CMB fluctuations related to dark matter and normal matter?

[bcrowell]

I came across this http://scienceblogs.com/startswithabang/2010/11/the_simplest_argument_for_dark.php cutesy pop-sci-ish explanation of how we know there's dark matter. If you scroll about 1/3 of the way down, it shows plots of the CMB's power spectrum as a function of multipole moment, l. (Note that the observed plot and the theoretical plots have mismatched x axes.) The observed spectrum has strong oscillations in the region of l=500-850. If you assume only dark matter, or only normal matter, then you don't get these strong oscillations; you mainly get a rapid fall-off. Why is it that you need a mixture of dark matter and normal matter to get the strong oscillations, but if you assume only one or the other you get a rapid fall-off?

 

[pervect]

There's some more info on this at http://background.uchicago.edu/~whu/intermediate/intermediate.html

 

[bcrowell]

Thanks, pervect! I'll have to spend some time studying that.