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Energy has mass & therefore gravity. Yet dark energy is repulsive ?

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  1. Mar 23, 2013 #1
    Energy has mass & therefore gravity. Yet dark energy is repulsive !?

    Energy has mass & therefore curves space resulting in gravity. Yet dark energy is repulsive !
    Anyone have the answer to this contradiction?
    Thanks
     
  2. jcsd
  3. Mar 23, 2013 #2

    marcus

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    I haven't seen any evidence reported that dark "energy" actually corresponds to an energy.
    What I see repeatedly stated in the recent literature is that evidence is mounting that the observed slight acceleration in the expansion of distances appears to be the result of a constant in the Einstein equation (a reciprocal area, i.e. a slight constant curvature) called the cosmological constant.

    It is probably misleading to call the cosmological constant an "energy", just as it is misleading to call the start of expansion the "Big Bang". But phrases appealing to the imagination get picked up by the media or for whatever reason come into widespread use and are then embedded in our language. So even if they give the wrong idea they are hard to replace.
     
  4. Mar 23, 2013 #3

    phyzguy

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    Why a positive energy density in the vacuum leads to repulsion in GR is a confusing point. This web site of Ned Wright gives a good explanation of the mechanism. Read this, and if you have more questions, ask.
     
  5. Mar 23, 2013 #4

    phyzguy

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    But isn't it true that one can move the Cosmological Constant from the left side of Einstein's equations to the right side, where it is equivalent to a constant vacuum energy density? Since the two views give the same physical results, it is basically a matter of taste which one you choose to accept. I think the reason people started referring to it as "dark energy" instead of a "cosmological constant" is that if it is a cosmological constant, this assumes that it is in fact constant, whereas referring to it as dark energy allows that possibility that it is not constant, but might vary in space and/or time. Until we know wheter or not it is truly constant, it seems prudent to keep all possibilities open.
     
  6. Mar 29, 2013 #5

    ftr

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  7. Mar 29, 2013 #6
    false vacuum, negative pressure, repulsive gravity

    Let me quote from this link: http://ned.ipac.caltech.edu/level5/Guth/Guth3.html

    The peculiar properties of the false vacuum stem from its pressure, which is large and negative (see box on the right). Mechanically such a negative pressure corresponds to a suction, which does not sound like something that would drive the Universe into a period of rapid expansion. The mechanical effects of pressure, however, depend on pressure differences, so they are unimportant if the pressure is reasonably uniform. According to general relativity, however, there is a gravitational effect that is very important under these circumstances. Pressures, like energy densities, create gravitational fields, and in particular a positive pressure creates an attractive gravitational field. The negative pressure of the false vacuum, therefore, creates a repulsive gravitational field, which is the driving force behind inflation. [...]

    The resolution to the energy paradox lies in the subtle behavior of gravity. Although it has not been widely appreciated, Newtonian physics unambiguously implies that the energy of a gravitational field is always negative a fact which holds also in general relativity. [..] The possibility that the negative energy of gravity could balance the positive energy for the matter of the Universe was suggested as early as 1932 by Richard Tolman, although a viable mechanism for the energy transfer was not known.

    During inflation, while the energy of matter increases by a factor of 10^75 or more, the energy of the gravitational field becomes more and more negative to compensate. The total energy - matter plus gravitational - remains constant and very small, and could even be exactly zero. Conservation of energy places no limit on how much the Universe can inflate, as there is no limit to the amount of negative energy that can be stored in the gravitational field.
     
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