Late-time cosmological evolution in f (R ) theories with ordinary and collisional matter

We study the late-time cosmological evolution of f(R) theories of modified gravity, with the matter content of the Universe being that of collisional self-interacting matter. We assume that the Universe is described by a flat Friedmann-Lemaitre-Robertson-Walker metric and that it is matter and dark energy dominated. The results of our numerical analysis for a collisional matter f(R) theory are compared with those resulting from pressureless matter f(R) theory and from the model. As we shall demonstrate, the resulting picture can vary from model to model, indicating that the effect of collisional matter in f(R) theories is strongly model dependent. In all studied cases, the effective equation of state parameter does not cross the phantom divide, both in the collisional matter and pressureless matter f(R) theories. Finally, we thoroughly study the effects of collisional matter on one of the f(R) models that is known to provide a unified description of early time inflation and late-time acceleration. The overall picture of the evolution of the Universe is not drastically affected, apart from the matter era, which is further enhanced with an additional matter energy contribution. However, a fully consistent description of the Universe's evolution requires the introduction of a dark energy compensate in the total energy density, a concept very well known from the literature.