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  1. #1 questions 
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    hello there,
    i am interested in science in a lay way,when i read about a topic or see a documentary,it always throws up more questions...for example,i saw a programme on the universe expanding...if that is so,why isn't the earth moving further from the sun,or is it?


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    Universal Mind John Galt's Avatar
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    The expansion does not effect concentrations of matter on the local level, like atoms, solar systems and galaxies. There's not much more to say. The wikipedia article might be of interest to you.


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    Brassica oleracea Strange's Avatar
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    You can think of the Earth-Sun system as being held together by their gravity. The same is true right up to the level of galactic clusters (which is why we see Andromeda flying towards us, rather than away). It is only at the really large scale that the distances between things is increasing.
    ei incumbit probatio qui dicit, non qui negat
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  5. #4  
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    the programme did not explain that...does that mean dark matter/energy only exists between galaxies,if it exists? and if scientists know the universe is expanding,shouldn't they be able to work backwards to find where it began?
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  6. #5  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by tkshots View Post
    the programme did not explain that...does that mean dark matter/energy only exists between galaxies,if it exists? and if scientists know the universe is expanding,shouldn't they be able to work backwards to find where it began?
    Dark matter and energy are not (directly) related to the expansion. Dark matter is thought to explain the unexpected rotational velocities of stars and other material within galaxies. Dark energy is hypothesized to explain the apparent acceleration of expansion.

    And, yes, you can trace the expansion back. The big bang model suggests that the universe was in a hot dense state 13.7 billion years ago and has been expanding and cooling since then.
    ei incumbit probatio qui dicit, non qui negat
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  7. #6  
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    yeah,i know when it began,i was wondering do scientists know where it began? shouldn't they be able to work that out?
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  8. #7  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by tkshots View Post
    yeah,i know when it began,i was wondering do scientists know where it began? shouldn't they be able to work that out?
    Oh, sorry. I misread it. It "began" everywhere. The universe is expanding uniformly. Wherever you are, you will see everything (beyond a certain distance) moving away from you.

    Similarly, if you run it backwards and imagine everything getting closer together then wherever you are you will see things getting closer to you. So everywhere appears to be the centre. Everywhere is the centre.
    ei incumbit probatio qui dicit, non qui negat
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    i was thinking in terms of the big bang,if there was one...i thought if you worked back,you would be able to trace it to a location...
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  10. #9  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by tkshots View Post
    i was thinking in terms of the big bang,if there was one...i thought if you worked back,you would be able to trace it to a location...
    You can: everywhere!

    The important thing to get your head around is that it was not an explosion in space; it is an expansion of space. And the expansion is homogeneous and isotropic (the same in all directions and from all locations). This means that, wherever you are, you will appear to be in the centre.
    ei incumbit probatio qui dicit, non qui negat
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  11. #10  
    Quagma SpeedFreek's Avatar
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    Yes, this applies to the Big-Bang too.

    If you trace time backwards, everything comes back towards us right here. So, the Big Bang happened right here.

    The same would apply wherever you were in the universe - you would track the expansion of the universe right back towards you.

    EDIT: I see Strange beat me to it!
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  12. #11  
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    so are you saying it did not explode/expand in one point(a la the big bang),but everywhere? if so,where does the theory of the infinitesmal point with infinite density fit in?
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  13. #12  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by tkshots View Post
    so are you saying it did not explode/expand in one point(a la the big bang),but everywhere?
    That is pretty much what the big n=bang theory says.

    if so,where does the theory of the infinitesmal point with infinite density fit in?
    Poor journalism and over-simplified analogies, I think. It is easier, and more dramatic, to describe it that way rather than take the time to get across a more realistic picture.
    ei incumbit probatio qui dicit, non qui negat
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    is it like a balloon that has been filled with air,and if scientists try to trace the origin,letting the air out,it looks like you are the origin,because you are on the "surface"? if so,is there any way to trace it further,or is there no further?
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  15. #14  
    Quagma SpeedFreek's Avatar
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    As far as we know, there is no further. In the balloon analogy, it is only the surface of the balloon that represents the universe. Inside and outside of the balloon are not part of the universe.
    Last edited by SpeedFreek; August 29th, 2012 at 03:12 PM. Reason: typo
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  16. #15  
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    i understand it much better now,thanks...
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