Challenging bulk viscous unified scenarios with cosmological observations

TL;DR

The paper investigates a unified dark matter/energy framework with bulk viscosity, modeled by a power-law η(ρ_D) = α ρ_D^m, and analyzes two BVF families (BVF1 with γ fixed, BVF2 with γ free) across four subcases (m = 0 or m free). Using Planck 2015 CMB, Pantheon SNIa, and cosmic chronometer data, the authors constrain the model parameters via MCMC, reveal nonzero bulk viscosity (β ≠ 0) in the data, and find that allowing m to be negative improves cross-dataset consistency and can modestly alleviate the H_0 tension. However, Bayesian evidence consistently favors ΛCDM over the bulk-viscous scenarios, indicating that while these models can fit current data and reduce H_0 tension to an extent, they are not preferred when penalized for model complexity. The results highlight the potential of bulk-viscous UDM as an alternative cosmology, while underscoring ΛCDM’s robustness in light of Bayesian model comparison.

Abstract

In a spatially flat Friedmann-Lemaître-Robertson-Walker universe, we investigate a unified cosmic fluid scenario endowed with bulk viscosity in which the coefficient of the bulk viscosity has a power law evolution. The power law in the bulk viscous coefficient is a general case in this study which naturally includes several choices as special cases. Considering such a general bulk viscous scenario, in the present work we have extracted the observational constraints using the latest cosmological datasets and examine their behaviour at the level of both background and perturbations. From the observational analyses, we find that a non-zero bulk viscous coefficient is always favored and some of the models in this series are able to weaken the current tension on $H_0$ for some dataset. However, from the Bayesian evidence analysis, $Λ$CDM is favored over the bulk viscous model.

Figures (12)

• Figure 1: Qualitative evolution of the density parameters for the BVF1 model with $m =0$ have been shown for different values of $\beta$, namely, $\beta =0.5$ (upper panel), $\beta =0.55$ (middle panel), $\beta = 0.6$ (lower panel), and also compared with no bulk viscous scenario (corresponding to $\beta =0$).
• Figure 2: We show some general behaviour of the BVF1 model considering the fact that $m \neq 0$. In the upper panel we fix $\beta =0.6$ and consider the density parameters for $m =0.4$ and also compared to the constant bulk viscous scenario (corresponding to $m =0$). In the lower panel we fix $m =0.2$ and consider three different values of $\beta$ in order to depict the evolution of the density parameters.
• Figure 3: Qualitative evolution of the density parameters for the BVF2 model with $m =0$ have been shown for different values of $\beta$, namely, $\beta =0.5$ (upper panel), $\beta =0.55$ (middle panel), $\beta = 0.6$ (lower panel), and also compared with no bulk viscous scenario (corresponding to $\beta =0$). Let us note that for all the plots we have fixed $\gamma = 1.01$.
• Figure 4:
• Figure 5: 68% and 95% c.l. contour plots for the BVF1 model with $m =0$, using the observational data from different sources. The figure also shows the one dimensional marginalized posterior distributions for some selected parameters.