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How do we know that 74% of the universe is dark energy?

  1. Jun 26, 2012 #1
    The WMAP satellite found that the universe consists of 74% dark energy, 22% dark matter, and 4% normal matter. I can understand how they found the dark matter and normal matter, but how do find something that doesn't interact with anything and is a property of space itself? And how did it measure that to a percentage?
  2. jcsd
  3. Jun 26, 2012 #2


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    Technically there is a constant curvature term that occurs naturally in the basic GR equation. There is no especially convincing reason to measure it by an equivalent (fictitious) "energy".
    WMAP did not find any "dark energy" as such. It found that at large scale space is flat or nearly so---it measured overall curvature and within some error bounds found it to be quite small. Could even be zero.

    This means that the constant curvature term in the basic equation has to have some small positive value to compensate for the estimated matter densities being too low to ensure flatness, or near flatness.

    That's a rather technical point. We can see the rate of spatial distance expansion and any given expansion rate has to be MATCHED by some combination of matter density (both kinds, dark and ordinary, combined) and the constant. By itself, without a small positive cosmological constant, the matter density was not enough to match the observed expansion rate.

    It's an oversimplification to say they found suchandsuch amount of "dark energy".
    What they found was approximate flatness AND an inadequate density of matter to match it given:
    (1) our basic GR equation which is the tested accepted law of gravity and largescale geometry
    (2) the observed rate of expansion of distances.

    Some people like to imagine that there is some mysterious undetectable constant energy throughout space which causes the slight compensatory curvature. For others it is simply a constant Lambda which appears naturally in the Einstein GR equation---the same way that the Newton constant G also appears naturally. It's presence is dictated by the symmetries of the theory, it could have turned out to be zero but it didn't. How you chose to think of it is somewhat a matter of taste. Here is a discussion of this issue along the lines I've indicated:

    Why all these prejudices against a constant?
    Eugenio Bianchi, Carlo Rovelli
    (Submitted on 21 Feb 2010)
    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
  4. Jun 26, 2012 #3


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    It's basically something like this

    1) Choose your ingredients for the universe: Amounts of matter, dark matter, dark energy, and radiation.

    2) Use your best understanding of the early universe to see what this predicts the CMB anisotropy spectrum will look like. I couldn't possibly explain here how this is done - it would take reading a graduate level cosmology textbook to understand it.

    3) Use the results of 2) to compare to observations

    4) Repeat 1-3 over and over, and find the ingredients that fit the observations the best
    Last edited: Jun 26, 2012
  5. Jun 26, 2012 #4
    Oh. So does the fact that Lambda is 0.3 is approximately the amount of normal matter plus dark matter percentage and that dark energy is approximately 0.7 and 70% mean anything? I think that was a contributing factor to the advocation of dark energy, right? Just an observation.
  6. Jun 26, 2012 #5


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    I know what you mean, but the Lambda that Einstein put in the basic GR equation is not a number like 0.3 or a percentage. It is a small constant instinctive bias towards curvature that space has and which has been estimated. I can dig up the estimate if you are interested.

    The exact figure is not so important but in a rough way it is comparable to the very slight curvature along a circle whose radius is, say, 15 billion light years. Miniscule!

    You can also express that curvature by talking about the constant energy density that there would have to be throughout space in order to PRODUCE that amount of curvature. It is bewilderingly small. If you know what a joule of energy is. (The size thump when you drop a book onto your desk from about 1 foot high off the table, or the work it took to lift it in the first place.)
    If that unit is familiar, then the energy that would produce the curvature Lambda I'm talking about is only 0.6 NANOjoules per cubic meter of volume. 0.6 BILLIONTHS of a joule in a cubic meter.

    However that energy might be a fiction. Just a way of describing this very slight curvature (using the fact that in GR energy bends space in a predictable way). If you LIKE thinking in those terms then converting Lambda from a curvature into an energy density gives 0.6 nanojoules per m3

    It is THAT 0.6 nanojoules which is the 73% (of the total "critical" density) which is people are always talking about.

    "Critical" energy density, a somewhat artificial notion, is the density which in the absence of the tiny bias Lambda
    would be needed to balance our expansion rate and give exact spatial flatness.
    It s about 0.82 nanojoules per m3.

    But actually what we HAVE by way of ordinary and dark matter plus radiation is only 0.22 nJ/m3.

    This is only about 27% of critical---in the absence of Lambda it would not be enough to balance the expansion rate and result in the (approximate) flatness which instruments like WMAP observe!

    So there has to be something else! Either an constant inherent curvature bias Lambda, or it's equivalent in energy terms amounting to 0.6 nJ/m3.

    Which in a roundabout way kind of answers your question :biggrin: WMAP did not "see dark energy". Nobody did. But we think we see the effects of a slightly positive curvature term in the main GR equation. We see it both in the observed (near) flatness and in a very very slow increase in rates of distance expansion (in absolute, not percentage rate terms).
    Last edited: Jun 26, 2012
  7. Jun 26, 2012 #6
    Thank you! That solves it! :D
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