### angular power spectrum

(method of characterizing the CMB)

An **angular power spectrum** is a characterization of
a function on the surface of a sphere, or analogously,
all directions in space from a point. It captures variance
as a function of scale. An **angular two-point correlation**
does this, showing a correlation for each possible angle,
but the spectrum more often termed the *angular power spectrum*
is the correlation over each **multipole moment**
(i.e., **spherical harmonic order**), *l*
of a multipole expansion (analogous to a **Fourier series expansion**,
but for functions over the surface of a sphere).

The cosmic microwave background is often characterized by such an
*angular power spectrum* of its anisotropy,
showing the temperature variation for coefficients
of a multipole expansion of the temperature over the celestial sphere.
The temperature is calculated by Wien's displacement law
from the received EMR.

The most commonly cited CMB *angular power spectrum* is
of its temperature, but similar spectrums of CMB polarization
and/or gravitational lensing are also used.

(*harmonics,cosmology,CMB,function*)
http://en.wikipedia.org/wiki/Cosmic_microwave_background

**Referenced by:**

CMB lensing

CMB polarization

initial fluctuation spectrum

magnetic field

N-point function

index