Back reaction of a long range force on a Friedmann-Robertson-Walker   background
  It is possible that there may exist long-range forces in addition to gravity. In this paper we construct a simple model for such a force based on exchange of a massless scalar field and analyze its effect on the evolution of a homogeneous Friedmann-Robertson-Walker cosmology. The presence of such an interaction leads to an equation of state characterized by positive pressure and to resonant particle production similar to that observed in preheating scenarios. 
Martina Brisudova and William H. Kinney (Univ. of Florida)