Viscous cosmologies with variableΛin higher derivative gravity

Abstract

We obtain cosmological solutions in higher derivative theories of gravity in the presence of an imperfect fluid and a time varying cosmological parameter ($\ensuremath{\Lambda}$). We explore both power-law and exponential expansion of the universe considering an imperfect fluid described by the Eckart, truncated, and full causal theories proposed by Israel and Stewart. We discuss the physical and geometrical features of the models in the presence of a known time varying $\ensuremath{\Lambda}$. The interesting feature of the models is that they admit accelerating universes in some specific cases. The evolution of the temperature of the universe in the framework of the full causal theory is also determined.