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Speed of light vs universe expansion

  1. Apr 2, 2013 #1
    After reading a few threads about the speed of light and the expansion of the universe, I noted one a message which mentioned that even though light may be headed our way it can appear to be going away from us, if the universe were expanding faster than the speed of light.

    How does that make sense though?
  2. jcsd
  3. Apr 2, 2013 #2
    Nature doesn't need to make sense to us...only to her!!

    Have you noted dark spots in the night sky? Those represent dark spots from which insufficient light can reach you. [So something prevents light from reaching us.]

    That's a crude one liner...a better explanation results from the expansion of the universe which causes the energy of distant emitted light to be reduced via redshift....

    At the Hubble radius, where we observe a 'Hubble sphere' surrounding us, the universe is expanding at 'c'...the speed of light....but that is not the usual measure of relative velocity.....so it turns out that some light can actually reach us from beyond that distance. There is a limit: The standard cosmological model has accelerated expansion with a horizon of about 15 billion Light years; light from beyond can’t reach us.

    For more, try reading about OLBER's paradox in Wikipedia.
  4. Apr 2, 2013 #3
    Thanks for the reply. But what I was thinking, is that if the speed of light relative to anything is still at the constant of C, then if we suppose the "edge of the universe" (for the sake of example) is expanding away from as at C and light from that edge is travelling to us at C, shouldn't it still reach us, since C is always constant?
  5. Apr 2, 2013 #4


    Staff: Mentor

    No, because "C is always constant" doesn't mean what you think it means in a curved spacetime. In flat spacetime the statement has an invariant meaning as it stands; but in curved spacetime, you have to rephrase it as "light always moves along the local light cones" (and similarly, objects with nonzero rest mass always move inside the local light cones).

    In flat spacetime, the two forms of the statement are equivalent because the light cones at different events all "line up" with each other. But in curved spacetime, they don't; so what look like "differences in the speed of light" at different events are really differences in the way the light cones are pointing at different events.

    In an expanding universe, "different events" can mean events at different times, not just different spatial locations. So a light beam emitted towards us by a distant galaxy might never reach us because the expansion of the universe keeps tilting the light cones away from us.
  6. Apr 2, 2013 #5


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    Gold Member

    To put Peter's excellent explanation into English without "light cones", think of it this way ... if the space between us and an object were not expanding, then it wouldn't matter how fast it was moving away from us since light emitted from it STILL would move towards us at c and would reach us. But when it is emitted at c but the space between us and the photon keeps increasing, then the situation looks as though (well, IS as though) the photon is moving away from us. No speeding tickets are issues because neither we nor the photon are moving FTL, it's just that the distance between us is expanding FTL. It's VERY counter intuitive.
  7. Apr 2, 2013 #6
    What's also inherent in the prior two explanations is that in flat spacetime the speed of light is 'c' all over the place, locally and distant; in curved spacetime, meaning in the presence of gravity, light travels at 'c' only locally....that is, in a small enough space around you that spacetime may be taken as flat....
  8. Apr 2, 2013 #7
    Thanks for the replies guys. Think it makes sense now... kinda.
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