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Thread: Is space expanding or is spacetime expanding?

  1. #1 Is space expanding or is spacetime expanding? 
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    Within the framework of General Relativity, what is the correct view:

    Space is expanding

    or

    Spacetime is expanding

    or are both correct, or is neither correct?


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  3. #2  
    Brassica oleracea Strange's Avatar
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    Space is expanding. (Space-time is static.)


    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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  4. #3  
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    Quote Originally Posted by Strange View Post
    Space is expanding. (Space-time is static.)
    That is a strange answer.

    If one is talking about the standard cosmological model, AKA the Big Bang theory, then the correct answer is to say that, in one spacetime solution that is a good approximation to the dynamics of the universe, the mean distance between ideal points gets larger over time.

    This is not the only spacetime solution that one can use to describe the universe. There are many solutions that can be used in standard cosmology that give different divisions of spacetime into space and time.

    The expansion is something that happens to the spacetime. It is just a nice feature of the universe that we can identify a nice class of spacelike hypersurfaces that show the expansion isotropically and homogeneously to a good approximation.
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  5. #4  
    Brassica oleracea Strange's Avatar
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    That may be slightly more information than the OP was looking for. Or maybe not.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    Forum Professor Daecon's Avatar
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    So would the question "Can time expand?" be meaningless?
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    Quote Originally Posted by PhysBang View Post
    Quote Originally Posted by Strange View Post
    Space is expanding. (Space-time is static.)
    That is a strange answer.
    Should I get your coat?
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
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  8. #7  
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    Quote Originally Posted by Daecon View Post
    So would the question "Can time expand?" be meaningless?
    You would have to provide a clear meaning. In a sense, one can say that time expands in certain foliations of spacetime. See Don Page's "How big is the universe today?" Sadly, behind a pay wall: How big is the universe today? - Springer
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  9. #8  
    KJW
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    Quote Originally Posted by Daecon View Post
    So would the question "Can time expand?" be meaningless?
    The FLRW metric of cosmology has a degree of freedom that allows one to coordinate-transform a time-expanding metric to a non-time-expanding metric, making these two physically indistinguishable.
    Markus Hanke and dan hunter like this.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  10. #9  
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    " Is space expanding or is spacetime expanding?"

    How about neither?
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  11. #10  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by JoeSixPack View Post
    How about neither?
    Then how are you going to explain away all the evidence?
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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  12. #11  
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    Am I to conclude that both "spacetime is expanding" and "space is expanding" are both true?
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    Quote Originally Posted by polygon6 View Post
    Am I to conclude that both "spacetime is expanding" and "space is expanding" are both true?
    No.
    Post #2 was a clue...
    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
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  14. #13  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by polygon6 View Post
    Am I to conclude that both "spacetime is expanding" and "space is expanding" are both true?
    No. You can have either space expanding or time expanding ( these are physically indistinguishable ), but you can't have space-time expanding. The scale factor is always found in just one of the two parts of the metric.
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  15. #14  
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    That is very interesting. It feels almost intuitive. Maybe misleadingly so .
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  16. #15  
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by JoeSixPack View Post
    How about neither?
    Then how are you going to explain away all the evidence?
    in a streaming, limited observable, universe.

    we might conclude that space is expanding, just because we don't/can't see the whole picture.
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