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What is moving faster

  1. Jul 28, 2009 #1
    What is moving faster

    The speed of light
    The expansion of the universe

    If the universe is expanding faster then the speed of light, then light be traveling away from us?
  2. jcsd
  3. Jul 28, 2009 #2
    It is true that at certain redshifts (z>1) the universe is expanding away from us faster than c, so yes it is correct to say that there are photons that are emitted beyond this horizon (the observable universe) that will never reach us because space is expanding faster than they can travel.
  4. Jul 28, 2009 #3
    In which case I would suppose that there are stars, et al that we can see now at the edge of the observable universe that will appear to fade and then disappear due to this expansion over time?
  5. Jul 29, 2009 #4


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    1. It is not actually possible to compare a rate of expansion to a speed. It's a unit problem: the expansion rate has units of speed per distance. You can't compare a speed per distance to a speed. The comparison makes no sense.

    2. That said, there are many objects in the universe that are receding at faster than the speed of light. That is to say, if we consider a photon traveling from some far away galaxy, if that galaxy is far enough away, then more space will have been created between us and the photon than the photon is able to travel. So yes, there are photons out there that are traveling in our direction but are nevertheless getting further away.
  6. Jul 29, 2009 #5


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    As noted, expansion of the universe is not a velocity; the units are different. But it does imply that distant parts of the universe are receding at speeds proportional to distance, and these can be arbitrarily large.

    However, you need to be careful. It is only in a case of accelerating expansion that there is an "event horizon" marking regions from which no photon can ever reach us no matter how long we wait. This is not the same as the observable horizon, which marks the regions from which photons can reach us up until now.

    The other tricky point is that it is quite possible for a photon to reach us, even if coming from regions that are receding faster than the speed of light. Basically, a photon passes into regions with smaller and smaller recession velocities, and can eventually pass into regions where the recession velocity is less than light speed again. In this case, the "proper distance" between us an an approaching photon actually increases at first; but then eventually this reverses and starts to reduce again.

    An extreme example of this is the cosmic background radiation, which was emitted at cosmologically small distances in a very rapidly expanding universe. Those photons were intially at a "proper distance" of something like 50 million light years or so; but were being carried away from us (or from the matter from which we were eventually formed) as the universe expands and disperses. It has taken 13.7 billion years for those photons to reach us.

    Technical caution: distance is actually ambiguous on these scales; I am using what is called the "proper distance" co-ordinate in this post.

    Cheers -- sylas
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