A Singularity Problem with f(R) Dark Energy
  In this paper, I point out that there is a curvature singularity problem appearing on non-linear level that generally plagues f(R) models that modify Einstein gravity in the infrared. It is caused by the fact that for the effective scalar degree of freedom, the curvature singularity is at a finite field value and energy level, and can be easily accessed by the field dynamics in the presence of matter. This problem is invisible in linearized analysis, except for the tell-tale growing oscillatory modes it causes. In view of this, viability of many f(R) models in current literature will have to be re-evaluated. 
Andrei V. Frolov