The effective dynamics of loop quantum $R^2$ cosmology

Abstract

The effective dynamics of loop quantum $f (R)$ cosmology in Jordan frame is considered by using the dynamical system method and numerical method. To make the analyze in detail, we focus on $R^2$ model since it is simple and favored from observations. In classical theory, $(\phi = 1, \dot{\phi} = 0)$ is the unique fixed point in both contracting and expanding states, and all solutions are either starting from the fixed point or evolving to the fixed point; while in loop theory, there exists a new fixed point (saddle point) at $(\phi \simeq 2 / 3,\dot{\phi} = 0)$ in contracting state. We find the two critical solutions starting from the saddle point control the evolution of the solutions starting from the fixed point $(\phi = 1, \dot{\phi} = 0)$ to bounce at small values of scalar field in $0 <\phi < 1$. Other solutions, including the large field inflation solutions, all have the history with $\phi < 0$ which we think of as a problem of the effective theory of loop quantum $f(R)$ theory. Another different thing from loop quantum cosmology with Einstein-Hilbert action is that there exist many solutions do not have bouncing behavior.