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Picturing the history of the world's expansion

  1. a straight-line ramp (steady expansion rate)

    0 vote(s)
    0.0%
  2. an exponential growth curve (increasing rate)

    2 vote(s)
    66.7%
  3. a convex ramp (decreasing slope)

    0 vote(s)
    0.0%
  4. silhouette of a woman's breast down to the nipple

    1 vote(s)
    33.3%
  1. Aug 2, 2003 #1

    marcus

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    The scale factor function (they write it R(t) or a(t)) gives the history of the expansion of the universe, as calculated from observational data. The slope at any moment is the expansion rate at that moment. Which of these best describes how you picture this curve?

    1. straightline ramp (constant expansion rate)
    2. exponential growth (increasing slope)
    3. convex ramp (positive but decreasing slope)
    4. silhouette of a woman's breast down to the nipple

    http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure14.jpg

    the solid dark line is the curve giving the best fit to current astronomical data
     
  2. jcsd
  3. Aug 2, 2003 #2
    Oh, LOL!!

    And so, would it then be best to describe some of the slopes as "perkier" than the others? LOL! :0)
     
  4. Aug 2, 2003 #3

    marcus

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    It was a conversation with you in "newton's" cosmological constant thread that sparked the particular idea so I have to share credit with you on it, and this discussion of the shape of the scale-factor curve in general.
     
  5. Aug 3, 2003 #4

    marcus

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    Well?

    Is everybody scared off by the anatomical comparison?

    Would you rather the curve were described as a

    "convex-then-concave ramp"?

    afterthought: or maybe a "tilde ramp"?

    the tilde symbol has the right kind of hump-then-dip, like a horizontal version of the ESS curve, so, if you make an upwards-sloping tilde, it looks about right.

    the real curve starts out at time zero essentially vertical---infinite slope, and is convex and rounds off, then has an inflection point and becomes concave. being vertical at time zero (a time when expansion was very rapid) is essential to the shape
     
    Last edited: Aug 4, 2003
  6. Aug 4, 2003 #5

    Phobos

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    you only get one of these per 1,000 posts, marcus. :wink:
     
  7. Aug 4, 2003 #6

    marcus

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    Yes, we have to keep it on the up and up!
    I believe in fact that she is wearing a low-cut evening gown.
    So everything is perfectly proper after all.

    BTW Phobos, how about doing us a favor and responding to
    Shadow's question in the Astronomy QA game. I believe Shadow
    is a comparative newcomer and he just posted a nice one. I'm
    refraining from answering because its time for some other people to get involved, with fresh interests.
     
    Last edited: Aug 4, 2003
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