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Size of the Universe Early On

  1. Oct 14, 2008 #1
    The Universe is supposed to be 13.73 billion years old +/- 120 million years. (Source: http://en.wikipedia.org/wiki/Age_of_universe).

    The furthest seen galaxy so far is supposed to be: "Named Abell 1835 IR1916, the newly discovered galaxy has a redshift of 10 [3] and is located about 13,230 million light-years away. It is therefore seen at a time when the Universe was merely 470 million years young, that is, barely 3 percent of its current age." (Source: http://www.universetoday.com/2004/03/01/record-for-furthest-galaxy-is-broken-again/).

    I assume from this that the light has travelled 13.23 billion years to reach us from that galaxy travelling at the speed of light. In so doing it has travelled 13.23 billion light years of distance to reach us. Please correct me if this is wrong.

    I further take from this that the light began on its journey when the Universe was only 470 million years old.

    So I gather from this that the light began on its journey towards us 13.23 billion years ago from a distance of 13.230 billion light years away when the Universe was only 470 million years old.

    This leads me to assume that the galaxy was 13.23 billion light years away from us when the light began its journey when the Universe was only 470 million years old.

    If I take this as correct then it leads me to conclude that for the light to have travelled from 13.23 billion light years away the Universe was at least 13.23 billion light years across when the Universe was 470 million years old.

    Well actually if we are at the centre of the Universe (which I know we are not) then it must be at least twice that size and have been at least 26 billion light years across when the Universe was just 470 million years old; as you have to measure to the opposite side too.

    From this I would have to assume that this means that the Universe has to have expanded at the very least at the speed of 26 billion / 470 million x the speed of light for the first 470 million years (on average).

    The result I get is that the Universe had to have expanded on average at the rate of 55x the speed of light for the first 430 million years?

    That is going on a big bang theory. Is the expansion theory different to this?
     
  2. jcsd
  3. Oct 14, 2008 #2

    mgb_phys

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    The universe has been expanding since the light left the distant galaxy.
    So the galaxy and us have been moving apart as the light travelled.
    It's just like a footballer running backwards as a pass is thrown to them, it takes longer for the ball to get there.
     
  4. Oct 14, 2008 #3
    Hi mgb. Thanks for your reply.

    I thought that light was supposed to travel at the speed of light relative to the observer and not be affected by the speed the source is moving away?
     
  5. Oct 14, 2008 #4
  6. Oct 14, 2008 #5
    Thanks robertm

    Does this link mean that we can have parts of the expansion who's light will never reach us because the expanse stretches faster than the light can traverse the expanse between us and the source?

    Taking the football analogy, the football field expands faster than the pass is travelling so that the ball never reaches us?

    "A common misconception is that the expansion of the Universe cannot be faster than the speed of light." (Source: http://arxiv.org/PS_cache/astro-ph/pdf/0310/0310808v2.pdf)
     
  7. Oct 14, 2008 #6

    mgb_phys

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    Yes, the vast majority of the universe is unobservable.
    The observable universe is about 90Billion lightyears across.

    The reason you can have a 90B light year wide observable universe that is only 14Billion years old, is that the light was emitted by a distant object when it was closer to us. The universe then expanded to move us and that object further apart so that us and the most distant observable objects are now about 90B light years apart.

    You can move 90B light years apart in 14Billion years because although no object can travel faster than light, the universe can expand faster than light (at least for non-flat universes).
     
  8. Oct 14, 2008 #7
    Cool. Thanks for that link guys.

    Though I must admit I continue to admire the ability of 'science' to develop 'work arounds' so as to asway any conflicts with steadfastly adhered to tenets such as SR's constant speed of light.

    Well done!
     
  9. Oct 18, 2008 #8
    Something just occurred to me today.

    Our entire Universe is supposed to be expanding isn't it?

    That would mean that light travelling anytime anywhere would be travelling through expanding universe.

    So wouldn't the speed we 'measure' for light already take into account the expansion of the Universe as light is always moving through an expanding Universe?

    If light is always moving through an expanding Universe then why would it take any longer - due to expansion - to travel from a distant galaxy to us than it would to travel between any objects moving apart or towards each other?

    Why the exception for the length of time it takes light to travel the distance for galaxies moving apart under expansion when expansion is the rule anyway?
     
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