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If the universe was closed

  1. Aug 14, 2011 #1
    Would it be possible to observe our own Milky Way Galaxy developing in its early stages?

    Or at least possible for a very old galaxy to observe itself developing in it's infant stages?
  2. jcsd
  3. Aug 14, 2011 #2


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    I can't answer your question for a matter filled closed universe because I haven't done the calculation but for a radiation filled closed universe even a photon that is emitted at the big bang only gets halfway across the universe before the the big "crunch" so I doubt there would be enough time for a photon to go completely "around" the universe such that we could observe ourselves from early stages.
  4. Aug 15, 2011 #3


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    Well, you would need a rather small radius of curvature for that to work, much smaller than the observable universe. So you don't just need a closed universe, but an overwhelmingly closed one.
  5. Aug 15, 2011 #4
    Can you guys also explain why?

    For example, WannabeNewton talks about a Big Crunch: is this an established fact? I thought the acceleration of expansion implied there won't be a Big Crunch? (I might be ill-informed)
  6. Aug 15, 2011 #5
    This statement is what prompted the question:

    "The particle horizon differs from the cosmic event horizon in that the particle horizon represents the largest comoving distance from which light could have reached the observer by a specific time, while the event horizon is the largest comoving distance from which light emitted now can ever reach the observer in the future.[59] At present, this cosmic event horizon is thought to be at a comoving distance of about 16 billion light years."

    Would this 16 billion light years (which is greater than 13.7 billion) allow me to view our galaxy when it was (16 - 13.7) billion years old (or 2.3 billion).

    It's more likely that my understanding of "co-moving" is severely flawed.
  7. Aug 15, 2011 #6


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    Ah. Co-moving distance is the distance in a coordinate system that moves "along with" the expansion. So think about it this way: your coordinate grid points are expanding along with the universe. So if two objects start out at a distance of three grid points away from each other, and assuming that they have no movement relative to each other through space (i.e. no relative motion that is not due to the expansion) then some time later, their co-moving distance from each other is still going to be 3 units (since they are still 3 grid points away from each other on this expanding or "co-moving" grid).

    The particle horizon (often just called the horizon) is the distance beyond which light has not yet had time to reach you. It defines the radius of the presently observable universe.

    The event horizon is the distance beyond which light will never reach you (no matter how long you wait). You can thus never obtain information about events beyond this horizon (just like the event horizon of a black hole).

    Where did you get your number of 16 billion light years? Can you post the source? It sounds wrong. Under the presently accepted model, the radius of the observable universe (i.e. the particle horizon) is thought to be around 46 billion light years.

    Now, to address the actual source of your confusion: it's perfectly possible to receive light from distant sources that are presently farther away from us than 13.7 billion light years. It doesn't mean that the light would have to have left the source "before" the beginning of the universe. IF the universe were static, then light that had been travelling for 13.7 billion years would have to come from a source that was now 13.7 billion ly away, and we wouldn't be able to see any farther. However, the universe is expanding. Therefore, sources from which light has been travelling for 13.7 billion years are actually now about 46 billion light years away, and that is the distance to the edge of our observable universe. So don't be puzzled by distances greater than 13.7 billion ly. The distance to an object can be greater than the light travel time for photons arriving from that object.

    EDIT: what I said in my last paragraph is also explained in this FAQ entry: https://www.physicsforums.com/showthread.php?t=506987 [Broken]
    Last edited by a moderator: May 5, 2017
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