Perturbed dark energy: classical scalar field versus tachyon
  The evolution of scalar linear perturbations is studied in gauge-invariant
approach for 2-component models with nonrelativistic matter and minimally
coupled scalar fields, the potentials of which were constructed for either
constant dark energy equation of state (EoS) parameter $w$ or its adiabatic
sound speed $c_a^2$ equal to zero. The numerical solutions show that such
fields are almost smoothed out on subhorizon scales. However they cause the
scale dependent suppression of the nonrelativistic mater density perturbations
and the decay of gravitational potential, which can be used for choice of the
dark energy model. We discuss 2 types of the Lagrangian: classical and
tachyonic ones. As our results show, the fields with $w=const$ are almost
indistinguishable, while for fields with $c_a^2=0$ the difference of dark
energy effective sound speeds $c_s^2$, which is caused by the shape of
Lagrangian, affects the evolution of perturbations significantly. We present
also the transfer functions for both components.
