Critical collapse and black hole formation within an expanding perfect
  fluid
  Following on after three previous papers discussing the formation of
primordial black holes in the early universe during the radiation dominated
era, we present here related results considering the theoretical possibility of
having a fluid with a different sound speed. In practice we vary the parameter
w of the equation of state p = we between 0 and 1, where e is the energy
density, and p the pressure of the fluid. We consider also the possibility of
having a cosmological constant interpreted as a vacuum energy component
(corresponding to w = -1) revising the results presented in our first paper of
this series, having now a more refined numerical code available. To perform our
simulations we have fine tuned our previous computer code allowing us to
observe that the nature of the critical collapse is conserved for all the
different values of w considered, in agreement with previous studies of ideal
collapses done in a different context from PBHs. As already done in our
previous works, we start the simulations with initial supra-horizon scale
perturbations of a type which could have come from inflation, characterized by
a pure growing mode component.
