Spacetime expanding within galaxies?

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I understand that spacetime is expanding between galaxies, as seen with the cosmological redshift. But, is it also expanding inside galaxies, such as the Milky Way?

I'm not sure if the galaxy is gravitationally bound enough to where spacetime might actually be contracting within the galaxy, or if it is still partially expanding with the surrounding void, or if it might start contracting some time in the future.

In other words, would we say that spacetime within the galaxy is contracting or expanding? And is there any evidence for either?
 

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  • #2
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Yes, space time is expanding at all levels of scale.
 
  • #3
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Yes, space time is expanding at all levels of scale.
Can you expand on that a little?

How do we know that spacetime is not contracting within galaxies?
 
  • #4
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Not as I understands it, the expansion is thought to work where the gravitational potential is low, as between galaxies? And the reason why galaxies don't expand would then be gravity. But If you mean that the expansion is a 'background process' of some kind acting 'everywhere', with gravity acting as a cohesive 'force' (buoys) keeping galaxies together?

Maybe?

But what about all the 'space' in matter then? If it was so and we assumed that space 'expanded', as points growing into 'spheres of more space' as I see nothing forbidding each new point to 'expand' too, we would have something where every point of 'space' would act as some inflationary process. It's a weird thought, then again, maybe 'space' just is a construct from what we see? In the other case we now have 'space' expanding inside us too it seems :)
==

There is one process that is complementary to this, 'matter shrinking'
 
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  • #5
tom.stoer
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Yes, space time is expanding at all levels of scale.
No, space within galaxies does not expand, simply b/c of the gravitational attraction of matter within galaxies
 
  • #6
phinds
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No, space within galaxies does not expand, simply b/c of the gravitational attraction of matter within galaxies
And I believe it's correct to say that there are even larger gravity-bound objects than galaxies, namely local clusters such as the one we are in.
 
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Bill_K
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The Friedmann-Robertson-Walker metric is defined for matter that has been uniformly smeared out, and applies on a large scale but does not apply on the scale of individual galaxies. Within gravitationally bound systems such as the Milky Way a metric related to the local distribution of matter must be used. In this local metric, distances do not increase with the overall Hubble expansion. So in that sense, space is not expanding within the Milky Way.

HOWEVER.. At each point the cosmology determines a cosmic time t, and a unique rest frame, namely the rest frame of the cosmic microwave background. And you can compare the rest frames at any two points by parallel transport along a spacelike geodesic of constant t. The rest frames will be different at every point, and recede from each other uniformly, in agreement with the Hubble expansion, even on an arbitrarily small scale. So in this sense, space is expanding within the Milky Way.
 
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Are you saying that we can measure a 'expansion' in the cosmic microwave backgrounds redshift? And that we see it 'expand' inside galaxies too? That's interesting, do you have a link to it?
 
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HOWEVER.. At each point the cosmology determines a cosmic time t, and a unique rest frame, namely the rest frame of the cosmic microwave background. And you can compare the rest frames at any two points by parallel transport along a spacelike geodesic of constant t. The rest frames will be different at every point, and recede from each other uniformly, in agreement with the Hubble expansion, even on an arbitrarily small scale. So in this sense, space is expanding within the Milky Way.
Right, I guess I should have expanded on what I said, but I was right when I said space does expand at all levels, just it is not apparent in systems bound strongly to one another?
 
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  • #10
pervect
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Didn't we have another thread on this recently?

In any event, you can't really tell if space is expanding or not by any local physical measurement. But it's still popular to use the "expanding space paradigm", and it can be made to work correctly. It can also be a source of confusion.

But rather than dismiss the idea, I'll point to a few papers that talk in depth about the issue without being too technical. (I already posted links in the othere thread, but I can't seem to find it).

"Expanding space: the root of all evil?" http://arxiv.org/abs/0707.0380

"Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe"
http://arxiv.org/abs/astro-ph/0310808
 
  • #11
tom.stoer
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And I believe it's correct to say that there are even larger gravity-bound objects than galaxies, namely local clusters such as the one we are in.
Agreed - but for clusters it may become 'fuzzy' whereas for galaxies it seems to be rather strict
 
  • #12
tom.stoer
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The Friedmann-Robertson-Walker metric is defined for matter that has been uniformly smeared out, and applies on a large scale but does not apply on the scale of individual galaxies. ...
OK

Within gravitationally bound systems such as the Milky Way a metric related to the local distribution of matter must be used. In this local metric, distances do not increase with the overall Hubble expansion.
OK

The rest frames will ... recede from each other uniformly, in agreement with the Hubble expansion, even on an arbitrarily small scale. So in this sense, space is expanding within the Milky Way.
How do you prove this?
 
  • #13
phinds
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Agreed - but for clusters it may become 'fuzzy' whereas for galaxies it seems to be rather strict
Yes, that is what I have read also. My statement was itself a little 'fuzzy'.
 
  • #14
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I understand that spacetime is expanding between galaxies, as seen with the cosmological redshift. But, is it also expanding inside galaxies, such as the Milky Way?

I'm not sure if the galaxy is gravitationally bound enough to where spacetime might actually be contracting within the galaxy, or if it is still partially expanding with the surrounding void, or if it might start contracting some time in the future.

In other words, would we say that spacetime within the galaxy is contracting or expanding? And is there any evidence for either?
Spacetime expansion does not apply to bound systems. Thus, there is expansion between galaxies, but there is not expansion within galaxies, planets, atoms...
 

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