[The CMB Angular Power Spectrum, picture credits NASA/WMAP Science Team]
Above you see the most-often-shown plot in seminars I attended this year. It depicts an analysis of the measured temperature fluctuations the Cosmic Microwave Background (CMB):
To obtain the CMB power spectrum one roughly speaking decomposes the above shown colorful picture. It contains many structures of various sizes that one takes apart into an overlay of pictures with specific sizes. These are labeled by a parameter l, the multipole moment (see also our earlier post on Anomalous Alignments in the CMB). Below you see as an example l=2 and l=16. Pictorically speaking, l is something like the number of (red) blobs on the equator. Since the equator corresponds to 360°, the size of structures in degrees for a given l is ~ 360°/l.
[Picture from Ned Wright's very recommendable Cosmology Tutorial]
With such a decomposition one gets rid of information like the actual position of blobs, and can analyze the structures of the pattern. In the power spectrum plotted on the x-axis is this multipole moment l, the upper x-axis shows the corresponding scale in degrees. The y-axis (the up-down one) shows the intensity of the temperature fluctuations with the dimension of a squared temperature, micro Kelvin (μK). It is rescaled by a factor l (l+1)/2π (which I think is there to not obscure the intensities with the angular dependent drop in the multipole expansion, please correct me when I am wrong). Note that according to the above the left side of the plot (small l) corresponds to large structures, whereas towards the right, with increasing l, structures become smaller.
If you look at the WMAP picture you would guess that most of the blobs seem to have sizes somewhere around one degree, and indeed you see in the power spectrum the largest peak somewhere around l=200 or so.
Now what is that important for? The matter content of our universe affects the way it expands. The CMB photons can travel freely from the surface of last scattering on. The power spectrum we observe today carries information about what the universe has done since then. For example consider the fraction of dark matter. If one changes it, one obtains a different curve. The amount of dark matter e.g. affects the power of the even to the odd peaks relative to each other:
[The pink bar shows the fraction of dark matter. Increasing it lowers the power of the even peaks relative to the odd ones. Picture from this website]
Likewise, changing the cosmological constant would shift the first peak around. In such a way, one can find out which parameters fit best the observed spectrum, which then allows us to draw conclusions about the matter content of our universe.
This post is part of our 2007 advent calendar A Plottl A Day.