We compute the one-loop polarization tensor Π for the on-shell, massless mode in a thermalized SU(2) Yang–Mills theory being in its deconfining phase. Postulating that [Formula: see text], we discuss Π's effect on the low-momentum part of the black-body spectrum at temperatures ~2,…,4 T CMB where T CMB ~ 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.