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Question on Dark Energy

  1. Jun 24, 2011 #1
    I have a question and/or a thought on dark energy. I have just 2 years of college Physics so I apologize if my question seems stupid. However, I was thinking about dark energy and the weakness of gravity relative to the other forces and the idea of living in a finite universe. Then I read an artice on discovery news about the gravity wave detector. Is it plausible that gravity is weak compared to the other forces because gravity waves propigating out from a large enough source such as an entire galaxy, reach the edge of the universe and reflect back, like waves in a pool? There fore effectively cancelling some of the waves out and decreasing its strength as a whole? Or, if a star goes Super Nova, and the "dent" the star makes in space springs back like a trampoline effectively sending out a ripple through space that creates a similar effect(after further thought maybe this could include galactic centers with massive stars rapidly orbiting a super massive black hole moving a bunch of these"dents" around quickly causing waves)?

    Secondly, could dark energy simply be an effect of the relative weakness of gravity? Meaning that the energy in the universe left over from the big bang and rapid expansion is slightly above the maximum expansive energy gravity could overcome, and the larger the universe gets gravity gets "spread so thin" the expansion accelerates?

    Could Dark Energy possibly be detected or explained by studying its effect on gravity?

    Again I apologize if my question is alittle dumb.
     
  2. jcsd
  3. Jun 24, 2011 #2

    marcus

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    tkav,
    no problem with your question! But it comes within the scope of ordinary classical GR and Cosmology. It is not really a "beyond the Standard Model" question.

    Your question is a mix of on-target conceptions, and off-target. I'd say you have a good pictorial idea of how gravitational waves---ripples in geometry---can be made by massive bodies collapsing or orbiting each other rapidly.

    But in the usual cosmo models there is no "EDGE" of the universe that could reflect waves back.

    The "observable universe" is merely the part that we currently have the opportunity to see because light-etc has had time to reach us. The boundary of your observable is not a physical real boundary, it just depends on your location. So it can't reflect anything.

    The currently observable portion is assumed to be just a small part of the whole thing, which could indeed be infinite. If the whole U is spatially finite then it still has no "EDGE" because it is all there is, so there is no "OUTSIDE". Picture the 2D analogy of the surface of a sphere. If all existence were concentrated on that 2D surface then there would be no edge.

    Secondly, could dark energy simply be an effect of the relative weakness of gravity? Meaning that the energy in the universe left over from the big bang and rapid expansion is slightly above the maximum expansive energy gravity could overcome, and the larger the universe gets gravity gets "spread so thin" the expansion accelerates?

    Could Dark Energy possibly be detected or explained by studying its effect on gravity?

    Dark Matter (which is different, and collects in clouds) has been detected by its gravitational lensing effects. Concentrations of DM bend lightrays and distort the background galaxies.

    You could say that Dark Energy has been "detected" primarily by measuring its effect on expansion of distances. You could say that this was an "effect on gravity" because it overcomes the tendency of gravity to slow expansion down. But that kind of detection still leaves a lot of questions and one wants more of an observational handle on DE.

    I would not call DE merely "an effect of the relative weakness of gravity"---relative to the initial kick-off expansion. No matter how weak, gravity would still be slowing expansion down, not speeding it up!

    The simplest explanation of DE is simply that it corresponds to a constant term in the law of gravity that we did not know was there until 1998.

    Since 1915, our law of gravity has been the Einstein Field Equation, and that equation has a natural place in it for a constant Lambda. But until 1998 most people thought that constant was zero. Then they found evidence that it wasn't zero.

    You don't really have to think of it as an "energy". It is just as natural to think of it as a very slight intrinsic spacetime curvature. That was how Lambda originally appeared in the equation.
    "Dark" makes it sound mysterious, and "Energy" is a bit misleading---we don't know for sure that it corresponds to any field or to anything we would ordinarily consider an energy.

    Media tend to hype stuff up---stimulate the public's imagination. Give the illusion that stuff is known when it is just one possible conjecture. there is a lot of uncertainty in Cosmology---things that could be understood and explained in several different ways--and where people need more observations to decide between the different models. that uncertainty would bore the audience and would not play well on television. So it doesn't come through in the media.

    Here's a professional article that I like about "Dark Energy". You could see if you could read parts of it. Parts at the beginning and end are not too technical, I think. Here is the summary HTML. To get the full article, click on PDF in the upper right corner of this summary page:

    http://arxiv.org/abs/1002.3966
    Why all these prejudices against a constant?
    Eugenio Bianchi, Carlo Rovelli
    (Submitted on 21 Feb 2010 (v1), last revised 11 Apr 2010 (this version, v3))
    The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We argue that these arguments are unconvincing, or ill-founded.
    9 pages, 4 figures
     
    Last edited: Jun 24, 2011
  4. Jun 24, 2011 #3
    Marcus, thanks for the explination. I guess this train of thought came from "Through the Worm Hole", and a theory on the Multiverse that was probably dumbed down to a degree that really does not represent the full theory. It was the episode that talked about a possible dodecahedron shape to the universe which has no real edge but a finite size. Granted I understand that some of these theories are on the fringe of Physics, and quite probably wrong, but they're good to get you thinking.
     
  5. Jun 24, 2011 #4
    I was going to edit my initial post but thought it might just be better to post this here. IF the universe is finite, in my initial post i put the idea of the wave reflecting, however, I should have explained it more like this. the wave travels out away from its source, encircling a giant baloon and eventually following a path that will have it re-cross its origion.
     
  6. Jun 24, 2011 #5
    there is a spherical distribution of matter in each galaxy, which is not what we see. Therefore we infer that there is a certain amount of Dark Matter in each galaxy. This could be some exotic particles,
     
  7. Jun 25, 2011 #6

    marcus

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    That would be a highly relevant point (IMHO) if the U did not expand. We don't know whether we should think of the U as spatially finite---say a giant balloon surface but 3D instead of 2D, as you suggest.

    But IF it were, you say, then wouldn't wavefronts emanating from a point converge at the antipodal location on the other side, and then spread out again and reconverge back where they originated?

    I think you are picturing it right, except you leave out expansion. Gen Rel allows distances to expand---geometry itself is dynamically changing. This is unintuitive to creatures like us who live on a rock with our geometry locked in and crystallized around us :biggrin:
    But according to the best model of the cosmos we have the waves never get around because the damn thing is expanding too fast.

    I'm saying this very badly. Sorry about the sloppy language. Maybe the best thing would be just to watch the wright balloon model animation. (and then use your imagination to factor in the recently discovered acceleration).

    In the movie, you can see little colored "photons" wiggling across the face of the expanding balloon. As long as the balloon is expanding, they make it even halfway around. It is a simple computer animation, but very nice.

    Just google "wright balloon model". If that does not get the movie, please let me know.
     
  8. Jun 25, 2011 #7
    It seems that I didnt really think the situation through in my initial post. I did see the Wright balloon animation, which was verry helpful. So to be clear, im picturing G waves propigating away from one of those galaxies on the baloon at C but relative to our location some parts of the universe are expanding awya from us at greater than C therefore the waves cannot ever make such a trip. It would be the same reason that the Big bang was 13.7 Billion years ago but the light is 45 billion light years away.

    I think the way information is presented in the media is a bit glamorized and distorted for us lay people. take the big bang for instance. I pictured a tiny primordial atom so to speak expanding at a verry high rate until the universe is its present size. the big bang happening everywhere at once but everywhere was a verry small area. However its is my understanding that that particular picture only relates to the observable part of our universe and not the universe as a whole.
     
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