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 local $H_0$ measurements, SNe Ia, observational Hubble data, BAO, and CMB to put cosmological constraints on the model. The special case of the non-dissipative unified dark fluid (UDF) is also studied, while a similar analysis is performed on the $\Lambda$CDM model for comparison. We got an $H_0$ value of $70.02$ $km$ $s^{-1} Mpc^{-1}$ for DUDF and $70.25$ $km$ $s^{-1} Mpc^{-1}$ for the UDF model. Our analyses revealed that the DUDF model has the lower $\chi_{\rm min}^2$-value. Based on model selection statistics in the form of the Akaike Information Criterion (AIC), we compare different models to select the favored one due to the observational data used. Our results revealed that the UDF model has the minimum AIC, with the conclusion that it is the most favorable model for the data. $\Delta AIC$ value of other models are then measured to this model. This difference indicated that the DUDF model is a substantial model on the level of empirical support. Examining the evolution of the deceleration parameter, the effective equation of state parameter, and the density parameter indicated that our unified fluid 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 at the late times.