### 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 Electromagnetic Radiation.

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

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