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If the observable universe were the entire universe, would the mass make it expand?

  1. Feb 28, 2008 #1
    Kind of a newbie question that may have an easy answer, but if the observable universe IS the entire universe, and assuming 96% dark matter (which is the current estimate, no?), would the resultant mass (I saw 3 x 10 to the 55th power g/cm3 somewhere) make the universe continue to expand, or eventually collapse?

    Many thanks,

  2. jcsd
  3. Feb 29, 2008 #2


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    the equation that governs the rate of expansion depends on the average density

    regardless of the overall size, so if the size were only that of the observable chunk, then other things being equal it would keep on expanding.

    be careful with terminology, you say "dark matter is 96 percent" but it isn't

    common estimates are dark energy 73 percent
    dark matter 23 percent
    ordinary matter and radiation 4 percent.

    dark energy is very different from dark matter----totally, you would not believe how different! ask about it if you want. they should not be mixed up.

    A good way to keep track of the estimated average density is to convert everything to energy terms (E = mc^2, you can always convert grams to joules)
    and then the density (including dark energy and dark matter) is around
    0.85 joules per cubic kilometer.

    your figure for the density does not make sense-----it is 3 x 1055 times the density of water. the density of water is one g/cm3
  4. Mar 1, 2008 #3


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    Observational evidence overwhelming supports the existence of a huge amount of exotic, dark [invisible] matter in the universe. Google on gravitational lensing / dark matter for examples.
  5. Mar 18, 2008 #4
    dark matter is not supposed by all or necessary

    Not everybody believes in dark matter as a concrete object.

    If we suppose that the universe is expanding at twice the speed of light it would have 8 times the expected volume which is approimately the ratio for actual to dark matter.

    The conflict between Sandage and de Vacoleurs gives two values for the hubble constant with one double the other. If our universe is within an outerverse that is itself expanding this could account for why there appears to be a need for dark matter when in fact there does not appear to be any.

    M-Theorists propose the existence of such an outerverse.

    I propose an expanding outerverse as a solution to the dark matter conundrum

    Ed Joyce
  6. Mar 18, 2008 #5


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    Welcome to PF!

    We observe the rate of expansion from which to infer the amount of dark matter (along with other parameters). We also have a wealth of other data, so this simple proposal cannot resolve the dark matter problem. If the Universe was expanding at twice the rate we thought we would simply observe it do be doing so, and hence would never have though it was expanding at a lower rate in the first place.

    Current differences in the measurement of the Hubble constant are between a value of ~72 and ~62 which comes from different distances measured to the LMC which calibrates the cepheid period-luminosity relationship. This is much less than a factor of two.

    You are going to have to define 'outverse' here much more clearly. This is certainly not in the vocabulary of string (or M) theories I've read about?
    Last edited: Mar 18, 2008
  7. Mar 18, 2008 #6
    When the 'Bullet Cluster' data came out I was convinced it locked the case for dark matter. Then John Moffat published;

    The objection that the modification is ad hoc is a very serious and legitimate objection. However, the data fit to rotation curves, and now cluster data, is not automatically garranteed as is sometimes claimed. In fact even maintaining the dark matter hypothosis MOG appears to make a good model on which to define expected distributions of dark matter.

    I don't have the answer and the evidence for dark matter remains very powerful. I would say using the term "overwhelming" is premature.
  8. Mar 19, 2008 #7


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    Indirect evidence for DM is the real 'bullet'. Large scale structure in the early universe is impossible to accomodate without a huge amount of DM. The infinite universe hypothesis does not resolve this dilemma.
  9. Mar 19, 2008 #8
    Here you are using model specific assumptions to justify assumptions of the model. Granted it is very strong evidence but you don't strengthen that evidence by circular logic. Indirect evidence can't be construed as a "bullet". That's the kind of myth creationist chase. I don't know where you got the infinite universe hypothesis (IUH) unless you assumed it from the Weyl model I spoke of. However, would the dilemma even exist in an IUH model? I don't hold any given model in high enough regard to say what an ideal model would look like. It certainly seems like way too many coincidences to boot the Big Bang entirely. The only thing I concern myself with is specific mechanisms from which empirical consequences can be defined. The standard model of cosmology is certainly the most probable of any given model, but within the domain of all possibilities its singular chances are probably not what they are perceived to be.

    I'm not objecting to Dark Matter as such, my objection was the characterization as "overwhelming". None of the choices of hypothesis are really very satisfying wrt the standard model, either of cosmology or physics, or both. I'm hedging my bets and will continue to object to certainty under these circumstances.

    If you have more than "indirect evidence for DM is the real 'bullet'" I am certainly interested. If that is all you have fine, it's still powerful evidence just not "overwhelming". The major advantage Dark Matter has over no Dark Matter models is the degree Dark Matter has been presupposed in the standard Big Bang model. Powerful, yet a retrodiction by design and not a difficult feature to save with a MOND type interpolation function, possibly even the same one used to fix galaxy rotations. Perhaps even mute it with some kind of IUH model as you suggested. Burn me as a heretic, I'm not the one demanding non-empirical certainty.
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