Dynamical Systems Analysis in Post-Friedmann Parametrizations of Modified Theories of Gravity
We carry out a dynamical analysis of first order perturbations for Cold Dark Matter, $Λ$ Cold Dark Matter, and a couple of Modified Gravity models using the Parametrized Post-Friedmann formalism. We use normalized variables to set the proper dynamical system of equations through which we make the analysis in order to shed some light on the dynamics of such perturbations inside these models. For Modified Gravity models, we use the scale-independent and -dependent parametrizations, in particular, two $f(R)$ and two Chameleon-like models are considered within the quasi-static approximation. Given the employed formalism, we found that the critical points and stability features of the dynamical systems for Modified Gravity models are the same as those found in the standard $Λ$ Cold Dark Matter model. However, the behavior around the critical points suffers important modifications in some specific cases. We explicitly find that signatures of these Modified Gravity models mainly arise on the velocity perturbations, while the density contrast and the curvature potentials turn out to be less sensitive to the parametrization taken into consideration. We also provide a percentage estimation of the extent of modification in the perturbations in the Modified Gravity models considered in comparison to the standard $Λ$ Cold Dark Matter model along the expansion history and for a couple of wavenumbers.