  We compute the one-loop polarization tensor $\Pi$ for the on-shell, massless
mode in a thermalized SU(2) Yang-Mills theory being in its deconfining phase.
Postulating that SU(2)$_{\tiny{CMB}}\stackrel{\tiny{today}}=U(1)_Y$, we discuss
$\Pi$'s effect on the low-momentum part of the black-body spectrum at
temperatures $\sim 2... 4$ $T_{\tiny{CMB}}$ where $T_{\tiny{CMB}}\sim 2.73 $K.
A table-top experiment is proposed to test the above postulate. As an
application, we point out a possible connection with the stability of dilute,
cold, and old innergalactic atomic hydrogen clouds. We also compute the
two-loop correction to the pressure arising from the instantaneous massless
mode in unitary-Coulomb gauge, which formerly was neglected, and present
improved estimates for subdominant corrections.
