Cosmic microwave background radiation temperature in a dissipative
  universe
  The relationship between the cosmic microwave background radiation
temperature and the redshift, i.e., the $T$--$z$ relation, is examined in a
phenomenological dissipative model. The model contains two constant terms, as
if a nonzero cosmological constant $\Lambda$ and a dissipative process are
operative in a homogeneous, isotropic, and spatially flat universe. The
$T$--$z$ relation is derived from a general radiative temperature law, as
appropriate for describing nonequilibrium states in a creation of cold dark
matter (CCDM) model. Using this relation, the radiation temperature in the late
universe is calculated as a function of a dissipation rate ranging from
$\tilde{\mu} =0$, corresponding to a nondissipative $\Lambda$CDM model, to
$\tilde{\mu} =1$, corresponding to a fully dissipative CCDM model. The $T$--$z$
relation for $\tilde{\mu} =0$ is linear for standard cosmology and is
consistent with observations. However, with increasing dissipation rate
$\tilde{\mu}$, the radiation temperature gradually deviates from a linear law
because the effective equation-of-state parameter varies with time. When the
background evolution of the universe agrees with a fine-tuned pure $\Lambda$CDM
model, the $T$--$z$ relation for low $\tilde{\mu}$ matches observations,
whereas the $T$--$z$ relation for high $\tilde{\mu}$ does not. Previous work
also found that a weakly dissipative model accords with measurements of a
growth rate for clustering related to structure formations. These results imply
that low dissipation is likely for the universe. The weakly dissipative model
should be further constrained by recent observations.
