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Concerning spatial expansion

  1. May 5, 2015 #1
    If the space between any two objects is expanding at a rate faster than light itself can travel, how is it that we're observing light from things so far away?
    One would think that the expansion of space affects light too, such that the space between the emitted light and the destination of said light expands so quickly that the light is stuck in this expansion and never reaches its destination. If I am mistaken, please correct me.
  2. jcsd
  3. May 5, 2015 #2


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    Hi, Ghostcrown. Since you seem to have a good grounding intuition, rather than directly answer, I'd like you to consider what would be observed in a universe where the expansion rate is not constant.

    Say, it goes down for at least a period in the history of the universe. What would happen to the light emitted from an object just outside the radius beyond which objects recede faster than light?
  4. May 5, 2015 #3
    I'm interested in the answer to this question too. I'm gonna try to answer it myself then someone more knowledgable can correct me.

    Objects can not travel away from each other faster than light, no matter what space is doing. As you get further and further apart, space expands faster and faster, causing time to get slower and slower. If space expands at such a rate that two objects are flying away from each other faster than light, from the perspective of one of them, the other one is not emitting light and must be traveling backwards in time, they no longer exist in the same spacetime? Wait... that doesn't sound right...

    Lemme try Bandersnatch's thought experiment: a photon leaves an object and zooms across the expanding universe. From every perspective other than the photon's it's traveling at the speed of light, so as space expands, its like walking up a down escalator, it'll make progress until the down speed equals it's travel speed. If it's expanding fast enough, the outside viewpoint would never see the photon hit the other object. From the point of view of the photon though, traveling at the speed of light, the entire universe has no length (limit (1 / sqrt(1 - v^2/c^2)) as v -> c = 0?), so it should be able to reach any position instantaneously. Wait... that's a paradox. :/

    My intuition is that things will accelerate away from us closer and closer to the speed of light, but time at the edge will get slower and slower, preventing any physical objects from moving at >c in relation to each other.

    So... what would actually happen?
  5. May 5, 2015 #4


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    @newjerseyrunner you shouldn't try to apply special relativity to a general relativistic effect such as the expansion of space. The latter, being more general, already includes all space-time related effects.
    There's also some reason to suspect you may need revising SR - there is no such thing as 'the point of view of a photon'. There is no frame of reference in which a photon is at rest.
  6. May 5, 2015 #5
    Oh, ok that makes sense.
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