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Direct Evidence of Dark Energy (?)

  1. Aug 3, 2008 #1
    Direct Evidence of “Dark Energy” (?)

    Hawaii Scientists Find Direct Evidence of “Dark Energy” in Supervoids and Superclusters
    superclusters & supervoids


    A team of astronomers at the University of Hawaii Institute for Astronomy (IfA) led by Dr. István Szapudi has found direct evidence for the existence of “dark energy.” Dark energy works against the tendency of gravity to pull galaxies together and so causes the universe’s expansion to speed up. The nature of dark energy (what precisely it is, and why it exists) is one of the biggest puzzles of modern science.

    This is arguably the clearest detection to date of dark energy’s stretching effect on vast cosmic structures: there is only a one in 200,000 chance that the detection would occur by chance.

    “We were able to image dark energy in action, as it stretches huge supervoids and superclusters of galaxies,” Szapudi said. Superclusters are vast regions of space, half a billion light-years across, that contain an unusually high concentration of galaxies, while supervoids are similarly sized regions with a below-average number of galaxies. They are the largest structures known in the universe. The team made the discovery by measuring the subtle imprints that superclusters and supervoids leave in microwaves that pass through them.

    “When a microwave enters a supercluster, it gains some gravitational energy, and therefore vibrates slightly faster,” explained Szapudi. “Later, as it leaves the supercluster, it should lose exactly the same amount of energy. But if dark energy causes the universe to stretch out at a faster rate, the supercluster flattens out in the half-billion years it takes the microwave to cross it. Thus, the wave gets to keep some of the energy it gained as it entered the supercluster.”

    “Dark energy sort of gives microwaves a memory of where they’ve been recently,” postdoctoral scientist Mark Neyrinck said. The team also includes graduate student Benjamin Granett, the first author on the paper, which will be published in the Astrophysical Journal Letters in August or September.

    The team compared an existing database of galaxies with a map of the cosmic microwave background radiation (CMB), the faint hiss of microwaves left over from the Big Bang. As predicted, they found that the microwaves were a bit stronger if they had passed through a supercluster, and a bit weaker if they had passed through a supervoid.

    “With this method, for the first time we can actually see what supervoids and superclusters do to microwaves passing through them,” Granett said.

    The signal is difficult to detect, since ripples in the primordial CMB are larger than the imprints of individual superclusters and supervoids. To extract a signal, the team averaged together patches of the CMB map around the 50 largest supervoids and the 50 largest superclusters that they detected in extremely bright galaxies drawn from the Sloan Digital Sky Survey, a project that mapped the distribution of galaxies over a quarter of the sky.

    preprint: http://arxiv.org/abs/0805.3695

    Source:http://www.ifa.hawaii.edu/info/press-releases/szapudi-7-08/
     
  2. jcsd
  3. Aug 3, 2008 #2
    Real evidence ?

    What do you think ?
    Is this a credible evidence ?
     
  4. Aug 3, 2008 #3

    marcus

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    Re: Real evidence ?

    I thought it was a good study. I logged it in our bibliography 26 May around the time it appeared on arxiv.
    The idea that there is a positive cosmological constant Lambda rests on several different kinds of supportive evidence.

    so far I havent seen any evidence that one should think of it as an energy, or as some exotic field. that remains optional. (or am I missing something?)

    To me this study was admirable and persuasive, as far as I am able to judge. But all it shows is another reason to include a positive constant Lambda in the picture----unexplained acceleration by a certain uniform amount.

    It doesn't make it any more or less likely AFAICS that the positive Lambda can be traced to an exotic form of energy. Maybe there is some other explanation (several have been proposed).
     
  5. Aug 3, 2008 #4

    turbo

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    Re: Direct Evidence of “Dark Energy” (?)

    I'm extremely suspicious of "direct evidence" of a phenomenon that bases its "evidence" on the same assumptions that gave rise to the phenomenon in the first place. Circular reasoning and lack of epistemology gets us into way more trouble than we might expect.
     
  6. Aug 4, 2008 #5
    Re: Direct Evidence of “Dark Energy” (?)

    The same might be said for the "direct evidence" for the age of the cosmic background radiation, or "direct evidence" of the big bang theory, and even "direct evidence" of cosmological theories in general! Of course, I wouldn't necessarily agree with you on having that high level of suspicion of the possibility of finding invisible "dark" energy.
     
  7. Aug 5, 2008 #6

    marcus

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    Re: Direct Evidence of “Dark Energy” (?)

    Marinas all the evidence you refer to here I find quite persuasive. As I said in my earlier post, I have little doubt that the acceleration associated with a positive Lambda is real.

    what I do find however is that the phrase dark energy doesn't have any clear meaning for me. what is it supposed to be? why is the term Energy appropriate? isn't that saying more than we actually know.

    would you allow reinterpreting their announcement to simply be about direct evidence of accelerated expansion?

    maybe it is too early to verbally commit to some presumed cause for the accelerated expansion---and this paper we are talking about does not seem to offer or favor any presumed cause----unless I'm mistaken, it just confirms the effect.
     
    Last edited: Aug 5, 2008
  8. Aug 5, 2008 #7
    Re: Direct Evidence of “Dark Energy” (?)

    To state it briefly, you mean : <cosmological evidences are model-dependant> ?

    I've always asked this question.. physical cosmology is kind of outsider in physics..
    (but, some results are quite model independent : CMB anisotropies for eg. no ?)
    Deriving results and experimental 'evidences' from models (like LCDM), is circular reasoning for me also. Some models (dropping the Homogeneity symmetry) do not even need Dark "energy"
    (I think the term comes from the fact it is not 'material'.. but ''something'' that fills the Universe > Energy).
    Any clarifications about this ?

    Is the reality what we observe, or what we want to observe ?
     
  9. Aug 5, 2008 #8
    Re: Direct Evidence of “Dark Energy” (?)

    hey now - how do dark energy and dark matter fit into the standard model?
     
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