Entropic cosmology in a dissipative universe
  The bulk viscosity of cosmological fluid and the creation of cold dark matter
both result in the generation of irreversible entropy (related to dissipative
processes) in a homogeneous and isotropic universe. To consider such effects,
the general cosmological equations are reformulated, focusing on a spatially
flat matter-dominated universe. A phenomenological entropic-force model is
examined that includes constant terms as a function of the dissipation rate
ranging from $\tilde{\mu} =0$, corresponding to a nondissipative $\Lambda$CDM
(lambda cold dark matter) model, to $\tilde{\mu} =1$, corresponding to a
fully-dissipative CCDM (creation of cold dark matter) model. A time evolution
equation is derived for the matter density contrast, in order to characterize
density perturbations in the present entropic-force model. It is found that the
dissipation rate affects the density perturbations even if the background
evolution of the late universe is equivalent to that of a fine-tuned pure
$\Lambda$CDM model. With increasing dissipation rate $\tilde{\mu}$, the
calculated growth rate for the clustering gradually deviates from observations,
especially at low redshifts. However, the growth rate for low $\tilde{\mu}$
(less than 0.1) is found to agree well with measurements. A low-dissipation
model predicts a smaller growth rate than does the pure $\Lambda$CDM model (for
which $\tilde{\mu} =0$). More detailed data are needed to distinguish the
low-dissipation model from the pure $\Lambda$CDM one.
