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BBT questions

  1. Jan 25, 2012 #1
    a)since universe is expanding is today's 1 meter different from tomorrow's 1m?
    b)since distant between any 2 points is increasing is the universe expanding into 4th dimension?
  2. jcsd
  3. Jan 27, 2012 #2
    [One might wish to argue today's meter IS different from tomorrow's because the time dimension, the fourth dimension, is different...but the length is the same.]

    The universe is 'expanding' only over intergalactic distances...on earth and in our solar system for example, gravity holds everything together. [I guess it might vary imperceptibly over time as masses [suns,planets,etc] change positions....but is way finer than our ability
    to observe (measure). For example, if a gravitational waves passes, curvature varies and so would distance.] Space and time are fixed and immutable.

    The usual 'fourth dimension' is time. I don't think anyone knows for sure what our universe of spacetime is expanding into, perhaps 'nothing', perhaps what existed before the big bang.
    We don't know for sure if the universe is infinite or finite, bounded or unbounded. A cyclic theory has us and our dimensions endlessly oscillating between branes which provide boundaries.

    In another sense the question about what we are expanding 'into' is like asking, what are we expanding 'from'?? Usually when you 'leave one spot' you 'enter a new spot'...not necessarily so in cosmology.
  4. Jan 27, 2012 #3
    I forgot: a good article on cosmology from Scientific American, Lineweaver and Davis:

    http://space.mit.edu/~kcooksey/teaching/AY5/MisconceptionsabouttheBigBang_ScientificAmerican.pdf [Broken]
    Last edited by a moderator: May 5, 2017
  5. Jan 27, 2012 #4
    If you accept time as a proper dimension, then yes, the universe is expanding into that dimension. In the ballon analogy, the center of the ballon would become time zero, the interior the past and the exterior the future (the surface would be the present).

    Your first question is slightly trickier. The mass, electromagnetic field and other properties of the ruler you use to measure the meter with will largely cause it to retain its shape. But even if it was smaller, you would be too and so have no real way of discerning if it has shrunk at all. As naty 1 notes, though, expansion only wins out in deep space, the above noted properties of matter stop expansion from gaining ground locally.

    Edited: to reflect the first sentence is a proposition, not a statement of fact.
    Last edited: Jan 27, 2012
  6. Jan 27, 2012 #5


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    While this may be a workable example, I think it does more to confuse people. Generally we say that while it is possible for the Universe to be expanding into a 4th spatial dimension, the model we use doesn't require it. It is possible, according to the model, for there to be intrinsic expansion without needing anything for the universe to expand into.
  7. Jan 27, 2012 #6
    Can you cite any source for that claim??

    don't bother trying...it's not accurate. ok, I'll say it: it's naive and wrong. The prior post is way too kind.
  8. Jan 27, 2012 #7
    a) no.
    b) no.

    Confusion resolved.
  9. Jan 27, 2012 #8
    good article
    Last edited by a moderator: May 5, 2017
  10. Jan 28, 2012 #9
    is there any analogy to describe that model?
  11. Jan 28, 2012 #10


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    Only ones that expand into something else, such as the expanding balloon analogy.
  12. Jan 28, 2012 #11
    so does that mean we cant imagine how it would be?
  13. Jan 28, 2012 #12


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    No, it just means you actually have to look past the normal analogies to understand it. After all, they are just analogies.
  14. Jan 28, 2012 #13
    Your own post does two things. 1) It acknowledges that time is usually treated as the fourth dimension. 2) That noone knows for sure what spacetime is expanding into. In view of this, the extended ballon analogy, while worded perhaps overly strongly with too many "is" and not enough "would be", is nothing more than an extrapolation. Who came up with it? I did after realising that two points in spacetime can, as they come closer together while rewinding time, be seen as a series of concentric arcs seperated by regular units of time, describing constant expansion (such expansion is used to calculate the age of the universe).

    I therefore do not see what, exactly, you need a source from me for. More to the point, your post becomes noting but rude.

    I apologise to the OP in one specific regard which is that I shouldn't have posted it at all in this thread. This is speculation, and although intended to remove a persistant issue people have with the ballon anology, should not be taken as a difinitive answer to your b) question.
  15. Jan 29, 2012 #14
    i dont understand. Gravity held matter together billions of years before but it dint stop the universe from expanding. When i said today and tomorrow i dint mean it literally. (say the difference between the lengths of 1 m today and after 10 billion years)
    if you are not sure what we are expanding into how do you answer 'no' to my question?
    are you telling me my questions are nonsensical?
    btw thanks for the article!
  16. Jan 29, 2012 #15


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    We measure the length of the meter in either how far light travels in a specific amount of time, or by objects we define as being 1 meter long. The expansion of space does not change either of those. Light still travels the same distance per second, and the object is still 1 meter long. The reason is that both the electromagnetic force, which holds atoms together, and gravity are both much stronger than expansion.

    Because the standard model says that it isn't required, which many take to be no.

    No, your question is fine. He is merely showing you that it is the result of an incorrect view on expansion. If you look at the universe, the expansion of space doesn't get anyone to any location, aka you never get anywhere. Our galaxy group doesn't get any closer to any other groups in ANY direction. Hence the phrase: "Everything is moving away from us". That is quite literally what is happening.

    Not only that, but to every galaxies viewpoint the result is the same. You can say that "Everything is getting further away from everything else". (Except within galaxy clusters/groups, they are gravitationally bound and do not experience expansion)
  17. Jan 29, 2012 #16
    is it safe to say that all galaxies were 1 during bigbang and were teared apart as time went?if so isn't the rod which you use to measure is tearing apart right now?hence the length varies. the article cited by Naty did seem to explain this but i find it ambiguous
    i am unable to convince myself that a nth dimension can loop back to itself without requiring n+1 th dimension may be i need some math what is it? Please suggest a link thx
  18. Jan 29, 2012 #17


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    The "force" from the expansion would be very small and would cause an object to be very very slightly larger than it would be without expansion, but it would not tear it apart. Not yet at least. If the acceleration of expansion continues, it may occur billions of years from now.

    Also, galaxies hadn't yet formed right after the big bang. There was just a very hot very dense universe full of light that created anti-particles which then annihilated to create light again, continuing the cycle. As expansion continued, the density decreased and eventually it was cool enough for stable atoms to form and for light to stop interacting with all that matter, thus creating the Cosmic Microwave Background.

    I don't have one myself. I'd say study up on general relativity and it should be there somewhere.
  19. Jan 30, 2012 #18
    Drakkith's post # 15 explains my prior comments just fine. Thnx.

    no way....Read here:


    Everything was initially some highly energetic radiation. No matter initially.

    After that, You'll read for example, that simple hydrogen and helium atoms did not begin to form until about 377,00 years after the big bang...and they gradually form stars, and those coalesce even later to form galaxies and so forth.
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