it might settle at a larger size due to a change in the rate of expansion).

Whereas with non-bound or unbound objects, the rate of expansion easily overwhelms their mutual gravitational attaction, causing distances to increase between them as the expansion continues. There will still be gravitational influence between those objects but not enough to counteract the expansion, just to slow it, and as time goes on their mutual influence slows the expansion less and less.

Gravity exerts its influence at the speed of light and so has similar properties to light in that, over a certain distance away, the gravitational influence of a distant object is actually receding from us from our point of view as it propagates towards us from the point of view of its source. In a universe with accelerating expansion there are galaxies out there whose gravitational influence will never reach us.

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OK so you agree that although there seems to be a tendecy for empty space to expand equally everywhere, however in reality the universe could not be expanding consistantly because gravity at all points counteracts the expansion at all points. Right?

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tommac: I think I may have linked to one of these papers elsewhere, but I am not sure you read it. I will link to three papers now, and I suggest you read them and the references therein.

Link 1

Link 2

Link 3

These links provide the abstracts of the papers, and also links to the full articles, which are available freely from the arxiv e-print links.

Tom,

The expansion appears to pull apart any two objects which are not
gravitationally bound to each other. Before the acceleration of the
expansion was discovered in 1998, it was not known whether the
Universe as a whole was gravitationally bound (that is, whether every
bit of matter is gravitationally bound to every other bit of matter).
Now that we know the expansion is accelerating, we know that the
Universe as a whole is not gravitationally bound, so galaxy clusters
will fly apart forever.

Whether galaxy clusters, galaxies, and all the things in galaxies hold
together forever depends on the nature of the "dark energy" that is
accelerating the expansion. If its strength increases with time, it
could eventually rip everything apart. If its strength is constant, it
won't rip anything apart that it hasn't already.

The strength of the dark energy might vary from place-to-place as
you suggest, but there is no clear evidence of that.

-- Jeff, in Minneapolis

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OK ... we can put dark energy into the equation if you like but my initial proof was manly focused on the expansion caused from the BB.

The point is that there are spaces between galaxies that are huge ... so one would think that the voids that the larger voids would expand faster than relatively tiny voids.

But it seems like you are saying that even in these large voids there is lots of dark energy.

So does the existence of dark energy negate any pushing apart of things left over from the BB?

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Sorry matt. I will read them today.

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Do I need to pay to read these?

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I am very interested in #2 as I see:

This seems consistant with my beliefs.

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I agree, Grant.

tommac, I agree with Grant. Instead of trying to reinvent the world of astrophysics over the last several decades (and perhaps 50,000 contributors), please take some time to learn what's already been accomplished before posting your ideas.

Thanks.

- Mugs

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I am human. Fully human.

OK I will accept it on blind faith ... Now can you tell me what are bound? what are not bound?

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Bound - Space with Mass that has a gravity wells influence.

Not Bound - Space without Mass or a gravity wells influence.

I am not really concerned with what you believe, that doesn't really matter too much in science unless it is based on a sound understanding of the theories and evidence supporting those theories. So, can you demonstrate to me your understanding of why objects which are receding due to cosmological expansion can be blueshifted, rather than just quoting a sentence from the abstract?

By the way, if you reread my intial post, you'll find the answer as to whether or not you have to pay for the articles.

And the answer is no, they are free. Just click on the link that says "arXiv e-print" and then click the link on the right that says "Download: PDF."

In response to the original question regarding whether "the expansion of space is constant", I would like to point out that the Hubble parameter (note I said parameter, not constant), which describes how the rate of change of distance between objects evolves with time (or redshift in the following) is described at the wikipedia website here (look about halfway down the page for the derivation of the Hubble parameter).

You'll also find that Tim Thompson has already defined "gravitationally bound" for you on one of your other threads, here: