$Λ$CDM Model Extension with Dissipative Dynamics and Sign-Switching Bulk Viscous Pressure

In this article, we extend the standard $\Lambda$CDM model by incorporating causal viscous dark matter (vDM) modelled using Maxwell-Cattaneo theory and obtain a novel Hubble parameter solution for the model, which can even be extended to incorporate both radiation and baryonic matter components. A detailed and comprehensive analysis of the model is carried out by deriving theoretical constraints on model parameters, comparing the model with the latest observational data sets and validating the behaviour of the model under different thermodynamic laws. Such an in-depth analysis of the model yielded several intriguing results, like the presence of sign-switching bulk viscous pressure which aids both, the decelerated expansion in the early universe and the accelerated expansion in the late universe, possibility of having negative specific entropy rate in the early Universe while still satisfying the covariant second law of thermodynamics, and a correlation between relaxation time parameter and sign-switching redshift. Finally, we propose a unified dark matter (UDM) interpretation for the dark sector in this model and hence showed that, under the said UDM interpretation, this model can satisfy the much-required near equilibrium condition associated with the background dissipative theory both in the early and late phase of the Universe. Based on the NEC trend showcased by the model, we conjuncture that the late-accelerating behaviour of Universe can be viewed as a means for the UDM fluid to attain a global equilibrium state.