  This lecture is a sketch of the physics of the cosmic microwave background.
The observed anisotropy can be divided into four main contributions: variations
in the temperature and gravitational potential of the primordial plasma,
Doppler effect from its motion, and a net red/blueshift the photons accumulate
from traveling through evolving gravitational potentials on their way from the
primordial plasma to here. These variations are due to primordial
perturbations, probably caused by quantum fluctuations in the very early
universe. The ongoing Planck satellite mission to observe the cosmic microwave
background is also described.
