  The spectrum of the radiation of a body in equilibrium is given by Planck's
law. In plasma, however, waves below the plasma frequency cannot propagate;
consequently, the equilibrium radiation inside plasma is necessarily different
from the Planck spectrum. We derive, using three different approaches, the
spectrum for the equilibrium radiation inside plasma. We show that, while
plasma effects cannot be realistically detected with technology available in
the near future, there are a number of quantifiable ways in which plasma
affects cosmic microwave background (CMB) radiation.
