Spacetime expanding within galaxies?

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Discussion Overview

The discussion revolves around the nature of spacetime expansion within galaxies, particularly questioning whether spacetime is expanding or contracting inside gravitationally bound systems like the Milky Way. Participants explore the implications of gravitational forces on spacetime dynamics and seek evidence for their claims.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that spacetime is expanding at all levels of scale, including within galaxies.
  • Others argue that spacetime does not expand within galaxies due to the gravitational attraction of matter, suggesting that expansion occurs primarily in regions of low gravitational potential, such as between galaxies.
  • A participant mentions the Friedmann-Robertson-Walker metric, stating it applies on a large scale but not within individual galaxies, where a different local metric must be used.
  • There is a suggestion that while local measurements cannot definitively show expansion, the "expanding space paradigm" remains popular and can be useful, albeit potentially confusing.
  • Some participants express uncertainty about the implications of expansion, questioning how it relates to the local distribution of matter and the concept of "matter shrinking."
  • A later reply discusses the idea that while distances do not increase with Hubble expansion in gravitationally bound systems, there may still be a form of expansion at a local level, as indicated by the behavior of rest frames in cosmology.
  • One participant requests evidence for the claim that expansion can be measured within galaxies, indicating a desire for further clarification on the topic.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether spacetime is expanding or contracting within galaxies. Multiple competing views remain, with some asserting expansion at all scales and others maintaining that gravitational binding prevents expansion within galaxies.

Contextual Notes

Participants note that the discussion is complex and may involve unresolved assumptions regarding the nature of spacetime and gravitational effects. The relationship between local measurements and cosmological expansion remains a point of contention.

cbd1
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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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Yes, space time is expanding at all levels of scale.
 
Vorde said:
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?
 
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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Vorde said:
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
 
tom.stoer said:
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.
 
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.
 
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?
 
Bill_K said:
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
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
phinds said:
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
Bill_K said:
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

Bill_K said:
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

Bill_K said:
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
tom.stoer said:
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
cbd1 said:
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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