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The Big Bang - acceleration

  1. Sep 27, 2004 #1
    Hi all,
    It's said that approximately 3 seconds after/into the Big Bang, the 'universe' expanded at a velocity > c.
    However, this breaks the laws of SR, right? "No mass can equal/breach c".

    So, could someone enlighten me on this subject?
    Why are BOTH "No mass can equal/reach c" and "the universe expanded at > c" (for those that believe in the BB) accepted? They contradict each other, right?

    Please enlighten me :)

    Thanks in advance,
  2. jcsd
  3. Sep 27, 2004 #2


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    It is a postulate of general relativity that in any spacetime the laws of special relativity apply locally. This means that the speed of light in vacuum cannot be exceeded by any object in a local experiment. Globally, the laws of special relativity apply only to a flat and static spacetime (Minkowski spacetime). The universe is assumed to be a Robertson-Walker spacetime (dynamic with spatial expansion) and thus the laws of special relativity apply locally but not globally. The fact that galaxies far away from us may appear to recede at speeds greater than c (which is also true today and not only during the first three minutes) is a consequence of Hubbles law (v = H d) which can be derived asuming a Robertson-Walker spacetime. This speed is merely due to expansion of space (galaxies are comoving with the expansion), although this galaxies may zero proper velocity "on" space. The proper velocity of galaxies "on" space is indeed limited by the laws of special relativity.
  4. Oct 4, 2004 #3


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    As hellfire said, a mass cannot move through space at a speed greater than c. But that law does not apply to space itself.
  5. Oct 4, 2004 #4
    I do not understand "space itself" vs "through space". I posted a question related to this in the "General Astronomy" section. What is meant by "space itself [can expand faster than c]"?
  6. Oct 5, 2004 #5


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    Picture this. If a car passes you faster than the speed of sound, will you hear it? Of course you will. The sound may not appear to be where you thought it was, but, you will still hear it go by you. Why would light appear to behave any different? The light has no idea how fast the source was traveling when it departed from the emitter. Do you think the sound will 'wink' out of existence once it passes you?
    Last edited: Oct 5, 2004
  7. Oct 6, 2004 #6


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    In Relativity, space is a physical part of the universe...not a void. Space is a thing that can be stretched, bent, compressed, etc.

    Picture the universe as matter & energy on a backdrop of space & time. "Spacetime" is sometimes called the fabric of the universe (although that can be a misleading name) - - it's what the universe is made of.
    Last edited: Oct 6, 2004
  8. Oct 6, 2004 #7
    I follow, but could you check out this thread I started last week or so :
  9. Oct 6, 2004 #8
    If it were assumed that the spatial component of c is not constant over time, then this would explain why the uiverse expanded at a rate greater than c (as it is here, now, in our current part of the universe), and it could explain why the fringes of the universe are expanding at a rate greater than c (as it is here, now, in our current part of the universe).

    If it is assumed that the spatial component of c has never changed, as is common and as you have seen on this forum, it is also possible to make things work by complex mathematical games and complex theories.
  10. Oct 6, 2004 #9
    Is that like how the Alcubierre (sp?) Warp Drive functions (in theory)?
  11. Oct 7, 2004 #10


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    Alcubiere and Space Expansion

    Closely related. But David Waite has pointed out that the naive view of the Alcubierre drive (which was Alcubierre's own original view) as expanding space behind the ship and shrinking it ahead, is wrong, and he has a counterexample in which the FTL effect is produced by a rotation on local spacetime.

    See this chapter of his online textbook on GR.
    Last edited: Oct 7, 2004
  12. Oct 7, 2004 #11
    Yikes, scrolling a few pages I have trouble understanding this math, some of the symbols and constants :cry:
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