Aspects of cosmological expansion inF(R) gravity models

Abstract

We study cosmological expansion inF(R) gravity using the trace of the field equations. High frequency oscillations in the Ricci scalar,whose amplitude increases as one evolves backward in time, have been predicted inrecent works. We show that the approximations used to derive this result veryquickly break down in any realistic model due to the non-linear nature of theunderlying problem. Using a combination of numerical and semi-analytic techniques,we study a range of models which are otherwise devoid of known pathologies.We find that high frequency asymmetric oscillations and a singularity at finitetime appear to be present for a wide range of initial conditions. We show thatthis singularity can be avoided with a certain range of initial conditions, whichwe find by evolving the models forwards in time. In addition we show that theoscillations in the Ricci scalar are highly suppressed in the Hubble parameter and scalefactor.