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Value of a measuring rod in an expanding universe?

  1. Oct 5, 2011 #1
    I have a question, but I'm a neophyte so please correct my premise if it is wrong. If it is true that in an expanding universe the space between any 2 points increases than wouldn't it be true that the space between the end points of a measuring rod (e.g. a light year, or more fundamentally a meter) would increase in like proportion to the objects it is measuring (e.g. 2 stars). Therefore wouldn't the distance between 2 stars measured today (using today's rod) be the same as the distance between those same 2 stars measured in the past(using the past rod). If that were true, wouldn't it appear (disregarding doppler effects and other such phenomena) that the universe was static?
  2. jcsd
  3. Oct 5, 2011 #2


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    No, only the space between comoving (coming with the expansion) points increases.

    A meterstick is a bound system, so at least one (an possibly both) ends of it will not be comoving.

    You are right in that changing all distances by a factor of two would be a scale change and thus unobservable, but that's not what the expanding space idea is about - though the popular version of "expanding space" isn't very clear, and confusion like yours is common.

    You might also look at the following papers:

    "Expanding space: the root of all evil?" http://arxiv.org/abs/0707.0380
    "Expanding confusion" http://arxiv.org/abs/astro-ph/0310808

    The first talks about some criticism of the expanding space idea, but explains how to interpet the paradigm correctly to avoid confusion, so it might be especially helpful. The second is a bit more advanced, I think, but relevant.
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