Viscous fluid dynamics with decaying vacuum energy density

Abstract: In this work, we investigate the dynamics of bulk viscous models with decaying vacuum energy density (VED) in a spatially homogeneous and isotropic flat Friedmann-Lema^{i}tre- Robertson-walker (FLRW) spacetime. We particularly are interested to study the viscous model which considers first order deviation from equilibrium, i.e., the Eckart theory. In the first part, using the different forms of the bulk viscous coefficient, we find the main cosmological parameters, like Hubble parameter, scale factor, deceleration parameter and equation of state parameter analytically. We discuss some cosmological consequences of the evolutions and dynamics of the different viscous models with decaying VED. We examine the linear perturbation growth in the context of the bulk viscous model with decaying VED to see if it survives this further level of scrutiny. The second part of the work is devoted to constrain the viscous model of the form $\zeta \propto H$, where $\zeta$ is the bulk viscous coefficient and $H$ is the Hubble parameter, using three different combinations of data from type Ia supernovae (Pantheon), $H(z)$ (cosmic chronometers), Baryon Acoustic Oscillation and $f(z)\sigma_8(z)$ measurements with Markov Chain Monte Carlo (MCMC) method. We show that the considered model is compatible with the cosmological probes, and the $\Lambda$CDM recovered in late-time of the evolution of the Universe. Finally, we obtain selection information criteria (AIC and BIC) to study the stability of the models.