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Seeing galaxies

  1. Aug 23, 2010 #1
    If galaxies aer expanding fast than the speed of light how are we able to see them. At the moment this expansion surpassed the speed of light shouldnt these galaxies dissappear or remain unchanged visually?
     
  2. jcsd
  3. Aug 23, 2010 #2

    russ_watters

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    Staff: Mentor

    Galaxies are not expanding.

    Do you mean moving away from us due to the expansion of the universe? You can see galaxies that appear to be moving away from us faster than the speed of light for much the same reason you can hear an object moving away from you faster than the speed of sound. The speed of light is not dependent on the speed of the moving galaxy.

    There is a limit, though, based on the universe expanding so fast the distance is increasing faster than the light moves.
     
  4. Aug 28, 2010 #3
    Certain galaxies might be presently unobservable because they are receding from us at faster than light speed due to cosmological expansion. But the light of others might still be streaming on its way toward us and so we see them as they were prior to reaching that superluminal velocity. Keep in mind that the galaxies themselves cannot exceed the 186,000 speed limit. It's the space between galactic clusters that is expanding at that velocity and beyond and thus creating insurmountable distances between us and them and making their present light forever undetectable for us.

    Also, we are talking about areas of space where gravity doesn't cancel out cosmological expansion.
    Galactic clusters containing from as few dozen to millions of galaxies are not internally affected by that expansion. In fact, within these clusters, galaxies collide and merge. Some. like our Magellanic clouds which orbit our Milky Way are satellites of other galaxies. Andromeda, the largest member of our own galactic cluster is presently approaching us and seems destined to collide with our our galaxy in the far future. The the distancing is between super clusters of galaxies where gravity cannot predominate.

    BTW
    By unobservable I mean that the light being emitted by them now will never reach us.
     
    Last edited: Aug 28, 2010
  5. Aug 29, 2010 #4

    Chronos

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    Receeding galaxies merely redshift into obscurity, they will never suddenly 'disapperar' from view.
     
  6. Aug 30, 2010 #5
    so they will eventually fade away?
     
  7. Aug 30, 2010 #6

    Chronos

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    Yes, according to current models.
     
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