On a class of scaling FRW cosmological models

We study Friedmann-Robertson-Walker cosmological models with matter content composed of two perfect fluids $ρ_1$ and $ρ_2$, with barotropic pressure densities $p_1/ ρ_1=ω_1=const$ and $p_2/ ρ_2=ω_2=const$, where one of the energy densities is given by $ρ_1=C_1 a^α+ C_2 a^β$, with $C_1$, $C_2$, $α$ and $β$ taking constant values. We solve the field equations by using the conservation equation without breaking it into two interacting parts with the help of a coupling interacting term $Q$. Nevertheless, with the found solution may be associated an interacting term $Q$, and then a number of cosmological interacting models studied in the literature correspond to particular cases of our cosmological model. Specifically those models having constant coupling parameters $\tildeα$, $\tildeβ$ and interacting terms given by $Q=\tildeα H ρ_{_{DM}}$, $Q=\tildeα H ρ_{_{DE}}$, $Q=\tildeα H (ρ_{_{DM}}+ ρ_{_{DE}})$ and $Q=\tildeα H ρ_{_{DM}}+\tildeβ H ρ_{_{DE}}$, where $ρ_{_{DM}}$ and $ρ_{_{DE}}$ are the energy densities of dark matter and dark energy respectively. The studied set of solutions contains a class of cosmological models presenting a scaling behavior at early and at late times. On the other hand the two-fluid cosmological models considered in this paper also permit a three fluid interpretation which is also discussed. In this reinterpretation, for flat Friedmann-Robertson-Walker cosmologies, the requirement of positivity of energy densities of the dark matter and dark energy components allows the state parameter of dark energy to be in the range $-1.37 \lesssim ω_{_{DE}}<-1/3$.

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