Darboux class of cosmological fluids with time-dependent adiabatic indices

A one-parameter family of time-dependent adiabatic indices is introduced fo r any given type of cosmological fluid of constant adiabatic index by a mat hematical method belonging to the class of Darboux transformations. The pro cedure works for zero cosmological constant at the price of introducing a n ew constant parameter related to the time dependence of the adiabatic index . These fluids can be the real cosmological fluids that are encountered at cosmological scales and they could be used as a simple and efficient explan ation for the recent experimental findings regarding the present day accele rating universe. In addition, new types of cosmological scale factors, corr esponding to these fluids, are presented.