The dark energy as a natural property of cosmic polytropes -- A tutorial

Theoretical results on a conventional approach to the dark energy (DE) concept are reviewed and discussed. According to them, there is absolutely no need for a novel DE component in the Universe, provided that the associated matter-energy content is represented by a perfect fluid whose volume elements perform polytropic flows. When the thermodynamic energy of this fluid's internal motions is also considered as a source of the universal gravitational field, it compensates the DE needed to compromise spatial flatness in an accelerating Universe. The cosmological model with matter-energy content in the form of a polytropic fluid not only interprets the observations associated to the recent history of Universe expansion, but successfully confronts with all the current cosmological issues, thus arising as a viable alternative to $\Lm$CDM model.