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Current eff. universe limited to redshift 1.73

  1. Feb 26, 2010 #1


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    the figure 1.73 is approximate...given uncertainties, somewhere around 1.7.

    We see galaxies out beyond z = 7, of course, and the ancient background light has redshift 1090, so we see ancient matter emitting light, before it even formed stars and galaxies.

    But as of now, the standard model says, nothing that happens more than 15.7 billion lightyears away can affect us, from now on.
    And that 15.7 distance corresponds to redshift z = 1.73

    In a curious way our "from now on" causally effective universe only extends to the socalled "cosmic event horizon" at z = 1.73.

    All the photons that will ever reach us from OUTSIDE that horizon are already on their way to us and already inside that range. If they were not, they would be destined to not ever reach us.

    Most of the galaxies we can see are outside the z = 1.73 range. And we can look forward to admiring these millions of beautiful galaxies for a great many more years. But if something happens today in one of them--like a supernova--we will never see it. Nothing that happens to them or in them from now on can affect us.

    There is a good article about the cosmic event horizon by Charles Lineweaver here
    also a new article by Nobelist George Smoot et al.

    This leads to a curious question. Our model of the cosmos is the pair of Friedman equations. These run on "universe time" a universal time parameter t, and govern the evolution of the scalefactor a(t). What the model predicts will happen with a(t), what it says about future expansion, depends only on the present. That's just as we expect with any system of ordinary differential equations.

    Normally one thinks of the Friedman model as treating the whole boundless universe as an organic whole. But maybe we should modify the Friedman equations by introducing a boundary term representing the cosmic event horizon.

    This seems to be what the co-authors Smoot, Easson and Frampton are doing. I suppose it could be thought of as primarily a formal change: a change of interpretation or in how we think about the model. But right now I find it a bit puzzling. If other cosmologists go along with this, it will take some getting used to.
    Last edited: Feb 26, 2010
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  3. Feb 26, 2010 #2


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    Ok, I've been just skimming the Smoot paper. Not that I really understand much of this information/entropy thing, but their logic sounds extremely circular to me. As i understand it, they use the horizon (which is due to acceleration) as a cause of acceleration. That's fine, and circulary, IMHO. But when they use the Hubble scale to predict acceleration, independent of a horizon, as they do in eq. 12, that's ...weird.
    I know that I'm near to obnoxious with my constant use of non-cosmological coordinates, but in this case you see that (12) gives us arbitrary acceleration (measurable) in an empty universe, depending on how we choose H (not measurable).
    Either I haven't understood at all what they're proposing, or there is something really wrong with it. Maybe someone can enlighten me.
  4. Feb 26, 2010 #3


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    Relax, Ich. It is early days. This is a potential paradigm shift which will either catch on or not catch on. We just have to wait and see. Smoot might get a second Nobel, or he might not.

    It might help to get an idea of the personality of the man. Here is a 16 minute video lecture by him with some interesting slides including computer-simulation movies of the early universe. Google "Smoot TED"
    and you get

    It is a popular wide-audience lecture. You might not like it because it is so wide-audience. But it gives some glimpse of a real person instead of just an author's name.
    Last edited by a moderator: Sep 25, 2014
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