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Thread: Standard Candles in an Expanding Universe

  1. #1 Standard Candles in an Expanding Universe 
    Time Lord
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    I have a question. In expansion theory, if you have a ring of say.... stones, and they are arranged in a big circle - big enough for expansion to be important, wouldn't the stones move away from each other?

    If initially the density of the stones were say 10 stones per meter of the circle, and then the circle doubles in size, wouldn't the density diminish to 5 stones per meter of the circle?


    Now.... let us try applying this logic to an expanding spherical light wave. The photon density of the wave should be diminishing as the spherical wave expands due to the light traveling away from the point of origin. However, shouldn't it also be diminishing due to expansion carrying the spherical wave outward also? Wouldn't there be fewer and fewer photons per square meter, just like with the rocks?


    So I'm thinking the dimness of a "standard candle" should be due both to the distance the light has traveled, and due to the degree to which space has expanded behind it. Is that the way the calculation is done, or is it done differently?


    Some clocks are only right twice a day, but they are still right when they are right.
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    Quagma SpeedFreek's Avatar
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    Quote Originally Posted by kojax View Post
    I have a question. In expansion theory, if you have a ring of say.... stones, and they are arranged in a big circle - big enough for expansion to be important, wouldn't the stones move away from each other?
    Well they don't "move" as such. The space between the stones scales up whilst the stones all remain at rest in relation to local space (and the photons passing them).

    Quote Originally Posted by kojax View Post
    If initially the density of the stones were say 10 stones per meter of the circle, and then the circle doubles in size, wouldn't the density diminish to 5 stones per meter of the circle?
    If you take the radius, then the area of a circle is . If you want to do this for a sphere, the volume is

    Quote Originally Posted by kojax View Post
    Now.... let us try applying this logic to an expanding spherical light wave. The photon density of the wave should be diminishing as the spherical wave expands due to the light traveling away from the point of origin. However, shouldn't it also be diminishing due to expansion carrying the spherical wave outward also? Wouldn't there be fewer and fewer photons per square meter, just like with the rocks?
    Yes, there would be fewer photons due to the expansion.

    Quote Originally Posted by kojax View Post
    So I'm thinking the dimness of a "standard candle" should be due both to the distance the light has traveled, and due to the degree to which space has expanded behind it. Is that the way the calculation is done, or is it done differently?
    Yes, that is all taken into account in the calculation, as it performed using the change in scale factor.


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