In “Eddington did not predict the CMB”:

In “Eddington did not predict the CMB”:

a. Wright argues that Eddington’s argument for the “temperature
of space” applies at most to our Galaxy. But Eddington’s
reasoning applies also to the temperature of intergalactic
space, for which a minimum is set by the radiation of galaxy
and quasar light. The original calculations half-a-century ago
showed this limit probably fell in the range 1-6 K. [6] And

that was before quasars were discovered and before we knew
the modern space density of galaxies.
b. Wright also argues that dust grains cannot be the source of the
blackbody microwave radiation because there are not enough
of them to be opaque, as needed to produce a blackbody
spectrum. However, opaqueness is required only in a finite
universe. An infinite universe can achieve thermodynamic
equilibrium (the actual requirement for a blackbody spectrum)
even if transparent out to very large distances because the
thermal mixing can occur on a much smaller scale than
quantum particles—e.g., in the light-carrying medium itself.
c. Wright argues that dust grains do not radiate efficiently at
millimeter wavelengths. However, efficient or not, if the
equilibrium temperature they reach is 2.8 K, they must radiate
away the energy they absorb from distant galaxy and quasar
light at millimeter wavelengths. Temperature and wavelength
are correlated for any bodies in thermal equilibrium.