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Closed Universe?

  1. Jan 31, 2006 #1
    Hello, first post here, I hope someone can help me with this.

    While not having studied physics myself, I am reasonably familiar with many key topics, in this case I found myself thinking about the standard Big Bang model.

    The question is simple - 'What is at the edge of the Universe'.

    My understanding was that spacetime was curved such that the universe was 'closed' The result was that the universe had no edge and that if one travelled in one direction long enough, one would return to ones starting point.

    This, I realised, can not be true though because such a universe would effectively be infinite (like being in a mirrored elavator) which would result in the Earths sky being as bright as the Sun from horizon to horizon as stated under Olbers paradox.

    My gut feeling is that my understanding of what a closed universe is is flawed, but if not - what is at the edge of the universe?

    Thanks in advance
  2. jcsd
  3. Jan 31, 2006 #2
    Oh wait......

    Is it because the light wouldnt have had time to reach us yet from stars viewed in the 'mirror universes'? I think I have answered my own question! Comments still welcome!

  4. Jan 31, 2006 #3


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    Most or less correct Monoxide, and Welcome to these Forums!!

    Olbers Paradox says that if the universe were
    1. Infinite,
    2. Eternal (infinitely old) and,
    3. Static

    then the sky at night should be burning bright.

    The fact that it is expanding will dilute the radiation so that it is not bright but dark at night as observed, also it is (probably) not eternal, so there has not been enough time to circumnavigate the universe.

    Finally, as the universe is expanding, light will never be able to complete the circumnavigation it in the first place.

    The end result? The sky is dark at night!

  5. Jan 31, 2006 #4
    But what of this issue of the edge of the Universe - is the closed universe idea I mentioned valid (that by going straight in one direction onwe ends up at the starting point)?

    Or is it a moot point as expansion means that we would never reach the edge even if we set off at light speed?

    I tried explaining BB to a friend who immediately asked what is beyond the edge - a reasonable question that I struggle to find a reasonable answer to.
  6. Jan 31, 2006 #5
    No. So far, the evidence points to a flat, accelerating universe.


    The farther we look, the greater the redshift. Hubble Constant*Distance=Recession Velocity. The value of "Distance" is the distance from the original state (the big bang singularity). Since no location can necessarily claim to be the center of the universe (Cosmological Principle), it is ok to pick any location as the center point. Two things affect how far we can see. First is the Recession Velocity. Where recession velocity equals c, objects are redshifted so much that we cannot see them any more. The equation for redshift as a function of recession velocity (in special relavitity) is this:


    While v<<c, the value of z is finite


    1.37720275 × 10^10 light years

    The rate of expansion of the space-time continuum can exceed the speed of light.

    The second thing that affects how far we can see the brightness of the light of the era of the universe we are looking at. So far, we can image individual galaxies and their structure at a distance exceeding 13 billion light years. However, we cannot image clearly individual stars of those distant galaxies. Also, Hubble's instrumentation does not detect ultra high redshifts at sharp enough resolution to image the first stars. The resulting demand is the Next Generation Space Telescope (NGST), or James Webb Space Telescope (JWST) which will have a resolution higher than Hubble at the infrared range.

    If you look at the light from the very distant galaxies at the same position you are right now, there would be no way of seeing these galaxies as they are right now, because all of these galaxies have already rushed beyond the edge of the universe. Their comoving distance is greater than 1.37720275 × 10^10 light years. One by one, the entirety of a galaxy passes a region where it is 1.37720275 × 10^10 light years from us. 1.37720275 × 10^10 light years after a galaxy or star goes beyond that we cannot see it or reach it any more.
    Last edited: Jan 31, 2006
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