CMB anisotropies

Gold Member
Can someone help me understand what information we extract from such kind of figure?

In fact the $C_l^{TT}$ parameter gives us information on the amplitude of the temperature fluctuations $\Delta T/T$... However I don't understand why there is such a peak formation (1 very large, 2 smaller and 2 even smaller), or why this is given in terms of multipole index $l$.

As for while it's given in terms of multipole index, this is because the sky has the geometry of the surface of a sphere, and the equivalent to Fourier transforms on the surface of a sphere are Spherical Hermonic transforms. In spherical harmonics, the power spectrum $C_\ell$ is the variance of all waves on the sky which have a wavelength on the sky of approximately $\pi/\ell$ radians across the surface of the sky.