Why is the CMB power spectrum often plotted as l(l+1) C_l and not only C_L ?

[Anne-Sylvie]

Hello everybody, (sorry for the eventual Engrish)

I can't find any convincing answer for the following question :

Why do we always (or often) plot the CMB power spectrum in this way :
jb.man.ac.uk/research/cosmos/vsa/images/CMB_power_spectrum.gif

I mean the y-axis is $$C_\ell \ell (\ell+1)$$ and not only $$C_\ell. $$ Why ?

Thanks for your answer. I already heard that its because of we want show the plateau at low l. But if it is the answer, why is the plateau proportional to $$\ell (\ell +1) ?$$

Thanks :)

 

[George Jones]

From http://relativity.livingreviews.org/open?pubNo=lrr-1998-11&page=articlesu14.html

The reason for plotting \ell \left( \ell + 1 \right) C_\ell is that it approximately equals the power per unit logarithmic interval in \ell.

If a log scale is used for \ell, scale invariance is a flat line. Scale invariance is expected at large angular scales (low \ell), hence the plateau.