  In a recent article, Kleidis and Spyrou (2015) proposed that both dark matter
(DM) and dark energy (DE) can be treated as a single component, if accommodated
in the context of a polytropic DM fluid with thermodynamical content. Depending
only on the polytropic exponent, $-0.103 < \Gamma \leq 0$, this unified DM
model reproduces to high accuracy the distance measurements performed with the
aid of the supernovae Type Ia (SNe Ia) standard candles, without suffering
either from the age or from the coincidence problem. To demonstrate also its
compatibility with current observational data concerning structure formation,
in the present article we discuss the evolution of cosmological perturbations
in the $\Lambda$ CDM-like (i.e., $\Gamma = 0$) limit of the polytropic DM
model. The corresponding results are quite encouraging, since, such a model
reproduces every major effect already known from conventional (i.e.,
pressureless cold dark matter - CDM) structure formation theory, such as the
constancy of metric perturbations in the vicinity of recombination and the
(late-time) Meszaros effect on their rest-mass density counterparts (Meszaros
1974). The non-zero (polytropic) pressure, on the other hand, drives the
evolution of small-scale velocity perturbations along the lines of the
root-mean-square velocity law of conventional Statistical Physics. As a
consequence, in this model "peculiar velocities" slightly increase, instead of
being redshifted away by cosmic expansion. What is more important is that, upon
consideration of scale-invariant metric perturbations, the spectrum of their
rest-mass density counterparts exhibits an effective power-law dependence on
the (physical) wavenumber, with the associated scalar spectral index being
equal to $n_s = 0.970$; a theoretical value that actually reproduces the
corresponding observational (Planck) result (Ade et al. 2016).
