A plot displaying the amplitude of
→ cosmic microwave background anisotropy
as a function of angular size or → multipole index.
Same as → angular fluctuation spectrum.
The plot, based the on WMAP and other data, shows a plateau at large angular or length
scales (→ Sachs-Wolfe plateau),
then a series of peaks at progressively smaller scales.
These features arise from the gravity-driven
acoustic oscillations of the coupled photon-baryon fluid in the early
Universe (→ baryon acoustic oscillation).
In particular, a strong peak is seen
on an angular scale (at l ~220), corresponding to the physical
length of the → sound horizon at the
→ recombination era.
It depends on the curvature of space. If space is
positively curved, then this sound horizon scale will appear larger on
the sky than in a flat Universe (the first peak will move to the
left). The second peak (l ~ 550), which is the first harmonic of the
main peak, relates to the baryon/photon ratio. The third peak can be
used to help constrain the total matter density.

The plot that gives the portion of a signal's power falling within given frequency bins.
The most common way of generating a power spectrum is by using a discrete Fourier transform.