...$U$

There is also the possibility in general of circular polarization, described by Stokes parameter $V$, but this is absent in cosmological settings.

... horizon

Recall that the comoving scale $k$ does not vary with time. At very early times, then, the wavelengh $k^{-1}$ is much larger than the horizon $\eta$.

... inflation,

$E_i^4 \equiv V(\phi)$, the potential energy density associated with the scalar field(s) driving inflation.

... here.

Aside from COBE, experiments to date have had a sizable calibration error ($\sim$ 5-10%) which must be factored into the interpretation of Plate 1.

... scheme

http://www.eso.org/science/healpix/