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 pressure-less matter $f(R)$ theory and from the $\mathrm{\Lambda}\mathrm{CDM}$ 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. Particularly, in a few cases, may give better fit to the $\mathrm{\Lambda}\mathrm{CDM}$ model. In all studied cases, the effective equation of state parameter does not cross the phantom divide, both in the collisional matter and pressure-less 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 density term, which is of leading order. 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. 
V.K. Oikonomou, N. Karagiannakis
gr-qc astro-ph.CO