Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Speed of gravitational waves vs Universe's expansion rate

  1. Jan 1, 2018 #1
    Is there any relationship between the Speed of gravitational waves and the Universe's "local" expansion rate?

    Speed of gravitational waves is supposed to be equal to the speed of light. Gravitational waves don't travel faster than light.

    But we can observe far galaxies moving away from us with high redshifts values that most people explain as the result of the creation of new space in-between us and the galaxy. We can call it the expansion rate.
    Other theories such as some kind of absorption exist.

    What sounds strange to me is that expansion rate can be faster than light, though not locally.
  2. jcsd
  3. Jan 1, 2018 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

    It is not creation of ”new space” as much as the distances becoming intrinsically larger.

    The expansion rate is not a velocity - it does not even have the correct units. Also, the limit of the speed of light is for actual relative motion. In the case of the expansion of the universe, it is not an issue of local motion (even if it may actually be viewed as such locally), but rather an intrinsic expansion of distances.

    Compare to an ant moving on the surface of a balloon. Even if the ant’s speed on the surface is limited to how fast it can run, its distance to another point on the balloon could grow faster if the balloon is inflated fast enough.

    Also, note that the thread level ”A” means that you indicate that you have knowledge of the subject equivalent to a graduate student in physics or higher and look for an answer/discussion at that level. Based on your question, I have assumed that this is not the case.
  4. Jan 1, 2018 #3


    User Avatar
    Science Advisor

    The "stretch and squish" effect of gravitational waves is quite similar to spatial expansion in some ways. Notably that the distance between objects changes at a changing rate without them accelerating.

    But I don't think there is any relationship between the speed of gravitational waves and the expansion rate. You can pretty much neglect expansion over distances less than a few megaparsecs, and gravitational wave wavelengths are much, much shorter than that.
  5. Jan 1, 2018 #4


    Staff: Mentor

    The expansion doesn’t produce gravitational waves. So there isn’t anything to limit to c.

    As @Orodruin mentioned, the expansion rate is in units of 1/time, while the speed of light is in units of length/time. So they really cannot be compared.
  6. Jan 3, 2018 #5


    User Avatar
    Science Advisor

    Gravitational waves propagate at the speed of light.

    The expansion isn't a creation of new space. Space isn't a "thing" that can be created. It is a scaling of all distances. Essentially, the distance between any two "comoving" galaxies is just the distance at some reference time times a scalefactor (notated a(t) in the Friedmann-Lemaitre-Robertson-Walker metric). So, if a(Jan 3, 2018) = 2 and a(5 billion years ago) = 1, then that means comoving* galaxies are twice as far now as they were 5 billion years ago. Comoving means that they are moving with the smooth average movement observed on cosmological scales. Any particular galaxy will have a small variation from this. The distance to very far galaxies grows faster than the distance to close galaxies simply because you are scaling a larger number.

    No matter how gradual the scaling, at sufficiently large initial distance between two galaxies, the distance grows faster than c. The rate at which distance is growing is not a velocity. Velocity is defined between two objects moving in the same frame of reference, i.e., two objects moving past each other in the same general place. Nothing can have a velocity greater than c, but there's no issue with the distance between two objects changing faster than c when the distance is measured between multiple frames of reference.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted