Challenging bulk viscous unified scenarios with cosmological observations

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 behavior at the level of both background and perturbations. From the observational analyses, we find that a nonzero 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, $\mathrm{\Lambda }$ cold dark matter is favored over the bulk viscous model.

Figure 1

Qualitative evolution of the density parameters for the BVF1 model with $m=0$ is shown for different values of $\beta$, namely, $\beta =0.5$ (upper panel), $\beta =0.55$ (middle panel), and $\beta =0.6$ (lower panel), and also compared with the no bulk viscous scenario (corresponding to $\beta =0$).

Figure 2

We show some general behavior of the BVF1 model, considering the fact that $m\ne 0$. In the upper panel, we fix $\beta =0.6$ and consider the density parameters for $m=0.4$ and also compare 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 $m=0$ is shown for different values of $\beta$, namely, $\beta =0.5$ (upper panel), $\beta =0.55$ (middle panel), and $\beta =0.6$ (lower panel), and is also compared with the no bulk viscous scenario (corresponding to $\beta =0$). Let us note that for all the plots we have fixed $\gamma =1.01$.

Figure 4

We show some general behavior of the BVF2 model considering the fact that $m\ne 0$. 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. Let us note that for all the plots we have fixed $\gamma =1.01$.

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.

Figure 6

The figure displays the 68% and 95% C.L. contour plots between various combinations of the parameters of the model BVF1 with $m$ free using the observational data from different sources. The figure also shows the one-dimensional marginalized posterior distributions for some selected parameters.

Figure 7

CMB temperature power spectra using the best-fit values of various free and derived parameters are shown for the BVF1 model with $m=0$ (i.e., the constant bulk viscosity).

Figure 8

CMB temperature power spectra using the best-fit values of various free and derived parameters of the generalized the bulk viscous model BVF1.

Figure 9

68% and 95% C.L. contour plots for the BVF2 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.

Figure 10

The figure displays the 68% and 95% C.L. contour plots between various combinations of the parameters of the model BVF2 using the observational data from different sources. The figure also shows the one-dimensional marginalized posterior distributions for some selected parameters.

Figure 11

We show the CMB TT spectra for the BVF2 model with $m=0$ using the best-fit values of the free and derived parameters.

Figure 12

Temperature anisotropy in the CMB spectra for different best-fit values of the free and derived parameters of the generalized bulk viscous scenario BVF2.