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?
I mean the y axis is $C_\ell \ell (\ell+1)$ and not only $C_\ell$. Why?
I know it's because of the scale invariance, but why do we absolutely want to show the flat line at low $\ell$? And I do not understand why the power spectrum is flat in this scale.
Thanks for any answer. :)