Shear and rotation in Chaplygin cosmology
  We study the effect of shear and rotation on results previously obtained
dealing with the application of the spherical collapse model (SCM) to
generalized Chaplygin gas (gCg) dominated universes. The system is composed of
baryons and gCg and the collapse is studied for different values of the
parameter $\alpha$ of the gCg. We show that the joint effect of shear and
rotation is that of slowing down the collapse with respect to the simple SCM.
This result is of utmost importance for the so-called unified dark matter
models, since the described slow down in the growth of density perturbation can
solve one of the main problems of the quoted models, namely the instability
described in previous papers [e.g., H. B. Sandvik {\it et al.}, Phys. Rev. D
{\bf 69}, 123524 (2004)] at the linear perturbation level.
