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Universe expansion speed vs gravitational waves speed

  1. Oct 6, 2013 #1
    From my own(simplistic)perspective,dark energy is expanding the universe by creating further spacetime at a velocity faster than the speed of light,if gravitational waves propagate at the speed of light how is it possible that M31 and the milky way are still bound by gravity when the fabric of spacetime is constantly being created between them at speeds faster than the speed of light?

    According to string theory gravitons are not attached to the membrane because of their shape (closed string).
    Does this explain why gravitons don't seem to be affected by the expansion of the fabric of spacetime ?
  2. jcsd
  3. Oct 6, 2013 #2


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    "dark energy", the name we give to whatever it is that is causing the acceleration of the expansion of the universe, is unbelievably weak and on the scale of galactic clusters and smaller, it has no effect. It's like an ant pushing on a house. It isn't that the ant's tiny force has a tiny effect on the house, it's that it has NO effect. It just isn't strong enough to shift the house on its foundations by any amount.

    String theory is at present nothing more than a mathematical exercise and has not been shown to be responsible for, or descriptive of, any real-world effects.

    "Fabric" is a terrible word to use when talking about space-time. It has an English-language meaning that does not translate well to cosmology.
  4. Oct 6, 2013 #3
    How do you feel about the use of color charge,flavor and black hole?
  5. Oct 6, 2013 #4


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    I think they are perfectly reasonable and widely accepted. "Fabric" is neither (unless you count television "science" in which case "fabric" is just fine)
  6. Oct 6, 2013 #5


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    Well, neither color nor flavor lend themselves to the sort of misunderstanding (no one is likely to think that quarks of different flavors will affect the taste buds on their tongue differently) that the "fabric of spacetime" metaphor does... And even though the space near the event horizon of a black hole may be excitingly bright, the black hole itself likely still appears in the visible wavelengths as a black disk, so that's not so bad.... But if you want an example of a really unfortunate term, it's hard to beat quantum-mechanical "spin".

    At some point, you have to move past the natural-language hand-waving and look at the math. The question you asked back in post #1 of this thread about the binding between our galaxy and M31 has a perfectly natural answer in the framework of general relativity.
  7. Oct 6, 2013 #6

    Vanadium 50

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    I remember the old days before the Fabric of the Universe was wash-and-wear. We were constantly ironing and pressing, but it never looked quite right.
  8. Oct 7, 2013 #7
    The rate that the distance is increasing between us and another galaxy is proportional to the distance to that galaxy. The Hubble Constant give this relationship precisely. A nearby galaxy like M31 is too close for its distance to be increasing faster than light. In fact, its distance is really not changing at all because it is gravitationally bound to us, like phinds said above.
    A galaxy more than 14 billion light years away is receding from us at a speed faster than light due to the expansion of the universe. So that galaxy can not affect us in any way, not by gravity or by light or anything else.
  9. Oct 7, 2013 #8
    To an observer in (some far away galaxy that is receding faster than the speed of light), we are receding at greater than the speed of light, but we still light that only travels at ... the speed of light ... from that galaxy. If it is no problem for light to overcome, it shouldn't be a problem for gravitational waves either ;)
  10. Oct 7, 2013 #9


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    Gravitational waves travel at c. This was demonstrated by Hulse-Taylor to within 1%.
  11. Oct 9, 2013 #10
    If a galaxy is so far away that it is receding faster than light, then we can't see it at all. No light or gravity from it will ever reach us. And no light or gravity from us with reach that galaxy. It is beyond our cosmic horizon.
    We are causally disconnected from that galaxy.
  12. Oct 9, 2013 #11


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    Galaxies WITHIN our observable universe (which we CAN see, by definition) are receding from us at about 3c, so you have that one wrong.

    The light from these galaxies was emitted before their recession velocity got so high, and they were closer.

    The point you are missing is that it does not matter what the speed of the object is, the light it omits is always traveling at c, so what matters is only how FAR it is from us.
  13. Oct 12, 2013 #12
    All good answers above:

    I would say "traveling at c locally" for clarity......in curved spacetime, different observers may record different speeds. As phinds implies, the light was originally emitted at c and remains at that fixed speed for all local observers along the way.

    Whether dark energy has ANY effect locally, that is cosmological expansion, say between adjacent galaxies, is still debatable. The expansion is based on the homogeneous and isotropic cosmological [FLRW} model. We debated this previously and a participating cosmologist said "The model doesn't apply locally." Others stuck with the description above...that it is too small to matter.....unimaginably too small to be observed so far.
    Last edited: Oct 12, 2013
  14. Oct 12, 2013 #13
    not really...depending on just what you mean: gravitons are 'not affected' [and neither are photons] in a sense. In another sense, they are affected: acceleration of either an observer or a cosmological horizon causes different particle counts to be measured.....see 'Unruh effect' for example.

    String theory lives in static spacetime, not the dynamic evolving space of cosmology.

    From Michio Kaku:

    From Lee Smolin:

    What these guys are implying is that just as in the Standard Model of particle physics we have to pick some free parameter values from observations, like electric charge size and electron mass, in string theory you have to pick a particular shape of the extra dimensions....say particular Calabi-Yau manifolds. So far as I know in the theory, when the geometry changes, change size or shape for example, the particle characteristics take on values we don't observe.

    One string theory with gravity is David Gross's heterotic string theory....I don't know much about it....but it utilizes a type of closed string
  15. Oct 12, 2013 #14
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