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Insights The Balloon Analogy ... the Good, the Bad, and the Ugly - Comments

  1. Sep 16, 2015 #1

    phinds

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  3. Sep 16, 2015 #2
  4. Sep 16, 2015 #3

    Geofleur

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    Nice article! I just used the balloon analogy today with my students, and I didn't say any of those wrong things. But I worry now that my students would draw those spurious parallels themselves. I may have them read this.
     
  5. Sep 16, 2015 #4

    phinds

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    The "first" ? AAAACCKK ! You want MORE? I don't know anything else :smile:
     
  6. Sep 17, 2015 #5
    well done and thumbs up! I sometimes find myself in sticking too close to the ballon model, but it helps in the beginning.

    perhaps in future one comes back to the balloon-model by taking it more by word like "what behaves on the surface like a balloon might behave in the inside like a balloon, as well"
     
  7. Sep 17, 2015 #6

    ShayanJ

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    I have the same feeling!
     
  8. Sep 17, 2015 #7

    timmdeeg

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    Hi phinds, I think you article is extremely helpful to the 'interested layman'.
    The only thing I would recommend to reconsider is the wording 'the rate of expansion' is slowing down or is accelerating, resp. This could confuse the layman who knows about the Hubble constant, which isn't accelerating. The expansion of the universe is either accelerating or decelerating. Or perhaps, but I'm not sure, it's more precise to say the universe expands at an increasing rate, in order to avoid the term 'the rate of expansion'.
     
  9. Sep 17, 2015 #8

    JBA

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    This is what younget when someone tries to explain something that no one actually understands.
     
  10. Sep 17, 2015 #9

    phinds

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    I don't get you. What is it that "no one actually understands" ?
     
  11. Sep 17, 2015 #10
    Many of those interpretation problems do not appear if we consider matter contracting instead of space expanding.
    Those are equivalent point of views as far as we know.
    All we can measure are ratios of distances.
    (If fraction a/b is increasing, is a increasing or b decreasing?)
     
  12. Sep 17, 2015 #11
    The point is that the so called "Hubble constant" is not a constant over time.
     
  13. Sep 17, 2015 #12

    phinds

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    And how would "contracting matter" explain the red shift of light from distant galaxies?
     
  14. Sep 17, 2015 #13

    PeterDonis

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    Perhaps not, but you now have a whole new set of problems. Such as, if matter is supposedly contracting, why is the size of the Earth not changing?

    On cosmological scales, perhaps this is true, since converting between the various cosmological distance scales is basically taking ratios of different indirect distance measurements. But ultimately all of those cosmological distance ratios are calibrated to distances that are not measured as ratios. I used the size of the Earth as an obvious example above, but perhaps a more relevant example for this discussion would be distances to stars measured by parallax. That gives an absolute reference for distance that cannot be interpreted as "contracting".
     
  15. Sep 18, 2015 #14

    JBA

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    What is actually driving the observed acceleration and expansion/inflation of our universe.
     
  16. Sep 18, 2015 #15

    timmdeeg

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    That's right and therefore it might be better to use the term Hubble parameter, which means the 'rate of expansion'.

    In short, during the epoch of inflation the universe expanded exponentially (driven by the cosmological constant only) and thus the 'rate of expansion' was constant (roughly). Since then it is decreasing and will be approaching asymptotically a constant value in the very far future again, due to the dominating dark energy then, at least according to the current model.
     
  17. Sep 18, 2015 #16

    phinds

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    OK. I agree w/ that. I wasn't sure if you meant what it doing as opposed to what causing it.
     
  18. Sep 18, 2015 #17

    JDoolin

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    I think there really are more than one model out there that you have to address. It's really not fair to just say "A lot of people think that the center of the balloon represents the big bang singularity, but that's just not true."

    The balloon analogy is primarily used to explain how redshift is NOT caused by relativistic Doppler recession of distant galaxies.

    However, I think if you take the balloon analogy, but only the TWO features you said were true, and "caveat" the FIVE features you said were false, there is no material way that your model actually conflicts with the idea of Doppler recession velocities.

    So is there a valid feature of the Balloon analogy that actually contradicts with Doppler recession?
     
  19. Sep 18, 2015 #18

    phinds

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    That has not been my experience at all. I have always found the balloon analogy used to simply give a graphic demonstration of how it is that the universe is expanding uniformly from every point and that there is no center. I'm not familiar with its use regarding any discussion of Doppler shift.
     
  20. Sep 18, 2015 #19

    JDoolin

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    Here, look at this page on wikipedia regarding the cosomological scale factor: https://en.wikipedia.org/wiki/Scale_factor_(cosmology)

    The article begins with the assumption that the scale factor exists... and comes down to
    v = H d; e.g.
    velocity = Hubble's Constant * Distance

    The implications here is that Hubble's constant is... well, a constant. And the velocity and distance relationship is... well; weird.

    However, a simple modification to the equation; stating that Hubble's constant is the reciprocal of the age of the universe, yields

    Distance = velocity * time

    which you teach to students in Junior High.

    In my experience, very smart people are very uncomfortable with the idea of DIstance = Velocity * Time being applied at cosmological scales. They will definitively say "No, that is NOT it." And usually, they will use some version of the balloon analogy to make their point. However, here, you seem to have debunked all aspects of the balloon analogy which would have actually conflicted with the kinematic description.

    My point, I guess, is that as soon as you invoke that scale factor, a(t), then you are strongly implying that space is (or at least could be) stretching over time--perhaps in an unknown and unpredictable way.
     
  21. Sep 20, 2015 #20
    I don't care for the balloon analogy. I like the "baking raisin bread" analogy.
     
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