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Expanding space thru matter vs matter thru space

  1. Sep 5, 2014 #1
    So the universe is expanding, rather space is expanding. By expanding we mean space is coming into existence at all points.

    Is that an equal rate of expansion everywhere?

    Now the expansion does not over come the force holding matter together, planetary systems, galaxies even galactic clusters to some extent. Larger scales than that and things are moving apart.

    Is it that space is not expanding within the smaller structures or is space expanding through these structures?
  2. jcsd
  3. Sep 6, 2014 #2


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    Expansion is generally believed to occur on cosmological distance scales - as in mega light years. There is some question if expansion is uniform everywhere in the universe, but, little evidence to suggest it is not isotropic and homogenous. Expansion is generally not viewed as having any effect at mere galactic scale distances.
  4. Sep 6, 2014 #3
    It is detectable at cosmological distances. If it is an emergent property, that does not mean it is not happening everywhere around us right now. I suspect there is not way to detect the creation or expansion of space other than red shift.

    You said that it was not homogeneous. What is preventing space from expanding homogeneously?

    If it is not homogeneous, perhaps as a result of dark matter. Dark matter is clumped with galaxy clusters and likewise expansion is not detected within these scales.
    Last edited: Sep 6, 2014
  5. Sep 7, 2014 #4


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    The easy answer is gravity overpowers expansion on 'small' scales.
  6. Sep 8, 2014 #5
    Perhaps there is a way to explore the affects of global expansion at small scales. If the dynamic properties of global expansion turn out to be the result of some local spatial phenomena that is uniformly present throughout the universe, then there may be a detectable collateral affect upon matter at small scales that is not seen as expansion, but as something else. At this point there seems to be no consensus as to what causes expansion, the issue being further complicated by the fact that the rate of expansion appears to change with time.
  7. Sep 8, 2014 #6
    Could space be growing rather then expanding? What's the difference? I find the term expanding a bit of a stretch. What if the material (space) between the galaxies is reproducing itself and carrying matter as it grows. This would be quite different then expansion.
    Last edited: Sep 8, 2014
  8. Sep 8, 2014 #7

    There seems to be no end to the number of theories, and correspondingly no predictions from any theory that can be tested. I would guess that most theorists lean in the direction of some form of linked entities, e.g., micro strings that somehow are capable of stretching or expanding. Einstein in later life shifted toward the idea of an ether, he seemed to think that General Relativity required something in the nature of reactance - but he was not willing to accord the idea of motion to it. I am not sure when he made that exception whether he was referring to uniform motion or accelerated motion. As is well known, uniform motion wrt to space is not detectable, and presumably uniform motion of space wrt to a mass is not detectable. But Einstein did convey the idea that an object at rest in the universe would feel a force if the universe were accelerated wrt to the object. So its hard to know exactly what he had in mind if in fact the two concepts are to be considered simultaneously.
  9. Sep 8, 2014 #8


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    That's one interpretation, but it's not the only possible one. One drawback of this interpretation is that it naturally leads to questions like the ones you're asking, which don't really have good answers.

    Another thing to keep in mind is that "space" in GR is not an invariant thing; it depends on how you split up spacetime into space and time, i.e., it depends on the coordinates you choose. We speak of the universe expanding in the standard coordinates used in cosmology, but the only aspect of that expansion that is invariant (i.e., that doesn't depend on the coordinates chosen) is the relative motion of the galaxies (on average). There is no invariant that corresponds to "space expanding" or "new space coming into existence".
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