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Galaxy acceleration

  1. May 14, 2015 #1
    Dear PF Forum,

    I have a question that puzzles me.
    Galaxies are moving away from us, except Milky Way which is in collision to us in some 4 billions years. I think there are other collisions as well from any other galaxies pairs.
    1. Galaxies are moving away from us. Are they also SPEEDING away? It's Dark energy is it, if it's true.
    2. If this is true, we CAN'T say that we are at rest and they are moving (or SPEEDING), right.
    But,... can they feel the acceleration? Or why can't we feel the acceleration?

    I know if A moves against B, from B point of view it's A that moves. What if one of them is accelerating? The one who's accelerating MUST know that he's the one who accelerates, right?

    Thanks for any explanations.
  2. jcsd
  3. May 14, 2015 #2


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    The 'movement' of galaxies due to the expansion of space does not produce any measurable acceleration. All objects moving with the Hubble flow are effectively in free fall in their local space.
    Similarly, you can't feel gravity accelerating you (imagine being in orbit). Its effect on you is only measurable insofar as tidal forces go.

    Distant galaxies are indeed accelerating away from us. Look at the Hubble law: ##V=HD## treat Hubble constant H as if it were an actual constant, and consider a galaxy at a distance ##D##. Its recession velocity will be ##DH##. Now consider the same galaxy after it had moved with that velocity for some time. It is now at distance ##D_1>D##, and its new velocity according to the Hubble law must be ##V_1=HD_1##, so ##V_1>V##.
  4. May 14, 2015 #3


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    Huh? We ARE (in) the Milky Way. Perhaps you mean the Andromeda galaxy?

    Also, you should look up the "Local Cluster". There are numerous galaxies that are not moving away from us. Andromeda is just the largest of them.
  5. May 14, 2015 #4
    Thanks Bandersnatch for your answer.
    It's very good of you to show me Hubble flow and Hubble law.
    Okay, ... it makes sense.
    But I'd like to ask this.
    1. Does the galaxies themselves move. Or,... they DON'T move, but only the space between them are expanding?
    2. If Galaxies don't move, only space that's changing, what about Andromeda? Does Andromeda and Milky Way really aproach each other. Or they are at rest, but the space is contracting, which is highly improbabel, right?
    3. Does space expand linearly or logarithmically?
    4. Does Dark Energy:
    A. Expands space only?
    B. Push Galaxies away only?
    C. Both?
    D. Neither?
    5. Why space expansion, as I suspect, happens in outer space? (intergalactic medium?) Why don't space expand, say, in my house, so I have a bigger one?
    6. Is there anybody care to show me "space expansion rate"? I mean how many cm/km/seconds? Or is it in Hubble Law?

    Thanks for any answer.
  6. May 14, 2015 #5


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    All of these questions have been answered here numerous times, over and over. I suggest a forum search on something like "universe expansion"
  7. May 14, 2015 #6


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    Shoo, @phinds . I need something to do.

    Yes, it's the space that is expanding, rather than objects being 'pushed'.

    The expansion of space can be noticed only on large enough scales, where average matter density is low enough. Where lots of matter is present, its combined gravity easily overcomes the effect of expansion. This effect is very tiny, and only accumulates over large distances. But since the observable universe is rather large, this results in quite drastic rates of expansion at sufficient distances.

    In those regions of high mass density, you can disregard expansion, but it doesn't matter objects won't move for other reasons. Andromeda and Milky Way being on a collision course are no different here than you going to buy groceries - neither cares much about expansion of the universe.
    In particular, the two galaxies are gravitationally attracted to each other, and may have some residual velocities* from their past history. Such motion in cosmology is called peculiar motion - it's all the components of observed motion that are not due to the Hubble flow.

    *you need to specify some reference frame to be able to name velocity

    How small an effect the expansion is can be seen by looking at the Hubble constant. It's around 68 km/s/Mpc. It means, that you need to venture as far as one megaparsec (3.26 million light years) to observe an object there receding at 68km/s. Twice as fast twice as far, and so on. Compare to Earth orbital speed of 30km/s and the current collision velocity of the two galaxies previously mentioned of 110 km/s (and they're closer than a megaparsec).

    Another way of thinking about the scale of the expansion is that every (sufficiently large) distance currently grows by about 1/144th of a percent per million years.

    As for whether the expansion is linear or not - I think you should be able to deduce that from the Hubble law.

    For more about the expansion, do what phinds said, and/or read this:
  8. May 14, 2015 #7
    Thanks Bandersnatch for snatching the opportunity to answer me. As Phinds already mentioned it. I should have searched the forum. :smile:
    And yes, Bandersnatch, it's logaritmic.
    V = HD.
    X KM/s = H * Y KM
    X KM = H * Y KM * s
    So, for every Hy KM for each seconds the speed always increases.
    In t0, for HY KM, the distance is HY+X
    In t1, for (HY+X) KM, the distance is ... (HY+X+something) well, it's rather difficult to calculate, but glancing at once, it's Zeno paradox, but in reverse.
    Last edited: May 14, 2015
  9. May 14, 2015 #8


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    I also should mention that you would likely find the link in my signature interesting.
  10. May 14, 2015 #9
    Dear Phinds, dear Bandersnatch,

    Yes, yes, Phinds.
    I've already clicked your link a week ago. It's a very good analogy to describe the expansion of the universe.
    1. It's the surface (2D) that we should imagine as (3D). There's no centre in the (2D) balloon surface, right. Just like there's no centre in the universe.
    2. The distance of the pennies changes proportionally on the balloon surface, THE PENNY DOESN'T CHANGES SIZE! (supposed it's pinned on a pin, an assuming the balloon doesn't leak).
    3. But what formula should we blow the balloon to describe hubble law?
    We just can't blow the balloon witih constant volume(V) each second. The diameter (and also the distance between penny, the distance is still 1D, right) would only increase by V1/3, right? So, with each passing seconds, the volume that we have to pump to the balloon should be dV3, right?
    in t0, we pump the balloon with volume d to increase the balloon volume, so it will change to V0 to V1.
    In t1, we should pump the balloon with d * (V1/V0) than we have V2
    In t2, d * (V2/V0), than we have V3,
    In t3, d * (V3/V0), etc..
    Your balloon analogy is a very good explanation, HOW the universe is expanding, but not WHERE?

    So, Bandersnatch, according to you explanations:
    So, are you trying to say that the rate of expansion are the same EVERYWHERE, whether it's in intergalactic space and inside a dense space, say earth atmosphere or inside say, the core of the sun. But because there's a strong gravity, the expansion is somewhat canceled? And only in intergalactic space that the space can freely expands?

    Thanks for your gentlemen responds.
  11. May 14, 2015 #10


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    It's a way to think about it, and it's the way I usually visualise what is going on. But that's only because I'm no good with General Relativity. I think it's not entirely correct, as it kinda suggest there's some force pushing objects apart, and gravity is holding them back, whereas it should be more thought of in terms of how density of matter and energy curve space-time so that objects remain in free fall but distances change.
    That's all the insight you'll get from me, as I'm afraid I might start spouting nonsense.
  12. May 14, 2015 #11


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    Yeah, I think you may have told me that in another post. I'm old. I forget things :smile:
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