Perturbations in a stringlike fluid

We study perturbatively a two-fluid model, where one of the fluids has an equation of state $p=\ensuremath{-}\ensuremath{\rho}/3$ (stringlike fluid), considering fluctuations in both fluids. Such a model can represent a closed universe whose dynamics is that of an open universe. We find strong instabilities at small scale in the perturbation of the stringlike fluid, whereas the ordinary matter fluid has a stable behavior. We discuss the consequences for the anisotropy of the cosmic microwave background.