i cannot understand everything (far from it) but here is the part where I believe is an alternative explanation for the CMB :
Equations (2.38), (2.39), (2.40), and (2.53) all illustrate that the creation rate of particles with energies larger than the
inverse expansion time, ρ, is exponentially suppressed. Parker [11] has noted that these exponential factor are similar
to those which appear in thermal spectrum at finite temperature.