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Expansion slows down and then speeds up

  1. Sep 18, 2003 #1


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    the version of GR used in cosmology is the Friedmann equations
    which are a radically simplified version of the 1915 Einstein GR equation (simplified by assuming large-scale uniformity "looks same in all directions" kind of thing)

    if you put a constant energy density into the friedmann equations you get that the scale-factor of the universe increases in a ramp that is convex for a while and then at some point turns concave (begins to look like accelerating exponential growth)

    here's a picture

    so expansion first slows for a while and then (the models usually put it at a billion or so years ago, fairly recent IOW) begins to speed up

    the constant energy density that they put into the friedmann equation (typically about half a joule per cubic km) is called various things like Lambda and "dark energy" and "cosmological constant"----main thing is it is just some energy density constant thru space and time, its effect on expansion derives mathematically in a simple way from its constancy.

    Back in "Archive" someone asked about this.

    There is a simple explanation why in General Relativity if you put in a constant Lamda then (as long as it is not unreasonably large) you get a slowing first, while Lambda is still small compared to the MATTER density, and then when matter has thinned out enough for the effect of Lambda to take over you get a speeding up.

    Lineweaver "Inflation and the CMB" goes into this
    http://arxiv.org/astro-ph/0305179 [Broken]

    Lineweaver's is still the clearest introductory explanation of mainstream cosmology, clearest diagrams, plainest talk

    His article is mirrored at the CalTech Knowledgebase site, I will get the link and edit it in:

    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Oct 18, 2003 #2
    Geez, a long explanation of a Simple concept. Why is the expansion of the universe speeding up? Easy, because our universe is in the transitional expansion of an eventually decaying orbit around an even more massive body.

    Here is a simple experiment:
    1) Attach one end of a swivel to a ball,
    2) Attach the other end of the swivel to a string. (At this point you should be able to spin the ball with minimal resistance to the swivel and no resistance in the string.)
    3) With the ball spinning and holding the topmost part of the string, push the spinning ball into a circular motion.
    4) Watch the fun!!!

    As the spinning ball starts off rotationally faster than the orbital velocity of the balls trajectory perpendicular to the locus of the string. The Orbit of the ball is stable. However, as the ball looses rotational velocity, equilibrium between orbital and rotational velocity is equalized, then the magic happens. The Kinetic rotational energy is transferred to the orbital energy and the ball increases its orbital distance from the locus. As the additional boost of energy is used up, the orbit decays to zero. (This also works on an atomic scale)

    The primordial energy source of the "Big Bang" was spinning at its inception. Thus proved by the elliptical shape of the universe. Since everything inside our bubble of Space/Time is derived from that initial energy source, and conversely, everything is reactive to it. The fact that the universe is expanding, shows that there is a temporal field effect outside of our universe as well as in, (the differential between the two controlling the expansion rate.) The transfer of kinetic energy within our universe shows that it is reacting to a greater energy source outside our universe. I could go on, but the point is that as our universe expands, the internal local gravitational field effect decreases, as does the temporal curvature of our universe. While the outside source of attraction continues to react to the sum of the energy within our universe, the universe must start an inevitable orbital decay.

    Wish I had more time, gotta go, bye.
  4. Oct 29, 2003 #3
    A recent survey pinpoints the "cosmic jerk" to happens 5 billion of years ago:
    http://www.bayarea.com/mld/cctimes/news/6996467.htm [Broken]
    Last edited by a moderator: May 1, 2017
  5. Oct 29, 2003 #4


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    great article
    Bigtimers like Frank Wilczek and Michael turner getting into the act
    Hard to understand how someone at a small paper like
    the Contra Costa Times could have done so much research
    maybe there was an attribution to some news service and I just didnt see it, the dateline was Cleveland where Adam Reiss announced the result

    I will do a search with author's names like Adam Reiss and
    Joseph Lykken and see if other articles come up

    how do you find articles like this, meteor?
    it is fantastic to think of people seeing Type 1A supernovae
    as far away as 7 to 10 billion LY.

    they should call it cosmic growth "inflection" point instead of "cosmic jerk"
    the place where convex changes over to concave
    Last edited by a moderator: May 1, 2017
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