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Question about Expansion Speed exceeding Ligth Speed

  1. Feb 15, 2013 #1
    I was recently watching "Into the Universe with Stephen Hawkings" (a general video about the universe on Science Channel) when I heard the statement "Just 10 minutes after the Big Bang, the Universe was 1,000s of light years across." How is this possible if nothing can travel faster than light. Even if two things traveled in the exact opposite directions from the initial start of the Big Bang at the speed of light, they would only be 20 lightminutes apart. Is this statement simply incorrect, or was it possibly to expand much faster than light in the Universe's infancy?
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  3. Feb 15, 2013 #2


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    Re: Question about Expansion Speed exceeding Light Speed

    A pattern of uniform expansion is not like ordinary motion.
    Nobody gets anywhere, nobody approaches a destination, everything just gets farther apart.

    So it is not governed by the speed limit of Einstein's 1905 (special) relativity.

    Think of expansion as a change in geometry, not as ordinary motion thru space.
    the relative positions of things don't change, distances just uniformly scale up.

    It may help you to watch the balloon model animation. That shows photons of light traveling at a constant speed across the expanding face of the balloon. The link is in my signature, or google "wright balloon model".
    by contrast, the galaxies do not move. they stay at the same longitude-latitude position on the balloon.

    The point of Einstein's 1915 (general) relativity is that we have no right to expect that distances between stationary observers remain constant. Geometry is dynamic. The theory explains WHY the angles of triangle add up to 180 in some places and add up to more or less in other situations. Why distances sometimes increase (or decrease) between stationary observers. You should be wondering what criterion for being stationary is used in cosmology. Ask about it if curious.

    Don't think of the universe expanding from a point outwards into empty space. Think of of all space uniformly filled with matter, no space "outside" of space, because there is no outside. And expansion is an internal process, internal to the universe. It does not expand "into" anything. Distances simply increase by a certain percentage each year.
    Last edited: Feb 15, 2013
  4. Feb 16, 2013 #3


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    Another way to state it is that General Relativity modifies the way the speed limit is stated: in General Relativity, nothing can outrun a light ray. That is to say, if a light ray follows some path through the universe, there is no way for any object to follow that same path any faster.

    Things in the universe that are already far apart can get further apart much faster than c because the light rays are carried along with the expansion just like everything else: nothing is moving faster than a local light ray, so there isn't any violation of the speed of light limit.
  5. Feb 17, 2013 #4
    Put simply, nothing can travel faster than the speed of light, as predicted by theory.

    But space (the medium through which everything is travelling) itself can expand faster than the speed of light.
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