Analytical solutions for cosmological perturbations in a one-component   universe with shear stress
  We construct explicit solutions for scalar, vector and tensor perturbations in a less known setting, a flat universe filled by an isotropic elastic solid with pressure and shear modulus proportional to energy density. The solutions generalize the well known formulas for cosmological perturbations in a universe filled by ideal fluid. 
Matej \v{S}kovran
gr-qc