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Dark Energy and CMB

  1. Jul 19, 2008 #1
    Is it possible to relate the hypothesized dark energy of the universe with the CMB wavelength?
  2. jcsd
  3. Jul 20, 2008 #2


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    What theoretical model do you have in mind?
  4. Jul 22, 2008 #3
    I don't have any particular theoretical model in mind. I've just been reading a bit on the inferred cosmic expansion acceleration and the presumed quantity of some sort of pervasive dark energy that has been hypothesized to account for the acceleration.

    I'm just supposing that the dark energy is the kinetic energy imparted during and remaining from some sort of Big Bang event.

    If so, then shouldn't there be some sort of useful, discoverable relationship between the light energy (CMB) and dark energy remnants of the Big Bang?
  5. Jul 22, 2008 #4


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    probably not, in any simple fashion
    the CMB temperature map is used to help get a handle on the universe's expansion history and thus to help estimate the size of the cosmological constant
    but the CMB temperature map is just one of several bodies of data that contribute---galaxy counts are also used, and supernovae studies.

    there probably is no direct connection of one energy with the other


    well for one thing the recent studies have tended to show that the cosmological constant is CONSTANT IN TIME----if you like to think of it as an energy density, it is 0.6 joules per cubic kilometer and it has always been and it stays that----that is just wht the acceleration record seems to show (people are still testing that one, but evidence is mounting that it is in fact constant)

    but obviously the temperature of the CMB is constantly declining. It used to be 3000 kelvin and now it is 2.76 kelvin. It has cooled continuously with expansion. And as expansion continues it will cool more.

    So you are trying to relate something that is constant with something that changes.

    The CMB has on one wavelength that stands out, it has a smooth blackbody spectrum of a certain temp---a mix of wavelengths.
  6. Jul 22, 2008 #5
    As Marcus implied, the CMB temperature map can indirectly tell us something about dark energy, here's why:

    The CMB has almost the same temperature anywhere you look in the sky, but there are 0.001% deviations (e.g. in one patch of the sky, T=2.731 and in another patch, T=2.732). Standard calculations say that the average size of a lot of these temperature fluctuations corresponds to structures (known as acoustic oscillations) that had a certain (calculated) size about 400,000 yrs after the Big Bang.

    So the next question you should ask is: "What determines the angular size of these structures?" That is, what says how big they should look to us? It turns out that the angular size of these structures depends on the expansion history of the universe, which in turn depends on the stuff in the universe: matter and dark energy. (The expanding universe causes something to look bigger than it would in a static universe.)

    A variety of methods have shown that all the normal matter and dark matter that we can detect does not give the right expansion history of the universe that is in concordance with the CMB temperature variations. But, when you throw in the amount of dark energy that the supernova results give, that gives you the right expansion history of the universe! In other words, with dark energy, the calculated angular size of those early universe structures is close to the actual observed angular size of the temperature variations.

    In other words, the CMB temperature variations give us a yardstick (with a length that we know) to look at. The angle that the yardstick subtends (i.e. how long it looks) depends on how much matter, dark matter, and dark energy there is in the universe. In the end, the CMB data meshes nicely with the supernova data regarding dark energy.
  7. Jul 22, 2008 #6


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    that's a nice brief explanation covering a lot of ground, neutralseer.
    I suspect more detail would be welcome anytime you wanted to expand on some part of it.

    Oh, what you said about looking bigger reminds me that when it emitted the light the last scatter surface was only 45 million LY away so the circumference was roughly 280 million LY.
    So on the map looking at those little red and blue hot and cold blobs, if one of those temperature fluctuation blobs is, say, one degree of angle wide.....
    then it originally corresponded to a blob of hot gas that was less than a million LY across! that is nothing in astronomical terms.

    more exactly it would be about 28/36 = 7/9 of a million LY across...assuming my 280 million LY circumference is approximately right

    so when we look at tht CMB temp map we are seeing a sky full of interesting small objects namely temperature variations in the ancient hot gas.

    and a lot else of course

    neutralseer you started the explaining of the CMB temp map, so why don't you continue. please correct anything wrong or misleading in what Ive said and provide any additional points you want. cool stuff
    Last edited: Jul 22, 2008
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