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Why 'dark energy'?

  1. Mar 13, 2007 #1
    This is my second question.

    As I understand the currently popular explanation for the accelerating expansion of the universe, it is based on the hypothetical existence of a uniformly distributed form of expansionary energy. Again, as I understand expansion, it is not the movement of bodies into previously 'empty' space, but the expansion of space itself. The analogy often used is the expansion of the membrane of a balloon as it is inflated. Dots on the membrane move apart, not because they are moving on the surface of the balloon, but because the surface is expanding.

    At the risk of over extending the analogy, one of the characterists of the expansion of the surface of a balloon is that the expansion is a property of the balloon, not a property of the material that composes the surface. My question is, why is there so much emphasis on explaining the expansion of the universe based on the interaction of the objects occupying space-time? Is it completely outrageous to suspect that the expansion of the universe may be a property of space-time itself and that it is independent of the 'suff' that resides within it?
  2. jcsd
  3. Mar 13, 2007 #2
    I don't think that this is a outrageous thought, fact is I think that we know that time is ever expanding and being part of space-time I would be surprised if it stopped. As for the 'suff' within space-time, I don't think of it as independent because it will always be a part of a one dimensional object, time, until we can separate time into more than one dimension it will always be part of the whole.
  4. Mar 13, 2007 #3
    It depends how you look at it. :smile:

    If we assume that general relativity is the correct theory that describes the "shape" of the universe over time then we have to realize that this "shape" is determined by the matter and energy distribution over time. So in that sense it is incorect to state something like "expansion is a property of space-time itself and independent of the "stuff" that resides within it".

    But, since the relationship between space and time does not satisfy the Pythagorean theorem, we could, but we don't have to, consider the consequences of that fact "properties" intrinsic to space-time.

    The inflating balloon model is far more confusing that helpful IMHO. The impression it gives that space somehows "blows up" is absurd, again IMHO.

    A much better educational tool for understanding the universe, again in my opinion, many will certainly disagree, is to start with the Milne model, it describes the development of a massless expanding universe filled with test particles.
  5. Mar 13, 2007 #4
    Perhaps I should not have used the term 'independent'. What I am trying to get at is that the rate of expansion of space time may not depend on the amount of 'stuff' that resides in it. Another way to look at it: What if the expansion of the universe is what produces the 'stuff' that resides in it? That is, the 'stuff' is an effect of expansion (or at least both expansion and the 'stuff' have a common cause).

    I agree with you that the balloon analogy is not very satisfying. I mentioned it only because it comes up regularly in this context and I thought it helped illustrate what I mean when I refer to a property of space-time rather than the 'stuff' contained in it. I'd love to find a better image and would welcome suggestions.
  6. Mar 13, 2007 #5


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    You will need a mechanism to produce this 'stuff' from the expansion otherwise your suggestion is just a wild speculation. :wink:

    You may care to take a look at http://en.wikipedia.org/wiki/Self_creation_cosmology [Broken])

    Last edited by a moderator: May 2, 2017
  7. Mar 13, 2007 #6


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    I don't think working cosmologists would consider it at all outrageous since many share that suspicion or point of view.

    1. one of your problems is you read NYT and think that represents reality. best to try to read technical papers even if you only get parts of them, than rely on popular journalist accounts.

    2. a big lesson from Einstein circa 1916 was that space and time have no reality apart from the dynamic GEOMETRY of space and time. that is, the evolving distance function or "metric" which is the variable governed by the Gen Rel equation.

    a "property of space-time" is logically a property of the equation describing the dynamics---the evolution of that metric.

    one possible feature of that equation is a small positive Lambda or cosmological constant.

    3. working scientists often do not BELIEVE theories, they find the most useful effective theory to use provisionally-----and they hope that the prevailing theory can be tested and perhaps shown to be unequivocally wrong, so that one can revise, or replace it.

    just because a NYT reporter, in a rather colorful article, suggests that working scientists believe this and that---not to take literally :-)

    4. DM and DE are convenient data-fitting ways of making a very useful effective model work. Folks are not naive. They know LambdaCDM could well be wrong. Let's hope it can be falsified soon and we get to revise it! But for the time being that is the best-fitting most useful picture and the one you use when writing papers. However there sure are alternatives to LambdaCDM being developed and considered and tested!

    Among alternatives relativisitc MOND is a big deal, but I see lots of papers being written about other ideas as well. NASA scheduled a major press conference with important people and several days advance publicity last year when they thought they had some HST observations that shot down MOND. That was some of the most important astro news of the year. Because MOND must be really really important, if they make such a fuss about finding evidence to refute it. But you don't hear MOND in NYT coverage.

    There are versions of relativistic MOND that do away with DM and DE by doing pretty much what you suggest, namely building them into the laws defining spacetime. So you dont need extra "stuff" because the observed behavior arises from some "property". Congratulations for having thought of something that some other people also thought was a good idea!:-)

    So it is not "outrageous" to think what you said. Smart professional research people consider those possibilities.

    Actually relativistic MOND is not the only---I keep hearing of others. I don't personally have a favorite. The various versions of MOND are just the most visible. Garth already mentioned it in post #2.

    the point is, though, that one can get a distorted perspective from the popular press.
    Last edited: Mar 13, 2007
  8. Mar 13, 2007 #7
    That the popular press is creating the distortion was one of my assumptions. (I included it as a possibility in my first question but didn't repeat it here.) I am glad to see you agree.

    Unfortunately, the popular press is the basis most people have for their understanding of science. The reason I raised these questions is that this topic is becoming very common in the popular press and there does not seem to be anyone (at least anyone with clout) who is tempering it. I get worried when just part of the picture is presented as if it were the whole, and the wildest speculations based upon it are featured as though they are established facts!

    Good point! But, by the same token aren't DE and DM just wild speculation too? Is there something that gives them more standing? (By the way, if DM and/or DE are actually confirmed, is there a mechanism for how they would have been produced?)
  9. Mar 13, 2007 #8


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    I have given the reasons why exotic non-baryonic DM is thought to exist in your "Why DM" thread. There are many 'mechanisms' suggested by particle physics by which DM might have been produced in the BB, one problem is reducing the number of candidates and the other is actually finding them in the laboratory (LHC?).

    Likewise DE, see my post #5 above.

    As a matter of fact I too consider exotic non-baryonic DM and DE to be questionable, and I do have an alternative, but then I am in a minority (of one) in the cosmological community....

  10. Mar 15, 2007 #9


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    The strongest case for DM to date is the 'bullet cluster'. It is very powerful - prompting some scientists to even mutter the 'P' word [proof].
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