Static Dark Fluid Thin Shells in Schwarzschild-de Sitter Spacetimes: Stability and Black Hole Shadows

We study the existence and radial stability of static, spherically symmetric thin shells joining two Schwarzschild-de Sitter (SdS) spacetimes $(m_\pm,Λ_\pm)$. Near the equilibrium radius $R_0$ the shell's surface density $σ$ and pressure $p$ obey the linearized barotropic law $p=p_0+c_s^2(σ-σ_0)$, w