The growth of fluctuations in Chaplygin gas cosmologies: A nonlinear
  Jeans scale for unified dark matter
  Unified dark matter cosmologies economically combine missing matter and
energy in a single fluid. Of these models, the standard Chaplygin gas is
theoretically motivated, but faces problems in explaining large scale structure
if linear perturbations are directly imposed on the homogeneous fluid. However,
early formation of a clustered component of small halos is sufficient (and
necessary) for hierarchical clustering to proceed, with the halos merging into
larger structures, as in the standard scenario, and the remaining homogeneous
component acting as dark energy. We examine this possibility. A linear analysis
shows that a critical Press-Schecter threshold for collapse can generally only
be reached for generalized Chaplygin gas models that mimic $\Lambda$CDM, or
ones where superluminal sound speeds occur. But the standard Chaplygin gas case
turns out to be marginal, with overdensities reaching order one in the linear
regime. This motivates a nonlinear analysis. A simple infall model suggests
that collapse is indeed possible for perturbations of order 1~kpc and above;
for, as opposed to standard gases, pressure forces decrease with increasing
densities, allowing for the collapse of linearly stable systems. This suggests
that a cosmological scenario based on the standard Chaplygin gas may not be
ruled out from the viewpoint of structure formation, as often assumed. On the
other hand, a 'nonlinear Jeans scale', constricting growth to scales $R \gtrsim
{\rm kpc}$, which may be relevant to the small scale problems of CDM, is
predicted. Finally, the background dynamics of clustered Chaplygin gas
cosmologies is examined and confirmed to be viable.
