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Gravitational Instability in Presence of Bulk Viscosity: Jeans Mass and the Quasi-Isotropic Solution

The talk focuses on the analysis of the gravitational instability in presence of dissipative effects both in the Newtonian regime and in the fully relativistic approach. In particular, the standard Jeans Mechanism and the so-called Quasi-Isotropic Solution are treated when bulk viscosity enters the dynamics. We express the viscosity coefficient $\zeta$ as a power-law of the fluid energy density: $\zeta=\zeta_0\epsilon^{s}$ and we analyze the dynamics up to first and second order in time. In the Newtonian regime, the perturbation evolution is founded to be damped by bulk viscosity and the top-down mechanism of structure fragmentation is suppressed. In such a scheme, the value of the Jeans Mass remains unchanged also in presence of viscosity. On the other hand, in the relativistic approach, we get, as a result, a power-law solution existing only in correspondence to a restricted domain of $\zeta_0$.

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