Dissipative Unified Dark Fluid: Observational Constraints
  We adopt a standard FRW cosmology with a unified scenario, where the usual
dark matter and dark energy sectors are replaced by a single dissipative
unified dark fluid (DUDF). The equation of state of such fluid can asymptote
between two power laws. As a result, it enables fluid to have a smooth
transition from dust at early times to dark energy at late times. The
dissipation is represented by a bulk viscosity with a constant coefficient,
whereas shear viscosity is excluded due to the isotropy of the universe. We
performed a Likelihood analysis using recent observational datasets from BAO,
CMB, Cosmic Chronometer measurements, and Type Ia supernovae to put
cosmological constraints on the model. We found that the viscosity coefficient
is not constrained by the data, whence the analysis is continued with this
coefficient fixed.
  The model yields a minimum $\chi^2$-value of $1357.91$ compared to $1363.34$
for the $\Lambda$CDM model. Using the Akaike Information Criterion (AIC) we
found that our model minimizes the AIC value with a difference of -1.4 compared
to the $\Lambda$CDM model, which indicates that DUDF model is viable according
to background observations. We studied the evolution of the universe due to
DUDF model by studying the evolution of different cosmological parameters like
the Hubble parameter, the distance modulus, the deceleration parameter, the
effective equation of state parameter, and the density parameter. We show that
DUDF model doesn't deviate from the standard $\Lambda$CDM model at early times,
with the ability to play the role of the cosmological constant by accelerating
the universe in the late times.
