Stochastic Background of Gravitational Waves as a Benchmark for Extended   Theories of Gravity
  The cosmological background of gravitational waves can be tuned by Extended Theories of Gravity. In particular, it can be shown that assuming a generic function f(R) of the Ricci scalar R gives a parametric approach to control the evolution and the production mechanism of gravitational waves in the early Universe. 
S. Capozziello, M. De Laurentis, M. Francaviglia