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The Chaplygin gas model, characterized by an equation of state of the type $p= - \frac{A}{\rho}$ emerges naturally from the Nambu-Goto action of stringtheory. This fluid representation can be recast under the form of a tachyonicfield given by a Born-Infeld type Lagrangian. At the same time, the Chaplygingas equation of state can be obtained from a self-interacting scalar field. Weshow that, from the point of view of the supernova type Ia data, the threerepresentations (fluid, tachyonic, scalar field) lead to the same results.However, concerning the matter power spectra, while the fluid and tachyonicdescriptions lead to exactly the same results, the self-interacting scalarfield representation implies different statistical estimations for theparameters. In particular, the estimation for the dark matter density parameterin the fluid representation favors a universe dominated almost completely bydark matter, while in the self-interacting scalar field representation theprediction is very closed to that obtained in the $\Lambda$CDM model.

Power spectrum in the Chaplygin gas model: tachyonic, fluid and scalar field representations