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The universe and such

  1. Mar 16, 2006 #1
    I have a question I've wondered about for some time. When astronomers say the furthest objects seen are roughly 13 - 14 billion light years away, does that mean they are currently 13 - 14 billion light years away (assuming they still exists) or they were 13-14 billion light years away when the light that has now reached us was emitted (i.e., 13-14 billion years after emission)?
  2. jcsd
  3. Mar 16, 2006 #2
    They were 13-14 billion years and light years away when they emitted the light that we see today.
  4. Mar 17, 2006 #3
    I think that lightyears (13-14 billion years ago) must have been shorter than they are now because of the stretching of the universe sins then. So, yes the distance is 13-14 billion lightyears, but lightyears have to be interpreted in a relativistic way? Am I right?
  5. Mar 17, 2006 #4


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    The only way the light year could have been shorter in the past is if the speed of light was less then. A light year is the distance light travels in a year; the year is an artificial unit of time that won't vary, so,.... The expansion of the universe stretches space, but not time, in the standard view.

    Now of course there ARE theories with varying light speed, but they are not contemplated in the standard cosmological model in which these estimates are calculated.
  6. Mar 17, 2006 #5


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    I would say neither. The most distant objects that we observe are now ~30 billion light years away and they were ~4 billion light years away at the time they emitted the light. Since the numbers you quote are roughly consistent with the age of the universe (~13.7 billion years), they must be referring to the distance traversed by light from the objects to us.
  7. Mar 17, 2006 #6
    Sheesh! The web sites I looked up for the most distant objects need to be seriously updated if they're ~30 billion!! Thanks, that's the answer I was looking ofr.
  8. Mar 20, 2006 #7
    To Self adjoint. This was what you stated:
    "but not time, in the standard view."
    Is this where we disagree? Is this where I am wrong, and why?
    Kind regards
  9. Mar 20, 2006 #8


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    Cosmology is not done with full-bore general realtivity (which is too complicated to solve in the large) but in very symmetrical models. Geometrically these models can be represented by the cartesian product of the Real Line, representing cosmological time, with a 3-sphere or Euclidean 3-space, representing space. And the cosmological expansion is about the spatial sphere, not the linear time. The expansion is expressed as the change in the radius of the sphere or units of the space with respect to the cosmological time.

    Unless is specifically working in a theory about the variation of c, which is not a standard thoery, then c will remain constant through all the expansion.
  10. Mar 21, 2006 #9


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    The problem with time is it is not absolute. When you stretch space, you also stretch time. This is the point of GR and SR. Space and time are fundamentally linked; like pi and the circumference of a circle.
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