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Universe expansions effect on gravity and einsteins theory of special relativity

  1. Aug 27, 2012 #1
    Firstly, apologies if this is the wrong thread for my post. Please direct me if wrong.

    My first observation was this!

    If the universe is expanding at all points away from each other. Should that not mean that everything including us, our instruments our measuring tools and, in fact, the earth too?
    If so. We would not perceive an expansion because us and our methods of measurement would be expanding proportionately.

    Therefore, my assumption is... the fact we are observing the expansion means our planet, and us are not expanding. I am not getting bigger, and the distance between the gradient on my ruler are not getting Wider. Doesn't this mean that objects with mass are not subject to the expansion. But the space between?

    If this 'BIG' assumption is correct. What does this hold for gravity, space and time?

    space and time will become 'stretched' therefore wont Gravities effect on the universe become diminished as objects become further away from each other? For example, our orbit of the sun being affected as its momentum 'being proportional to space and time' overcomes the progrssively weaker gravity?

    My question is. Does this have an affect on the theory of special relativity?
  2. jcsd
  3. Aug 27, 2012 #2


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    No, gravitationally bound systems (galactic clusters, galaxies, solar systems, planets, you, atoms, etc) are not affected by the expansion because it does not overcome gravity, the strong and weak forces, and electromagnetism.

    Yes, if it were true, that statement would be correct, but it is not.

    As I said, galactic clusters and smaller are not affected.

    Eventually, Galactic clusters will be alone, each in their own observable universe.

    No, why should it?
  4. Aug 27, 2012 #3


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    Yes and no. While SR doesn't need to be modified at all, we have to know when we can use it and when we can't. For example SR says that nothing can travel FTL, yet far away galaxies are receding from as at ever increasing velocities, which do in fact exceed c. All objects are receding from all other objects not bound to them. It is commonly explained that the galaxies aren't moving but are merely being dragged along by expanding space. This is...not quite accurate, but it gets the point across. This is allowed because SR does NOT deal with the geometrical shape of spacetime, which is what General Relativity deals with. This "recession" is NOT a violation of any laws, as GR says that this happens, so we simply cannot use SR in the case of very large distances.
  5. Aug 27, 2012 #4


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    Although this is a correct statement, it seems to be to be stated in a way that is misleading. It isn't the DISTANCE that makes SR unworkable for large distances, it is the EXPANSION that in our universe goes along with the long distances. I'd say "we simply cannot use SR in the case of objects that are moving with the expansion of the universe."
  6. Aug 29, 2012 #5
    Thank you all for your responses.

    Your answers have raised as many questions as have been answered! As seems to be the way in this field!

    If the expansion cannot overcome gravity, hence cluster remain. Then, as explained, only the space where no, or very little, gravity can exert its influence is expanding i have 2 questions.

    1. Is there a formulae/theory to explain the point at which gravity becomes weak enough for the expansion to become the greater influence.

    2. What does explain the geometrical shape of the universe and it's expansion?
  7. Aug 29, 2012 #6


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    Good question(s). I think a FLRW spacetime with a cosmological constant (CC) could be used to make an estimate. In this model we can find coordinates so that every 'dust' particle sees a spherically symmetric spacetime with itself at the centre and the other particles moving away under the influence of an effective potential caused by the expansion coefficient and the CC. In this model we assume that the dust particles are not interacting, but if we relax that condition a bit and allow them to attract each other, I'm sure a ball-park figure could be calculated.

    Of course, the dust particles would have to be clusters or super-clusters of galaxies to (approximately) preserve the homogeneity and isotropy assumptions that underlie the model.
    Last edited: Aug 29, 2012
  8. Aug 29, 2012 #7


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    No theory that I know of. Empirically, it appears pretty much conclusive that galactic clusters and smaller are unaffected, but I suppose it is possible that there are galactic clusters that have some galaxies so loosely bound that they might be subject to expansion.

    No one knows WHAT the topology of the universe is, although to within our ability to measure it the observable universe, it is flat. That's something like "flat to about 3%".

    Personally, I find it unlikely that out of a HUGE range of possible values, it is flat to within a very small amount but not totally flat. That is, I'm a believer in "flat" but that's an opinion, not backed up by fact.

    EDIT: In case you are not aware of it, the force of expansion is so utterly miniscule on small scales that it might as well not exist. My example of this is as follows:

    Even though the universe is expanding, it isn't going to get any easier to find a parking place. That is, if you could go out into space, away from galactic clusters and magically paint a pair of parking space lines, it would be 20 BILLION YEARS before they had moved far enough apart to park another car.

    On cosmological scales, however, the expansion has an enormous effect and galaxies that are now at the edge of our observable universe are receding from us at about 3c.
  9. Aug 29, 2012 #8
    Thanks for the parking space analogy phinds. Puts it into perspective. So the expansion force is massively weaker than gravity. But on such large scales has a huge effect.

    But... What about dark matter and dark energy?

    Dark matter plays a large part in galaxies. And is a major factor in the calculations explaining the gravitational affects of galaxies. But this and matter only make up 27% ( 4% matter 23% dark matter) of the known universe. It seems that dark energy (73%) makes up the The remainder and is the dominant force!

    So... Gravity. Being weak.. has a dominant effect on large scales. Has nothing on dark energy which is far far weaker but is the dominant effect on a universal scale.

    But Einstein proved that gravity was subject to SR. Is anyone currently trying to consolidate dark energy into SR
  10. Aug 29, 2012 #9


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    Well, scales are relative, but yeah, I'd agree.

    I don't have an answer except to say that I think you mean GR, not SR.
  11. Aug 30, 2012 #10
    Bizzare scaling factors there. Where is this dark energy most likely coming from? Is it being fed from a central region, as in blowing up a balloon, or is new space and new energy to fill it being created everywhere at the same time, like a growing bacteria colony assembling new scaffolding of the Higgs field?
  12. Aug 30, 2012 #11


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    We don't even know what it IS, much less where it's coming from, but the idea of it coming from any kind of "central region" is just silly since (1) there IS no "central region" to the universe and (2) even if there were, there were, essentially all of the universe would be causually disconnect from it, making distribution impossible.

    All we really know about "dark energy" is what it DOES. We CALL it "dark" because we don't know what it is.
  13. Aug 30, 2012 #12
    Lee Smolin thinks that the big bang may actually have arisen from the formation of a black hole in another universe. The black hole has a singularity, the big bang has a singularity, maybe there's a connection:tongue: In consciousness studies we call this the law of the minimization of mystery in regards to consciounsess collapsing the wave function. But I digress.

    I know the universe is homogeneous and isotropic and all that, but if it came from a singularity and it is expanding in some manner, what are some, even hypothetical, possible geometries of that expansion. Is it like a smoke ring being blown out of parsed lips? I guess the filling a ballon analogy is out. Can someone help!?
  14. Aug 30, 2012 #13


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    Sounds like something invented to sell books

    There is zero evidence that the singularity of the big bang was in any way related to the singularity of a black hole. "Singularity" just means "we don't know / math models break down"

    I'm not touching that one.

    It would be pure speculation.
  15. Aug 30, 2012 #14
    Thanks very much for all replies.

    As to the big bang being the result of a black hole. I'm very sceptical. Juries out on that for me.

    Phinds. My novice status shows through. As you say it's gr not sr.

    Although we don't know what dark energy is. I'm sure we can, and work is-a-foot to, measure the expansion and how this is affected by gravity. This surely must be an excellent foundation to give us some clue as to the properties of dark energy. Thus leading us to an understanding of what it may be.
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