One-parameter class of cosmological fluids with time dependent adiabatic indices for accelerating universes

A one-parameter family of time dependent adiabatic indices is introduced for any given type of cosmological fluid of constant adiabatic index by a mathematical method belonging to the class of Darboux transformations. The procedure works for zero cosmological constant at the price of introducing a new constant parameter related to the time dependence of the adiabatic index. I argue that these are the real cosmological fluids that are encountered at cosmological scales and that they can provide a simple and efficient explanation for the recent experimental findings regarding the present day accelerating universe. Draft of submission date: March 28, Yr 2·10 a.C. 1 Recent data from type Ia supernovae that can still be considered as meager provoked tremendous interest since they indicate that the universe has now an accelerating large scale expansion [1] (ρ+ 3p < 0 in eq. 1 Institute of Physics, Guanajuato State University, P.O. Box E-143, Leon, Gto, Mexico. International Center for Relativistic Astrophysics, Rome-Pescara, Italy, after April 2000