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Universe expansion beyond c at some distant point?

  1. Jan 4, 2013 #1
    After seeing a description on the exapnsion of the universe, I wrote a very simple program to see how the universe is expanding faster the further it is away from us (wherever we are). Playing with that got me thinking that at some distant point, planets and everything would be moving away from us faster then the speed of light...

    Why is this not true?

    Meaby the universe isnt big enough and the expansion not fast enough to ever reach the speed of light? Or meaby it is true, but the speed due to expansion is not actual velocity relative to the space it is in (it's frame of reference)?

    Please enlighten me!
  2. jcsd
  3. Jan 4, 2013 #2


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    It IS true. The stuff that is now at the edge of our observable universe is receeding from us at about 3c.

    No speeding ticket is issued though because nothing is moving faster than c.

    Look up "metric expansion". Basically, you have it right in saying " ... the speed due to expansion is not actual velocity relative to the space it is in (it's frame of reference)? "
  4. Jan 4, 2013 #3
    Awesome, thanks for the right term to search!
  5. Jan 12, 2013 #4
    Indeed, this is true. Based on Doppler Shift observations, some of the most distant objects (globular clusters at the edge of the observable universe) are receding from us with a recessional velocity in excess of c. However, this is not a violation of Special Relativity because these velocities really just follow from perspective. It is merely the isotropic expansion of space-time between two points of interest. The typical analogy is baking raisin bread. As the dough expands, the raisins move away from one another, but we tend not to think of the raisins themselves as being in motion. It's simply the dough (space) expanding. As a consequence, light emitted from these globular clusters 'today' will never reach the Earth, being subject to infinite gravitational red shift. With respect to all but our Local Group, we are in a slow crawl toward causal disconnection with the rest of the universe.But this won't happen for an almost incomprehensible measure of time.
  6. Jan 12, 2013 #5


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    Hmm, yes, but you should really look at our FAQ: https://www.physicsforums.com/showthread.php?t=508610 [Broken]

    to see some caveats about just how meaningful it is to talk about "the velocity of the universe", and just what it is, exactly, that is greater than c.
    Last edited by a moderator: May 6, 2017
  7. Jan 12, 2013 #6
    Thanks for the analogies and the extra clarifications, so my summary was pretty much spot on when I said:

    "It is true, but the speed due to expansion is not actual velocity relative to the space it is in (it's frame of reference)".

    Or is there some fundamental difference in the other explanations?
  8. Jan 16, 2013 #7
    It is true. That's exactly why the night sky is black instead of white.
  9. Jan 16, 2013 #8


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    Not true. The sky is black instead of white because of the finite age of the universe. Light from beyond a certain distance simply hasn't had time to reach us. However expansion does in fact make it impossible that the sky will ever become "nearly white" after a very long period of time.
  10. Jan 16, 2013 #9

    The universe is finitely old and the speed of light is finite, so only finitely many stars can be observed within a given volume of space visible from Earth. The light from some stars hasn't reached us yet; the light from more distant stars will never reach us. The density of stars within this finite volume is sufficiently low that any line of sight from Earth is unlikely to reach a star. And that sky will get darker as the acceleration of the expansion increases and will ultimately be black, empty, cold..

    Regarding the 'expansion' of the universe, yes using the assumptions and conventions
    included in the FLRW cosmological model, expansion beyond the Hubble sphere is 'faster than light'. Using different distance metrics, that is distance measures, many other expansion rates would result. In fact distance and velocity have arbitrary values in general relativity.

    Just keep in mind that right here in our solar system, we APPEAR to be expanding away from very distant observers that are a Hubble distance away! Locally, the speed of light is ALWAYS 'c', just like here.
  11. Jan 17, 2013 #10


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    I doubt there are any stars hiding behind the CMB.
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