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The physics of dark matter & energy

  1. Mar 23, 2006 #1
    i want to know, do we have a physics of this undefined term?
    as far as i can tell, the hypthesis is that dark matter differs from "ordinary" matter by the fact it doesn't ommit light, if that's right beside the point of how can you find it with astronomical tools (which depend mainly on light and its interaction with matter), it might raise the point that dark matter is actually really a black body situation, perhaps a perfect one, and perhaps one way to identify is by checking black body radiaton near its surroundings, but is my conjecture even right and really dark matter is a case of a perfect black body?

    and do we have tools in physics to anlayse its behaviour?
    and what sort of energy is dark energy and how does it differ from the energies we are acquianted to?
  2. jcsd
  3. Mar 23, 2006 #2


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    That's how it was originally defined, yes. What people usually mean by "dark matter" nowadays is non-baryonic, weakly-interacting, massive particles. Since these particles are thought to make up the majority of the truly "dark" mass, the definitions are similar.

    For the dark matter to have a blackbody distribution, it would have to be a relativistic species (behaving like radiation). For it to be relativistic at this epoch, it would have be of very low mass, even lower than neutrinos. Aside from not being expected from theory, this possibility can be ruled out from the fact that dark matter clumps around galaxies. Relativistic matter wouldn't remain bound to a galaxy or cluster of galaxies.

    We can observe the positions and motions of objects in the universe and infer some things about the dark matter. The simple classic example is the rotation curves of spiral galaxies.

    I wrote up a review of this a while back: Dark Energy
  4. Mar 23, 2006 #3
    ST -- or anyone who knows:

    Can you give a brief description of what it is in the WMAP observations / analysis that provides support for dark energy and for the estimate that it comprises ~70% of the stuff in the universe? Thanks.
  5. Mar 23, 2006 #4


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    WMAP alone is actually a very poor probe of dark energy because at the time that the CMB was made (z~1100), the dark energy density was negligibly small. However, there's more to the story. Roughly, WMAP is capable of measuring the total matter content of the universe (this contributes to the formation of the acoustic peaks in the power spectrum, especially the third one) and the flatness of the universe (roughly determined by the angular scale of the first peak). It's a little more complicated than that, but that's the basic idea.

    If the universe is measured to be flat, then the total density of everything must equal the critical density, so one way to infer the dark energy density is to say that it makes up the remainder of the closure density. This can be done with WMAP alone and is consistent with more direct measurements of accelerated expansion (from looking at SNe). If the dark energy is a cosmological constant, then we know the effect it has on the expansion rate of the universe. If we also know the density, then we can actually determine the distance to the surface of last scattering expected with that amount of dark energy. If this number is consistent with the measured distance to the surface of last scattering (and it is), then the model is self-consistent.
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