  In this paper we study the perturbations of a cosmic multi-fluid medium
consisting of radiation, dust and a Chaplygin gas. To do so, we follow the 1 +
3 covariant formalism and derive the evolution equations of the fluctuations in
the energy density for each species of fluid in the multi-fluid system. The
solutions to these coupled systems of equations are then computed in both
short-wavelength and long-wavelength modes. Our preliminary results suggest
that unlike most dark energy models that discourage large-scale structure
formation due to the rapid cosmological expansion (which gives little time for
fluctuations to coalesce), the Chaplygin-gas model supports the formation of
cosmic structures. This is manifested in the solutions of the perturbation
equations which show the growth of density fluctuations over time.
