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Is th U expanding?

  1. Mar 26, 2007 #1


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    Cosmography: Extracting the Hubble series from the supernova data
    Authors: Celine Cattoen (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
    Comments: 38 pages, 6 figures, 7 tables, uses iopart.cls

    We perform a number of inter-related cosmographic fits to the legacy05 and gold06 supernova datasets. We pay particular attention to the influence of both statistical and systematic uncertainties, and also to the extent to which the choice of distance scale and manner of representing the redshift scale affect the cosmological parameters. While the "preponderance of evidence" certainly suggests an accelerating universe, we would argue that (based on the supernova data) this conclusion is not currently supported "beyond reasonable doubt". As part of the analysis we develop two particularly transparent graphical representations of the redshift-distance relation -- representations in which acceleration versus deceleration reduces to the question of whether the graph slopes up or down.
    Turning to the details of the cosmographic fits, three issues in particular concern us: First, the fitted value for the deceleration parameter changes significantly depending on whether one performs a chi^2 fit to the luminosity distance, proper motion distance, angular diameter distance, or other suitable distance surrogate. Second, the fitted value for the deceleration parameter changes significantly depending on whether one uses the traditional redshift variable z, or what we shall argue is on theoretical grounds an improved parameterization y=z/(1+z). Third, the published estimates for systematic uncertainties are sufficiently large that they certainly impact on, and to a large extent undermine, the usual purely statistical tests of significance. We conclude that the case for an accelerating universe is considerably less watertight than commonly believed.

    It seems to be a matter of HOW it is measured.
    Last edited: Mar 26, 2007
  2. jcsd
  3. Mar 26, 2007 #2


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    I think the title of the thread you meant to write was "Is the Universe Accelerating".

    In any case this is an interesting argument, though I think most Cosmologists wouldn't disagree with their conclusions when it comes the the supernovae data alone. However when you include the CMB and galaxy survey data as well the case for acceleration gets much much stronger, to the extent that there is little doubt.

    However, an enormous caveat to this is that incorporating the CMB and galaxy data in this way to get tight parameter constraints and a high degree of confidence in acceleration assumes that the parameterization, and hence the physical model we use (general relativity) is correct. So if we assume GR (and other theories, such as axion-photon coupling limits etc) works the way we think it does then we can be very confident that the Universe is accelerating. If there are some more unexpected physics going on that we are unaware of then the tight parameter constraints make things seem more certain than they really are.

    Despite this it is remarkable how well the LCDM model fits completely different and independent data sets with the same set of parameters. Despite what some people may insist, there is no competing theory that explains things so well and so succinctly... yet. This may well change in the future however, which is why Cosmologist constantly try and think about ways in which we might be fooling ourselves!
    Last edited: Mar 26, 2007
  4. Mar 26, 2007 #3


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    Yes silly slip.
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