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Accelerated expansion without Dark Energy?

  1. Aug 22, 2009 #1
    There was an article in the Proceedings of the National Academy of Sciences where the authors derived a model which explained the accelerated expansion of the Universe without needing the ad hoc assumption of Dark Energy. I haven't really done any cosmology yet so I can't really appreciate the argument. What do you guys think? It seems that their theory goes against the Copernican Principle, which doesn't seem promising.

    Here's the article:
    http://arxiv.org/PS_cache/arxiv/pdf/0901/0901.1639v1.pdf" [Broken]
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Aug 22, 2009 #2
    The shockwave model of Smoller and Temple is a valid one mathematically, but to perceive accelerated expansion our galaxy must be in a rather unlikely place. It might be, but to convince other cosmologists they'll have to point to other observational evidence that their model explains better than the Lambda-CDM standard model. Not an easy task.
     
  4. Aug 22, 2009 #3

    Xnn

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    If the gravity behaved differantly at interglatic distances than it does on smaller scales, then there would not be a need to consider dark energy to explain the accelerating expansion of the universe.
     
  5. Aug 23, 2009 #4

    Chronos

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    Dark energy is not an 'ad hoc' contribution to the standard model. It is based on the supernova legacy study, which is well grounded. It is difficult to refute the dark energy proposition without new physics. No such models have been proposed to date.
     
  6. Aug 23, 2009 #5

    Xnn

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    Chronos;

    Could you explain what supernova legacy lead to the necessity of dark energy?
    My impression has been that it's the acceleration of the universe's expansion that leads to the need for dark energy.
     
  7. Aug 23, 2009 #6
    Having read and analyzed the data from the Supernovae Legacy Study, I too have my doubts about any need for 'dark energy'.

    Instead, it appears to me to be a last ditch attempt to save the General Relativity Standard Model.

    So exactly how is 'dark energy' said to be justified? Further, I do NOT see any need for 'new physics' in addressing this issue -- what is needed is to simply DROP this concept of a General Relativity determined cosmological redshift. All that is needed is a straightforward recession velocity expansion one.
     
  8. Aug 24, 2009 #7

    Chronos

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    See: http://arxiv.org/PS_cache/arxiv/pdf/0804/0804.4142v1.pdf
    Here is an excerpt:

    1. INTRODUCTION
    The evidence for dark energy has evolved from the first
    hints, for the case of a flat Universe (Perlmutter et al.
    1998; Garnavich et al. 1998; Schmidt et al. 1998),
    through the more definite evidence for the general
    case of unconstrained curvature (Riess et al. 1998;
    Perlmutter et al. 1999), to the current work which aims
    to explore the properties of dark energy (for a review see
    Perlmutter & Schmidt 2003). Several new cosmological
    measurement techniques and several new Type Ia supernova
    (SN Ia) datasets have helped begin the laborious
    process of narrowing in on the parameters that describe
    the cosmological model. The SN Ia measurements remain
    a key ingredient in all current determinations of
    cosmological parameters (see, e.g., the recent CMB results
    (Dunkley et al. 2008)). It is therefore necessary to
    understand how the current world dataset of SN Ia measurements
    is constructed, and how it can be used coherently,
    particularly since no one SN Ia sample by itself
    provides an accurate cosmological measurement.
     
  9. Aug 24, 2009 #8
    Good dataset -- or rather the best we have.

    However, the same dataset can be used to refute dark energy as unneeded. The problem is that the current data has a error that is still an order of magnitude too large to be able to choose between models.

    Dark energy is still well in the speculation zone -- far from acceptable mainstream or proved. In fact, the need for General Relativity on the cosmological scale is unproved and becomes more so as the apparent flatness of the universe gets more support.

    So far all I see is a declarative statement 'Dark Energy' and no compelling argument for its existence or need.
     
  10. Aug 24, 2009 #9
    First of all, I am not a cosmologist or scientist just an old engineer that is interested in cosmology. It seems to me that if there is a very small expansion force independent of distance between particles that is overwhelmed by gravity at less than intergalactic distances could explain the expansion of the universe without the need for "dark energy".

    The expansion force could be inertia from the "big bang" and be so small relative to the gravitation force in intragalactic distances that gravity keeps the galaxy together. However as the distance between galaxies increases the gravity force is too week to stop the expansion force.

    I am not trying to create "new physics" just a thought without any scientific basis.

    Peter Danforth
     
  11. Aug 26, 2009 #10
    The residiuals for high redshift supernovas clearly indicate the best fit curve overshoots those.
     
  12. Aug 27, 2009 #11

    Chronos

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    No worries, Peter, I too am an old engineer with an interest, but no particular expertise in cosmology. Dark energy is needed to account for the miniscule force causing space to expand between galaxies in deep space [a second law of thermodynamics thing]. The term 'energy' is misleading. It is not the kind of 'energy' we are accustomed to dealing with in mundane engineering calculations.
     
  13. Aug 29, 2009 #12
    If Newton knew what we know now would he have added another term to his gravitation equation that accounted for the expansion of the universe?
     
  14. Aug 29, 2009 #13
    Actually he may have done something very like that. In 'The Measure of the Universe' John D North says Newton does at one point mention something which is very much like a cosmological constant. Of course Newton didn't know about accelerated expansion, he wanted to get a static universe which doesn't collapse under gravity - which is the same reason why Einstein introduced the cosmological constant.
     
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