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Assigning Contributions of Dark Energy to the Cosmos

  1. May 16, 2009 #1
    Hello:

    It has been said that something on the order of 75% of the known universe is comprised of dark energy. But, unless one is assigning a negative value to the putative mass that would be associated with that dark energy, then I don't quite see how one can come up with the 75% figure -- or any figure.

    The gravitational energy from normal and dark matter would be negative, would it not? And since a repulsive force is at work with dark energy, would not the gravitational energy be positive? And if we are comparing amounts of matter associated with those energies, then don't we have to assign some sort of mass values for those energies? That would then indicate to me the need to invoke negative mass for the dark energy contribution.

    And I am more generally wondering if negative mass particles could indeed explain dark energy.

    Thank you for your help.

    RoKo
     
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  3. May 16, 2009 #2

    marcus

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    A positive mass can be associated with a repulsive effect, depending on the equation of state. You make a mistake in your post by reasoning that since the effect is repulsive therefore the mass must be negative. This does not follow, and is not the case.

    So there is no need to speculate about "negative mass particles" :biggrin:
     
  4. May 16, 2009 #3
    Okay, thanks. You say that a positive mass CAN be associated with a repulsive effect. But that does not indicate that it MUST be. It still seems to me that negative mass particles can produce a repulsive force. Can you say with 100% certainty that dark energy is not associated with negative mass particles? Classical ideas I learned in kindergarten tell me that there is nothing intrinsically wrong with what I said. Perhaps dark energy does not have anything to do with negative mass particles, but why should we so readily dismiss the possibility? Should one not consider all possibilities in the first instance?


    RoKo
     
  5. May 16, 2009 #4

    marcus

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    Well you have some pretty alternative offbeat ideas, so I will just try to explain what the standard picture of dark energy is.

    The usual dark energy is a positive energy density of about 0.7 nanojoules per cubic meter.

    It is assumed to be constant thru out all space and time. (They also explore cases where it varies, but the normal default case is constant.)

    According to normal classical Gen Rel, if you have a constant positive energy density this will automatically cause accelerated expansion (in the universe as we know it.)

    You can easily see why if you look at the main equations that govern (the scalefactor) of cosmology---the two Friedman equations. Look it up on Wiki.
    You will see that one of the two Friedman equations is called the "acceleration" Friedman equation.
    You will see that negative pressure can cause acceleration and that pressure is 3 times more important than mass-energy density.
    You understand that if there is a truly constant energy density associated with spatial volume---of 0.7 nanojoules per cubic meter---that this would automatically have a negative pressure of - 0.7 nanopascal.

    This is the normal thing to assume. We don't need to assume any PARTICLE for dark energy or especially any exotic particle with a "negative mass". That would be hard to imaging because mass is inertia, how something responds to force. And it gets very exotic trying to fantasize about negative inertia.

    We don't need anything bizarre like that, all we need is a simple constant positive energy density giving a constant pressure of - 0.7 nanopascal, throughout space, and that will fit the data.
     
  6. May 17, 2009 #5
    Hello. Thanks for your effort. Well, about a week ago I talked to the general relativity professor across the street and he said that negative mass particles would produce the opposite space-time curvature as positive mass particles. Also, Dirac introduced negative mass electrons in his relativistic theory of the electron and this eventually led to the discovery of the positron. So I would not say it's bizarre to think about negative mass particles. The whole theory of antiparticles followed from negative mass electrons.

    Well, obviously, the matter of dark energy is all wrapped up. You fellows know exactly what it is. As you have insisted, it can't possibly be negative mass particles. How do you know that? Well, because such an idea would be "bizarre."

    I am a former professor of physics, but have been working on technical problems and simply needed some advice. I thought that dark energy was a mystery. Now I see you fellows have it all figured out. Thanks for the information.

    RoKo
     
  7. May 17, 2009 #6

    Chalnoth

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    Well, the problem is that the other type of data that must be reconciled with the cause of the acceleration of the universe is that the universe is observed to be spatially flat. Matter with negative energy density would tend to cause an accelerated expansion by driving the universe towards being open instead of flat.
     
  8. May 17, 2009 #7
    Good luck on the problem. I had a fire in my home and must now attend to having all sorts of repairs done and also much cleaning.

    I wish you all well. Keep an open mind. That's what this old guy would suggest.

    Best wishes,

    RoKo
     
  9. May 17, 2009 #8

    Chalnoth

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    Keeping an open mind is fine, but the evidence is pretty conclusive that the cause of the accelerated expansion is not some negative-mass form of matter.
     
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