Phase-space of flat Friedmann-Robertson-Walker models with both a scalar field coupled to matter and radiation

Abstract: We investigate the phase-space of a flat FRW universe including both a scalar field, $\phi,$ coupled to matter, and radiation. The model is inspired in scalar-tensor theories of gravity, and thus, related with $F(R)$ theories through conformal transformation. The aim of the chapter is to extent several results to the more realistic situation when radiation is included in the cosmic budget particularly for studying the early time dynamics. Under mild conditions on the potential we prove that the equilibrium points corresponding to the non-negative local minima for $V(\phi)$ are asymptotically stable. Normal forms are employed to obtain approximated solutions associated to the inflection points and the strict degenerate local minimum of the potential. We prove for arbitrary potentials and arbitrary coupling functions $\chi(\phi),$ of appropriate differentiable class, that the scalar field almost always diverges into the past. It is designed a dynamical system adequate to studying the stability of the critical points in the limit $|\phi|\to\infty.$ We obtain there: radiation-dominated cosmological solutions; power-law scalar-field dominated inflationary cosmological solutions; matter-kinetic-potential scaling solutions and radiation-kinetic-potential scaling solutions. Using the mathematical apparatus developed here, we investigate the important examples of higher order gravity theories $F(R) = R + \alpha R^2$ (quadratic gravity) and $F(R) =R^n.$ We illustrated both analytically and numerically our principal results. In the case of quadratic gravity we prove, by an explicit computation of the center manifold, that the equilibrium point corresponding to de Sitter solution is locally asymptotically unstable (saddle point).