Dissipative unified dark fluid: Observational constraints

Abstract

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 H0 measurements, Type Ia supernovae, observational Hubble data, baryon acoustic oscillations, and cosmic microwave background 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 implemented to the ΛCDM model for comparison. Our analysis revealed that between the three analyzed models, the DUDF has the lower χmin2-value. Based on model selection statistics in the form of the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC), we compared different models to select the favored one due to the observational data used. Our results revealed that the UDF model has the minimum AIC, whence, due to the AIC, it is the most favorable model for the data. ΔAIC values of models are, then, measured to this model. This difference indicated that the DUDF model is substantial on the level of empirical support. On the other hand, the BIC results still prefer the ΛCDM model. Our dark fluid also alleviates the tension in the Hubble parameter. We got an H0 value of 70.02 kms−1Mpc−1 for the DUDF model and 70.25 kms−1Mpc−1 for the UDF model. These results alleviate the tension to ∼2.2σ for the DUDF model and ∼2.1σ for the UDF model. In addition, we studied the evolution of the deceleration parameter, the effective equation of state parameter, and the density parameter. We also estimated a value for the viscosity of the cosmic fluid. We found that our unified fluid doesn't deviate from the standard ΛCDM model at early times, with the ability to play the role of the cosmological constant by accelerating the universe at late times.