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What does expand: Space or Spacetime?

  1. Nov 30, 2015 #1
    Hello everybody,

    When we say that the universe expands, what exactly do we refer to: Space or space time? Is it only a spatial expansion or also a temporal expansion?

    If it is also a temporal expansion , what are the implications w.r.t. to inflation and accelerating expansion?

    Best regards,
    Robert
     
  2. jcsd
  3. Nov 30, 2015 #2

    Chalnoth

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    It refers to space expanding. Specifically, the distance between far-away objects gets greater over time. The time coordinate chosen does not expand.
     
  4. Dec 1, 2015 #3
    It concerns only space. It expands faster than it was before and now we have to find the reason why. Currently, we believe the inflation occurs because of the dark energy, but it remains to be a mystery o_O.
     
    Last edited by a moderator: Dec 2, 2015
  5. Dec 1, 2015 #4

    phinds

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    Saying that space expands is not exactly incorrect, but it is misleading. It implies that space is a "fabric" or something that can be stretched. Better to just say that things get farther apart from each other since there is no "thing" there that actually expands. This can seem like just semantics but it really isn't. Google "metric expansion".
     
  6. Dec 2, 2015 #5
    This is what I always thought, but then I saw a feature with a German astrophysicist where he argued that in the big bang, not only space but rather space time expanded.

    The statement appeared a bit strange to me, because expansion clearly seems to be a temporal process. You compare some measure at different points in time, and then you find out that the "later" values are larger. That's how I interpret expansion.

    Thanks for the clarification, but that was always clear for me. I do not think that expansion implies stretching
     
  7. Dec 2, 2015 #6

    Chalnoth

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    Saying that space-time expanded really isn't accurate. When you look at the whole space-time structure, the expansion appears as space-time curvature.

    I think the point is that if you have a clock at different points in time in the universe, that clock behaves the same according to the coordinate time we usually use to describe the expansion (that is, one second in the coordinate time is one second on the clock, as long as that clock is stationary with respect to the expansion).
     
  8. Dec 3, 2015 #7
  9. Dec 3, 2015 #8
  10. Dec 3, 2015 #9

    phinds

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    The amount of dark energy is always increasing. That's one of the mysteries of dark energy. It has a constant density per unit volume so when things get farther apart, there is more dark energy between them.
     
  11. Dec 3, 2015 #10

    Chronos

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    Expansion is most easily explained by the simplest version of dark energy - a cosmological constant. If the dark energy density of the universe is truly a constant then it constitutes the simplest model to explain expansion. This is why efforts to measure dark energy density during different epochs in the universe are a hotbed of research activity. If it turns out to vary over time, it will deeply impact cosmology. The results to date remain inconclusive, but, do not rule out a cosmological constant. The tricky part is DE, not unlike dark matter, is not something that lends itself to direct measurement . We are constrained to measure it by examining the motions of matter in the distant universe. It was suspected a few years ago that supernova data accumulated since the introduction of DE into our cosmological model suggested a value for w of less than -1, which would disfavor the cosmological constant version of DE. Last I heard on this front is uncertainty in the data was too large to permit any firm conclusions. For further discussion on why the cosmological constant is a simple, yet natural explanation for expansion, see http://arxiv.org/abs/1002.3966, Why all these prejudices against a constant?.
     
    Last edited: Dec 3, 2015
  12. Dec 3, 2015 #11
    So, the tendency to expand can be considered an intrinsic property of space, as long as there are no other interference factors that compensate this tendency to expand?
     
  13. Dec 3, 2015 #12

    phinds

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    No, I don't think it is "an intrinsic property of space", but since we dont' know WHAT it is, no one can say for sure. If you are asking whether or not it is the way things work in our universe, then obviously yes.
     
  14. Dec 3, 2015 #13
    I was just wondering about the simplest way to describe the existing observations. And if the density per unit volume stays constant, although space continuously expands, then it seems to me that the simplest description might actually be to treat it as an intrinsic property of space.
     
  15. Dec 4, 2015 #14

    martinbn

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    You mean accelerated expansion. For expansion you don't need dark energy.
     
  16. Dec 4, 2015 #15

    cristo

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    This isn't correct. Technically, inflation refers to the accelerated expansion in the very early universe (right after the big bang); dark energy is responsible for the accelerated expansion that the universe is perceived to be undergoing in the present epoch.

    Your first sentence also isn't technically correct: the "amount" of dark energy is not increasing, it's energy density is constant. The different is that all other energy/matter species decay with the expansion, so the relative contribution of dark energy to the energy budget is increasing.
     
  17. Dec 4, 2015 #16

    cristo

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    I think you're mixing up two concepts here - expansion and accelerated expansion. The former is just really an initial condition, while the latter requires some new explanation. This could either be some property of spacetime (such as a cosmological constant; but that itself brings other issues), or some new, weird matter species.
     
  18. Dec 4, 2015 #17

    phinds

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    Yeah, I guess my wording was sloppy. The amount of dark energy between us and a galaxy 3 billion light years away, in say a conceptual tube the diameter of the Earth, is increasing with time BECAUSE its density is constant but the distance increases. It's a messy concept and I'm never quite sure how to talk about it.
     
  19. Dec 4, 2015 #18

    timmdeeg

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    Being a vacuum energy the dark energy or the cosmological constant resp. might be an intrinsic property of the vacuum, rather than of space. From this point of view, what happens to the vacuum energy if the universe expands? Does it get "diluted"? I don't think so, whereas the density of matter definitively does. Of course, as we don't know the nature of the dark energy, we can't exclude a still unknown time dependence of its density.
     
    Last edited: Dec 4, 2015
  20. Dec 4, 2015 #19

    Chalnoth

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    The vacuum doesn't dilute. I.e., its properties remain the same regardless of the expansion.
     
  21. Dec 5, 2015 #20

    timmdeeg

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    So, in case the vacuum energy is such a property, it remains the same. If I understood you correctly.
     
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