A varying Dark Energy effective speed of sound parameter in the phantom
  Universe
  We analyse the phenomenological effects of a varying Dark Energy (DE)
effective speed of sound parameter, $c^{2}_{\textrm{sd}}$, on the cosmological
perturbations of three phantom DE models. Each of these models induce a
particular abrupt future event known as Big Rip (BR), Little Rip (LR), and
Little Sibling of the Big Rip (LSBR). In this class of abrupt events, all the
bound structures in the Universe would be ripped apart at a finite cosmic time.
We compute the evolution of the perturbations, $f\sigma_{8}$ growth rate and
forecast the current matter power spectrum. We vary the $c^{2}_{\textrm{sd}}$
parameter in the interval $[0,1]$ and compute the relative deviation with
respect $c^{2}_{\textrm{sd}}=1$. In addition, we analyse the effect of
gravitational potential sign flip that occurs at very large scale factors as
compared with the current one.
