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Originally Posted by Cougar
I don't understand it well enough to explain it -- perhaps someone can step in here with a clear explanation -- but apparently specific information can be drawn from careful measurements of the CMBR to enable the following conclusion:
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The clear detection of the predicted acoustic peak structure implies that the dark matter is non-baryonic.
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This statement comes from the Conclusion section of the Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology. |
See the tutorial "
Anisotrpies in the CMB", from
Wayne Hu. The basic idea is that the height of the 2nd acoustic peak, relative to the first acoustic peak, depends on the scattering of CMB photons due to matter. The height of that peak as observed implies that there should be much less mass in the universe than is implied by observed gravity. Hence, there must be more mass, but the mass must not scatter CMB photons. But any baryonic matter must scatter CMB photons. So, the missing mass cannot be baryonic.
As somebody else already pointed out, you can come to a similar conclusion, based on gravitational lensing.