that, as far as bulk viscosity is not dominant, the regularization provided by the index of the fluid preserves the same power-law behavior of the inviscid isotropic Universe. In other words, the bulk viscosity coefficient enters trough an effective equation of state, ranging the same parameters domain of an ideal fluid (i.e. between dust and stiff matter). Finally, we show how, if the bulk viscosity becomes sufficiently dominant, it is possible to get an equation of state having an effective polytropic index less than $\nicefrac{2}{3}$, leading to a power-law inflation solution. The latter is characterized by a massive entropy creation and no longer causal separation exists across the Universe regions.

Basic formalism {#basic-formalism .unnumbered}
===============

The Lichnerowicz original stress-energy tensor describing relativistic viscous fluids stands as follows [@LICH1] $$\begin{aligned}
\label{TmunuC}
\begin{split}
T_{\mu\nu}&=(\rho+p)u_{\mu}u_{\nu}+pg_{\mu\nu}-\left(\varsigma-\frac{2}{3}\eta\right)\pi_{\mu\nu}\nabla_{\alpha}C^{\alpha}\\ &-\eta\pi_{\mu}^{\alpha}\pi_{\nu}^{\beta}\left(\nabla_{\alpha}C_{\beta}+\nabla_{\beta}C_{\alpha}\right),
\end{split}\end{aligned}$$ where $\rho$ is the energy density, $p$ is the pressure, $g_{\mu\nu}$ denotes the metric tensor with the signature $(-+++)$ and $u^{\mu}$ is the four-velocity properly normalized as $$u_{\mu}u^{\mu}=-1\,.$$ The bulk and shear viscous contributions are represented by the $\zeta$ and $\eta$ coefficients, respectively. Here, $$\pi_{\mu\nu}=g_{\mu\nu}+u_{\mu}u_{\nu}$$ is the projection tensor. Furthermore, $C^\mu$ represents the so-called dynamical velocity which is related to $u^\mu$ by $$\label{C}
C^{\mu}=Fu^{\mu},$$ $F$ being 