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Does Dark Energy break the Conservation of Energy?

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  1. May 11, 2013 #1
    As the expansion of space accelerates, space is created and this seems to break the conservation of energy law. I've googled this but have not found a really good explanation of this.

    Imagine for a moment we're so far into the future that there are vast vast voids between what remains of galaxies. Some of these voids may be larger than the current known universe. Background radiation from the big bang has red-shifted to the point that it's wavelength is longer than the size of the known universe.

    How is the expansion of space at some mind-boggling rate with nothing in it and no background radiation not breaking the law of conservation of energy?

    Second question: Can light actually stretch that far or will it eventually be absorbed into the background quantum fluctuations and "die" so to speak?

    Thanks for helping me with my curiosity!
     
  2. jcsd
  3. May 11, 2013 #2
  4. May 12, 2013 #3

    marcus

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    Besides what yenchin said, there is a more basic question. So far we have no evidence that the CCC (cosmological curvature constant) corresponds to anything you could properly call an "energy".

    Einstein found that general covariance, the basic symmetry of GR, allows exactly two gravitational constants: the Newton G constant and another one that he called Lambda. Lambda is an intrinsic curvature expressed in units of reciprocal area.

    For many years most cosmologists supposed that Lambda was exactly zero--but there was no good reason to suppose this. All one could say, from the data available at the time was that it was very small. In 1998 new data became available and the parameters were adjusted to get the best fit. Lambda turned out to be a small positive curvature

    The best fit value of Lambda, combining all the data as of 2013, is
    1.007 x 10-35 (seconds)-2

    Here instead of using meters I'm using seconds as a unit of spacetime length, IOW "light-second" is my unit distance and square light-second is the unit of area. The reciprocal of Lambda is an area of approximately
    1035 square light-seconds.
    It's a big area because has to correspond to a very slight (almost undetectable) constant curvature, the reciprocal or "one over" the area.

    You can always turn one type of quantity into another type if you multiply by the right combination of physics constants. If you want to convert the original curvature form of Lambda into an energy density, a combination of constants that will work is:
    c2/(8πG)
    If you multiply 1.007 x 10-35 (seconds)-2 by that and do the algebra you will come out with an energy density
    0.53 nanojoules per cubic meter
    That does not mean there IS .53 nano joules of energy sitting in every cubic meter of space. It is just a different way of writing the cosmological curvature constant.

    People are always excited by the thought of energy and many of us like to think of the cosmological constant that way. Indeed there MIGHT BE some energy field actually there causing the curvature!!! Or there might not.

    Basically I think it is premature to assume anything more than that there's a slight constant curvature built into the geometry of spacetime, and it has now been measured. It's a constant that occurs naturally in the Einstein equation. It is jumping the gun to attribute it to some kind of exotic energy field---without a scientific basis for believing that.

    But the seductive phrase "dark energy" has infected our language, so people have gotten addicted to jumping the gun in this case. Still, quite a few cosmologists are careful to use the phrase "cosmological constant" in their professional writing, and avoid the popular term "dark energy".

    There's a 2010 article debunking the buzz over "dark energy" that is titled
    "Why all these prejudices against a constant?"
    To get it, google the title, or selected words like "why prejudices constant"
    Googling just those 3 words will get you
    http://arxiv.org/abs/1002.3966 which has a link to the PDF
     
    Last edited: May 12, 2013
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