Post-inflationary behavior of adiabatic perturbations and   tensor-to-scalar ratio
  We explain why it is so difficult and perhaps even impossible to increase the cosmological tensor-to-scalar perturbation ratio during the post-inflationary evolution of the universe. Nevertheless, contrary to some recent claims, tensor perturbations can be relatively large in the simplest inflationary models which do not violate any rules of modern quantum field theory. 
Andrei Linde, Viatcheslav Mukhanov and Misao Sasaki