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Expanding Space and Starlight

  1. Mar 20, 2006 #1
    I have 2 questions:

    1. Since space is expanding, shouldn't the distance between the earth and the sun be getting greater? And shouldn't that affect the orbit of the earth (greater distance between the earth and sun ->> less gravitational attraction)?

    2. Is it possible that the speed of light could have been faster at one point. For example I was looking at the following:

    t = t0(1-(v^2/c^2))^(1/2) (I think this is correct)

    I understand why this formula (and the others like it for mass and distance) would imply that no object could travel faster than the speed of light (can't take square root of a negative)...but that doesn't seem to limit what c can be. If c were greater that doesn't cause any problems in the formula. Are there other reasons for saying that the speed of light has always been a constant speed?

  2. jcsd
  3. Mar 21, 2006 #2
    Gravitationally bound masses do not separate because of cosmological expansion. It is only on the scale of the distances between galatic clusters that the separation distances increase
  4. Mar 21, 2006 #3
    Is that because the expansion is so small it is unnoticable? Or does the gravitational attraction (continuously) pull the earth back to the correct distance from the sun?
  5. Mar 21, 2006 #4
    To get a physical picture - imagine a balloon that is expanding - on the surface we place some objects that indent the expanding balloon because they have inertia (mass) ... two nearby objects would tend to fall together because each rolls toward the indentation produced by the other - these objects are the galaxies and the indentations are the local effect (curvature) of matter on space - if the objects are far enough apart they will separate faster than they roll toward one another - this occurs on the scale of galatic cluster - but if they are closer the effect of the indentation causing them to move toward one another will be greater than the expansion of the balloon as a whole - so we see local galaxies like M31 blue shifted (moving toward the Milky Way).
  6. Mar 22, 2006 #5


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    Both. Gravitational attraction pulls the earth back to the sun, but not exactly to the "correct" distance. And the difference is so small that it remains unnoticeable.
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