Tachyonic vs Quintessence dark energy: linear perturbations and CMB data
  We use linear perturbation theory to study perturbations in dynamical dark
energy models. We compare quintessence and tachyonic dark energy models with
identical background evolution. We write the corresponding equations for
different models in a form that makes it easier to see that the two models are
very hard to distinguish in the linear regime, especially for models with $(1 +
w) \ll 1$. We use Cosmic Microwave Background data and parametric
representations for the two models to illustrate that they cannot be
distinguished for the same background evolution with existing observations.
Further, we constrain tachyonic models with the Planck data. We do this
analysis for exponential and inverse square potentials and find that the
intrinsic parameters of the potentials remain very weakly constrained. In
particular, this is true in the regime allowed by low redshift observations.
