Dark energy

  1. A few years ago I came to this forum to ask about the issue of space expansion....and the answer I got then was that the expansion is the nature of the space (or space-time) according to relativity or some other more advanced theory.....no one used dark energy to explain the issue at that time......now because of the 2011 nobel prize, should we say that the assumption that the space would expand by itself is not correct but we need to introduce the dark energy to explain this?

    My interest to this issue is philosophical....to me, the self expansion and the dark energy theory are two different philisophies: the first treats the space as "something", but the second treats the space as the empty container and any symptoms of expansion could only be meaningful by the departure of the bodies in that container, which is caused by some energy inside the the container, and that some energy is dark energy....

    Would any one to kindly clarify the issue here for me? Thanks a lot in advance.
  2. jcsd
  3. phinds

    phinds 8,103
    Gold Member

  4. Phinds:

    Thanks for the response....however, first of all, there were a few people (more than one) responded, and none of them was trying to use cosmological constant to convince me that the universe is unchanging......second of all, the response was more involved....I even asked why we don't see the expansion in our solar system, and a senior replied that the reason that we don't see it here is because that the density of matter is high here therefore, the gravitation reduced the expansion.......

    i think now you would tell me that the reason that there is no expansion in this solar system is because there is no dark energy, right?

  5. phinds

    phinds 8,103
    Gold Member

    No, I absolutely would not. Dark energy, as far as I know, exists everywhere. It just has no effect on gravitationally bound objects.

    The analogy that I like to use is this. If an ant pushes on a tank, it does not have a tiny effect, it has absolutely no effect at all. It cannot overcome the inertia of the tank and the friction of the tank against the ground.

    Similarly, dark energy has no effect on gravitationally bound objects. It is INCREDIBLY weak on small scales. It is only on HUGE cosmological scales that it adds up to having an enormous effect.

    To give you another of my favorite analogies, even though the universe is expanding, it is still going to be hard to find a parking place. What this is a description of is that if you could magically go out into intergalactic space and paint parking lines, it would take a BILLION YEARS before the lines would get far enough apart to let you park one more car between the lines.

    BUT ... when you consider an amount of space billions of light years across, then dark energy has a huge effect because it is cumulative over distance.
    Last edited: Apr 2, 2012
  6. Phinds:

    Thanks a lot for the clear explanation!

    But to me, philosohically I would consider both dark energy and space self-expansion (i.e. space is something and that something itself stretch out without the need of any energy and it is meaningful not because the departures of the bodies inside it) would remain possible causes for the observed space-expansion until someone finally find the dark energy.......thanks
  7. Dark energy interacts only through gravity . The proposal of itself expansion got fallen apart when it was noticed that when graphs of expansion of universe by studying the supernova red shifts were sketched , they were not even near to the "farthest neighbourhood" as predicted by applying the self expansion theory. Also the self expansion started violating the Einstein's general theory of relativity .So the scientists felt that a driving force is present uniformly throughout the universe that is causing the exponential expansion of universe and that was termed as dark energy .
    Both theories are indeed different philisophies .
    I hope this informatiom might help you a little bit .
  8. .
    Would it help if you considered dark matter within the very basic concept that momentum is the observers past and that particle position is the observers present, and then develop ideas within this direction towards an understanding of dark energy.
    If dark matter has no momentum then it does not exist to the observer because the past cannot exist.
    Dark matter would be a particulate state that does exist but only in the observers present and so cannot be observed outside of the observers state of awareness.
    Although it is not relevant to your posting if you sometimes consider the double slit experiment you may find it helpful to introduce this concept.
  9. phinds

    phinds 8,103
    Gold Member

    huh ???
  10. There are two distinct facts here. The first is that the universe is expanding, and the second is that the expansion of the universe is accelerating.

    Dark energy is not needed to explain the first fact, that the universe is expanding. We can describe an expanding universe perfectly well with general relativity and what we know about normal matter. Actually, we need dark matter to explain how much mass galaxies seem to have, but don't confuse dark matter with dark energy.

    Dark energy came into the picture when recent observations revealed that the expansion of the universe was accelerating. The effect of the gravitational attraction of regular and dark matter in the universe ought to be to slow the expansion of the universe. However, the observations leading to the 2011 Nobel prize contradicted this prediction: they revealed that the expansion is not slowing down, but speeding up. To explain this we need dark energy.
  11. Thanks all for the introduction of how dark energy is getting into the picture as well as the reminder of the difference between dark matter and dark energy........

    Somehow I still feel as space_buster said that those two theories are still two viable philosophies........because even though now I know that based on previous self-expansion theory we could not count for the accelerated expansion so that the whole idea of dark energy comes about..........however, if the space (or space-time) itself is "something", then whatever behavior (accelerated expansion or unaccelerated expansion) could also be just the behavior of that "something" itself....therefore, what we need might not necessarily be some other thing in that container like dark energy, but a new descriptive model about the behavior of this "something" itself.......does that even make a bit sense at all?

  12. murongqingcao , I have a bit of similar thinking as yours . Currently I am working on my own space model . However , I have halted that to clear my basics and to solve the space-time and energy-mass distribution math and many other concepts . Anybody's interest into it is always welcome ...
  13. best wish to you...I wish I could help but I am not a physicist....my main interest in philosophy.....good luck with your math model!
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